AU9522098A

AU9522098A - Superconducting magnetic coil

Info

Publication number: AU9522098A
Application number: AU95220/98A
Authority: AU
Inventors: Dawood Aized; Robert E. Schwall
Original assignee: American Superconductor Corp
Current assignee: American Superconductor Corp
Priority date: 1994-01-24
Filing date: 1998-12-03
Publication date: 1999-04-01
Anticipated expiration: 2015-01-09
Also published as: AU739105B2

Description

Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

(ORIGINAL)

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Name of Applicant: Actual Inventors: American Superconductor Corporation, of Two Technology Drive, Westborough, Massachusetts 01581, United States of America AIZED, Dawood SCHWALL, Robert, E.

DAVIES COLLISON CAVE, Patent Attorneys, of 1 Little Collins Street, Melbourne, Victoria 3000, Australia "Superconducting magnetic coil" Address for Service: Invention Title: I a The following statement is a full description of this invention, including the best method of performing it known to us: -1 *is ,Zsr

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p rpEK'KATl5614$ I2 ?.2/7,93 1A- SUPERCONDUCTING MAGNETIC

COIL

STATEMENT AS TO FEDERALLY SPONSORED

RESEARCH

This invention arose in part out of research pursuant to Subcontract No. 86X-SK700V awarded by the Department of Energy.

BACKGROUND OF THE INVENTION This is a continuation-in-part of Aized. entitled SUPERCONDUCTING

MAGNETIC

S 10 COIL. filed Jan. 24, 1994. Ser. No. 08/186,328 abandoned.

The invention relates to superconducting magnetic coils and methods for 1 manufacturing them.

As is known in the art, the most spectacular property of a superconductor is tihe disappearance of its electrical resistance when it is cooled below a critical temperature

T,.

Another important property is the destruction of superconductivity by the application of a magnetic field equal to or greater than a critical field The value of for a given superconductor, is a function of the temperature, given approximately by ,5 .H H (1-T 2

T)

where the critical field at 0 K, is, in general, different for different superconductors. For applied magnetic fields less than ,-the flux is excluded from the bulk of the superconducting sample, penetrating only to a small depth, known as the penetration depth, into the surface of the superconductor.

The existence of a critical field implies the'existence of a critical transport electrical current, referred to more simply as the critical current of the superconductor. The critical current is the current which establishes the point at which the material loses its 1^ 25 superconductivity properties and reverts back to its normally conducting state.

S. Superconducting materials are generally classified as either low or high temperature superconductors operating below or at 4.2'K and below or at 108 K, ctr operating:'?^ 7 -2respectively. Higi. temperature superconductors

(HTS),

such as those made from ceramic-- r metallic oxides are anisotropic, meaning that they generally conduct better in one direction than another. Moreover, it has been observed that ue to this:anisotropic characteristic, the critical current varies asa function of ne 1. orientation of the magnetic field with respe- to the I crystallographic axes of the superconducting material.

High temperature oride superconauctors include general 10 Cu-0-based ceramic superconductors, members of the rareearth-copper-oxide family (YBCO), the thallium-barium- S calcium-copper-oxide family. (TBCCO), the mercury-bariumcacium-copper-oxide family (HgBCCO), and BSCCO compounds containing stoichiometric amounts of lead (ie.,(Bi,Pb) 2 Sr 2 Ca 2

CU

3 Olo).

High temperature superconductors may be used to fabricate superconducting magnetic coils such as Ssolenoids, racetrack magnets, multipole magnets, etc., in *I '.2hich the superconductor is wound into the shape of a S2'-coil. 'NWen the temperature of the coil is sufficiently Slow that the conductor can exist in a superconducting state, the current carrying capacity as well as the magnitude of the magnetic field generated by the coil is significantly increased.

In fabricating such superconducting magnetic coils, the superconductor may be formed in the shape of a thin tape which allows the conductor to be bent around relatively small diameters and allows the winding density of the coil to be increased. The thin tape is fabricated as a multi-filament composite superconductor including individual superconducting filaments which extend the length of the multi-filament composite conductor and are surrounded by a matrix-forming material, which is L typically silver or another noble metal. Although the matrix forming material conducts electricity, it is not

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3 superconducting. Together, the superconducting filaments and the matrix-forming material form the multi-filament composite conductor. In some applications, the superconducting filaments and the matrix-forming material are encased in an insulating layer. The ratio of superconducting material to matrix-forming material is known as the "fill factor" and is generally between and 50%. When the anisotropic superconducting material is formed into a tape, the critical current is often 10 lower when the orientation of an applied magnetic field is perpendicular to the wider surface of the tape, as opposed to when the field is parallel to this wider S" surface.

Summary of the Invention 15 Controlling the geometry and/or type of anisotropic superconductor wound around a superconducting coil, increases an otherwise low critical current characteristic, associated with a region of the coil caused by the orientation of a magnetic field, thereby 20 increasing the current carrying capacity and center S magnetic field produced by the superconducting coil.

Generally, for a superconducting solenoid having a uniform distribution of high temperature superconductor wound along its axial length, the magnetic field lines emanating from the coil at its end regions become perpendicular with respect to the plane of the conductor 'qi (the conductor plane being parallel to the wide surface of the superconductor tape) causing the critical current density at these regions to drop significantly. In fact, the critical current reaches a minimum when the magnetic field is oriented perpendicularly with respect to the conductor plane. Although the critical current density is relatively high at the regions more central to the coil, the sharp decrease in the critical current density 35 at the end regions provides an overall decrease in the -4 current carrying capacity of the coil in its superconducting state.

Increasing the critical current value at the regions where the magnetic field is oriented more perpendicularly to the conductor plane can be provided in a number of ways.

"Bundling" the amount of superconductor, by increasing the number of strands of the superconductor connected in parallel provides a greater cross section, thereby rt s 10 increasing the critical current at low I, regions. With S this arrangement, the same type of superconductor, S usually from the same superconductor tape manufacturing run, is used for the different sections of the coil.

Varying the bundling of superconductor can be S 15 accomplished along the axis of the superconducting coil, for example, from one pancake section to the next, as well as within the pancake itself where the conductor cross-sectional area changes radially from the inner part to the outer part of the coil.

On the other hand, different superconductors having different fill factors may be used to distribute the amount of superconductor to control the critical current at the different sections of the coil. In still another arrangement, altogether different high f 25 temperature superconductors having different I c characteristics may be used for the different sections of the coil.

Because the magnetic field associated with a superconducting coil is directly related to the current I^ carrying capacity of the coil, a concomitant increase in the magnetic field provided by the coil is also achieved.

Even in applications where the volume of superconductor used for the coil is desired to be maintained substantially constant, and bundling of the superconductor requires that the number of turns

-I-

associated with that section of the coil be reduced, the decrease in magnetic field at the regions of the coil associated with such sections does not significantly effect the magnitude of the magnetic field at the center region of the coil. Adjusting the geometry of the sections of the coil also provides, to some extent, a desired field distribution profile, while maintaining a higher critical current density of the coil.

Moreover, other problems commonly encountered with multi-sectioned uniform current density superconducting coils can be alleviated. For example, each section of a multi-sectioned uniform current density superconducting coil has an associated critical current value dependent on the orientation of the field incident on that section at any given time. In a uniform current density coil, where all of the sections are uniformly wound with the same amount of superconductor, certain sections (generally those at the end regions of the coil) will have critical current values significantly less than those positioned at the center of the coil. Unless the superconducting coil is operated at a critical current less than the lowest critical current value of the sections, the section with the lowest I. will operate in its normal non-superconducting state. In some situations, flawed sections of the superconductor, for i example, during its manufacture, will have an I c value 3 significantly lower than other sections of the Ssuperconductor. Current passing through a normally conducting section, generates I2R losses in the form of heat which propagates along the length of the superconductor to adjacent sections. Due to the heat generated in the normally conductive section, adjacent sections begin to warm causing them to become nonsuperconducting. This phenomena, known as "normal-zone 1 35 propagation" causes the superconducting characteristic of -6these sections to degrade which leads to the loss of superconductivity for the entire coil, referred to as a "quench".

Because the critical current values associated with each of the individual sections (measured with respect to the orientation of the field incident on that section) of a graded superconducting coil, in accordance with the invention, have I c values closer to each other, the coil can be operated at a higher overall critical current. An additional advantage of maintaining a small difference between the critical current values of the S: individual sections of the superconducting coil is that a oo" *relatively quick transition to the overall critical I current of the coil is obtained. Thus in the event that the coil reverts from the superconducting state to a normal state (quenches), the inductive energy stored in the coil is distributed uniformly throughout the coil rather than being localized where it might cause damage due to heating.

In one aspect of the invention, a magnetic coil features a plurality of sections positioned axially along a longitudinal axis of the coil, each section including a high temperature superconductor wound about the longitudinal axis of the coil, and having regions with i 25 critical current values, measured at a zero magnetic §I field, which increase in value from a central portion of the coil to end portions of the coil.

particular embodiments of the invention include one or more of the following features. The critical 30 current value of each section is dependent on the angular i orientation of the magnetic field of the coil and is

V

selected to provide a desired magnetic field profile for Sthe coil. The critical current value of each section can -i be selected by varying the cross-sectional area of the A- 35 superconductor of at least one section or by changing the -7type of superconductor of at least one section. The superconductor may be a mono-filament or a multi-filament 3 Bcomposite superconductor including individual superconducting filaments which extend the length of the multi-filament composite conductor and are surrounded by a matrix-forming material. The number of individual superconducting filaments associated with a first one of the plurality of sections may be different than the number of individual superconducting filaments associated 10 with a second one of the plurality of sections. The cross-sectional area of the superconductor is varied in a direction parallel to the longitudinal axis of the coil.

S* and increases for the sections positioned at the central portion of the coil to the sections positioned at the end portions of the coil. The cross-sectional area of the superconductor is varied in a direction transverse to the longitudinal axis of the coil and decreases from regions proximate to the inner radial portion of the coil to the outer radial portion of the coil. The orientation of the individual tape-shaped superconducting filaments is other |I than parallel with respect to a conductor plane defined by a broad surface of the tape. The sections of the superconductor are formed of pancake or double pancake coils and the cross-sectional area of the superconductor is varied by increasing the number of strand- '-f superconductor connected in parallel. The high temperature superconductor comprises Bi 2 Sr 2 Ca 2

C

3

O.

In another aspect of the invention, a superconducting magnetic coil features sections, positioned axially along a longitudinal axis of the coil, including a high temperature superconductor wound about the longitudinal axis of the coil, and each section having regions with critical current being substantially equal when a preselected operating current is provided S 35 through the superconducting coil.

-B-3 In another aspect of the invention, a method for providing a superconducting magnetic coil including a plurality of sections positioned axially along the axis, with each section being formed of a preselected high temperature superconductor material wound about a longitudinal axis of the coil and having an associated critical current value, and each section contributing to the overall magnetic field of the coil, features the i following steps: 0 a) positioning the sections along the axis of S the coil to provide a substantially uniform distribution of superconductor material along the axis of the coil; II*' b) determining critical current data for each of the sections on the basis of the superconductor material associated with each section and the magnitude and angle of a magnetic field; c) determining a distribution of magnetic field magnitude and direction values for a set of spaced-apart points within the magnetic coil; d) determining critical current values for each i of the points within the coil based on the distribution 'of magnetic field magnitude and direction values and the S critical current data; e) determining contributions toward the overall magnetic field of the coil from each of the sections; f) determining a critical current value for the coil and for each section positioned along the axis of the coil; and g) changing the critical current value of at least one section of the coil to provide critical current values fcr each section substantially equivalent to each other.

In preferred embodiments, the method features one or more of the following additional steps- Steps c) S 35 through g) are repeated until the critical current values a

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9 of each of the sections based on the distribution are within a desired range of each other. The step of changing the critical current value of at least one section of the coil includes changing the type of superconductor or increasing the cross-sectional area of the superconductor material associated with sections of the superconductor that are axially or radially distant

I;

j

I

C.

a 4* 4 from the center of the the coil. The step of 10 value for each section coil includes the step current value for each current value based on associated with points radially away from the critical current value coil for at least one section of determining a critical current positioned along the axis of the of determining an average critical section, the average critical values of critical current extending either axially away or center. The step of changing the of at least one section of the coil includes increasing the cross section of the superconductor material associated with sections of the superconductor that are away from the center of the coil.

The step of determining critical current data for each of S: the sections of the coil further features the steps of measuring the critical current of the superconductor material associated with each section at a number of different magnitudes and angles of an applied background S 25 magnetic field, and extrapolating critical current data for unmeasured magnitudes and angles of a background magnetic field.

With this method, a superconducting coil having a predetermined volume of superconductor may have sections in which their geometries (for example, cross-sectional area) are changed along both the longitudinal and radial axes of the superconducting coil, thereby increasing the current carrying capacity and center magnetic field without increasing the volume of superconductor in the coil.

i, r i 1 10 Other advantages and features will become apparent from the following description and the claims.

Brief Description of the Drawings Fig. I is a perspective view of a multiply stacked superconducting coil having "pancake" coils.

Fig. 2 is a cross-sectional view of Fig. 1 taken along line 2-2.

Fig. 3 is a graph showing normalized critical current as a function of magnetic field in units of Tesla.

Fig. 4 is a view of the coil showing the conductors partially peeled-away.

Fig. 5 illustrates a coil-winding device.

Fig. 6 is a flow diagram describing the method of making the superconducting coil of the invention.

Fig. 7 is a plot showing the total magnetic field distribution within a superconducting coil having a uniform current distribution.

Fig. 8 is a plot showing the distribution of a magnetic field oriented perpendicularly to the conductor plane within the uniform current density superconducting coil.

'i Fig. 9 is a plot showing the normalized critical c.urrent distribution within the uniform current density superconducting coil.

Fig. 10 is a graph showing the average normalized critical current distribution as a function of the axial length of the uniform current density superconducting coil.

Fig. 11 is a graph showing the voltage-current characteristic of a superconducting coil.

Fig. 12 is a plot showing the critical current distribution divided among regions for a superconducting coil.

11 Fig. 13 is a plot showing the magnetic field distribution within a non-optimum superconducting coil having a non-uniform current distribution.

Fig. 14 is a cross-sectional view of an exemplary one of the pancakes of Figs. 1 and 2.

Fig. 15 is a graph showing the average normalized critical current distribution as a function of the radius of the uniform current density superconducting coil.

Description of the Preferred Embodiment Referring to Figs. 1-2, a mechanically robust, high-performance superconducting coil assembly combines multiple double "pancake" coils 12a-12i, here nine separate pancake sections, each having co-wound composite conductors. The illustrated conductor is a high temperature metal oxide ceramic superconducting material known as Bi 2 Sr 2 Ca 2 Cu 3 O, commonly designated

BSCCO

(2223). In the coil assembly 10, each double "pancake" coil 12a-12i has co-wound conductors wound in parallel which are then stacked coaxially on top of each other, with adjacent coils separated by a layer of plastic .insulation 14.

Pancake coils 12a-12i are formed by continuously uwrapping the superconducting tape over itself, like tape on a tape recorder spool. An insulating tape of thin polyester film, sometimes with an adhesive, can be wound between the turns. Alternatively, the conductor can incorporate a film or oxide insulation applied before winding. Note that the superconductor may be completely processed to its final state prior to winding ("react and wind" coil) or may be exposed to a degree of heat treatment after the pancakes have been wound ("wind and react" coil), the method influencing the insulation system chosen. In one embodiment, the completed pancakes are then stacked and connected in series by bridging 35 pieces of conductive tape soldered between stacks.

r 12 Plastic insulation 14, formed as disc-shaped spacers are suitably perforated to permit the free circulation of refrigerant and are usually inserted between the pancakes during stacking. Pancake coils 12a-12i here are constructed as "double-pancake" coils with the tape appearing to be wound from the outside to the inside of the first pancake and then wound from the inside to the outside of the second pancake, thereby eliminating the soldered bridge between the two pancakes which would i 10 otherwise occur at the inner diameter of the coil.

i "An inner support tube 16 fabricated from a I S" plastic-like material supports the coils 12a-12i. A ;D g first end flange 18 is attached to the top of inner support tube 16, with a second end flange 20 threaded onto the opposite end of the inner support tube in order SI to compress the double "pancake" coils. In an alternate embodiment, inner support tube 16 and end flanges 18, can be removed to form a free-standing coil assembly.

Electrical connections consisting of short lengths S of superconducting material (not shown) are made to join the individual coils together in a series circuit. A Llength of superconducting material 22 also connects one t end of coil 10-to one of the termination posts 24 located on end flange 18 in order to supply current to coil assembly 10. The current is assumed to flow in a counter-clockwise direction, and the magnetic field vector 26 is generally normal to end flange 18 forming the top of coil assembly 10. i Referring to Fig. 2, the superconducting magnetic coil 10, has a magnetic field characteristic similar to a conventional solenoid in which the magnetic field intensity at points outside the coil (for example, point P) is generally less than at points internal to the coil.

:j For conventional magnetic coils, the current carrying Al. 35 capacity is substantially constant throughout the V_ v i; ag--E^ a r 13 windings of the conductor. On the other hand, with low temperature superconductors, the critical current is dependent only on the magnitude of the magnetic field and not its direction.

Further, as discussed above, the current carrying capacity of a high temperature superconductor is not only a function of the magnitude but the angular orientation of the magnitude field. In a central region 30 of the coil, the magnetic field lines 32 are generally parallel (indicated by an arrow 33) with the longitudinal axis 34 of the coil and become less so as the magnetic field lines extend away from a central region 30 and towards end regions 36 of coil 10. Indeed, the orientation of field lines 32 at end regions 36 (indicated by an arrow 37) become substantially perpendicular with respect to axis 34.

Referring to Fig. 3, the anisotropic characteristic of critical current as a function of :I magnetic field for BSCCO (2223) high temperature I 20 superconductor is shown for applied magnetic fields I oriented parallel (line 40) and perpendicularly (line 42) 'to the conductor plane. The actual critical current values have been normalized to the value of critical current of the superconductor measured at a zero magnetic field. Normalized critical current is often referred to 1 as the critical current retention. As shown in Fig. 3, the normalized critical current, at a magnetic field of T (tesla), drops significantly from about .38 for a parallel oriented magnetic field to .22 for a perpendicularly oriented magnetic field. In addition to being dependent on the magnitude and orientation of the magnetic field, the critical current of a high temperature superconductor varies with the particular type of superconductor as well as its I, 35 cross-sectional area. Thus, in order to compensate for iP q y 1 .r 14 the drop in critical current of the superconductor at end regions 36 of coil 10 due to the magnetic field becoming more perpendicular with respect to the conductor plane, those pancakes positioned at the end regions (for example, 12a, 12b, 12g, 12h) may be fabricated with a superconductor having a higher critical current characteristic, or alternatively, may be formed to have a greater cross-sectional area of superconductor relative to those regions more central to the coil.

1 0 For example, referring to Fig. 4, a graded superconducting coil assembly 10 is shown with one side of the three endmost double pancakes 12a, 12b, and 12c, peeled away to show that an increased amount of Ssuperconductor tape is used for the double pancakes S 15 positioned axially furthest from the central region 30 of Sthe coil. In particular, pancake 12a includes five wraps of conductor tape 44 between wraps of insulating tape as compared to only two wraps of conductor tape 46 for pancake 12c located more closely to the center region Pancake 12b, positioned between pancakes 12a and 12c, Sincludes three wraps of conductor tape 48 to provide a gradual increase of superconductor to compensate for the gradual decrease in the critical current, due to the generated magnetic field, when moving from pancake 12c to 25 pancake 12a. As will be discussed below, in conjunction with Figs. 13 and 14, the cross-sectional area of superconductor can be varied along the radial axis of the coil during its fabrication.

Referring to Fig. 5, in one approach for fabricating a superconducting coil, a mandrel 70 is held iin place by a winding flange 72 mounted in a lathe chuck i 71, which can be rotated at various angular speeds by a device such as a lathe or rotary motor. The multifilament composite conductor is formed in the shape of a i- 35 tape 73 and is initially wrapped around a conductor spool feMBagggga~iaffimsaa~aB^ P\PERKAT15614.95.1 2-lfqlN

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74. In a react-and-wind process for fabricating a superconducting coil, the conductor is a precursor material which is fabricated and placed in a linear geometry, or wrapped loosely around a coil, and placed in a furnace for processing. The precursor is then placed in an oxidizing environment during processing, which is necessary for conversion to the superconducting state. In the react-and-wind processing method, insulation can be applied after the composite conductor is processed, and material issues such as the oxygen permeability and thermal decomposition of the insulating layer do not need to be addressed.

On the other hand, in a wind-and-react processing method, the precursor to the Ssuperconducting material is wound around a mandrel in order to form a coil, and then processed with high temperatures and an oxidizing environment. Details related to the fabrication of superconducting coils are discussed in co-pending application Ser. No.

08/188.220 filed on Jan. 28, 1994 filed by M.D. Manlief, G.N. Riley, Jr., J. Voccio, and A.J. Rodenbush, entitled "Superconducting Composite Wind-and-React Coils and Methods I of Manufacture", assigned to the assignee of the present invention, and attached herewith as 15 Appendix 1.

-In the wind-and-react processing method, a cloth 77 comprising an insulating material is wrapped around an insulation spool 78, both of which are mounted on an arm 75. The tension of the tape 73 and the cloth 77 are set by adjusting the tension brakes 79 to the desired Ssettings. A typical value for the tensional force is between 1 5 Ibs., although the amount can be adjusted for coils requiring different winding densities. The coil forming procedure is accomplished by guiding the eventual conducting and insulating materials onto the rotating material forming the central axis of the coil. Additional storage spools 76 are also mounted on the -p S 16 winding shaft 72 in order to store portions of the tape 73 intended to be wound after the initial portions of materials stored on spool 74 on the arm 75 have been wound onto the mandrel.

In order to form a coil 80, the mandrel 70 is placed on the winding shaft 72 next to storage spools 76 and the devices are rotated in a clockwise or counterclockwise direction by the lathe chuck 71. In certain preferred embodiments of the invention, a "pancake" coil is formed by co-winding layers of the tape 73 and the cloth 77 onto the rotating mandrel 70. Subsequent layers of the tape 73 and cloth 77 are then co-wound directly on top of the preceding layers, forming a "pancake" coil having a height 81 equal the width of the tape 73. The 15 "pancake" coil allows both edges of the entire length of tape to be exposed to the oxidizing environment during the heat treating step.

In other preferred embodiments of the invention, a double "pancake" coil may be formed by first mounting the 20 mandrel 70 on the winding shaft 72 which is mounted in lathe chuck 71. A storage spool 76 is mounted on the S* winding shaft 72, and half of the total length of the tape 73 initially wrapped around spool 74 is, wound onto the storage spool 76, resulting in the length of tape 73 being shared between the two spools. The spool 74 mounted to the arm 75 contains the first half of the length of tape 73, and the storage spool 76 containing the second half of the tape 73 is secured so that it does not rotate relative to mandrel 70. The cloth 77 wound on l§ 30 the insulation spool 78 is then mounted on the arm The mandrel is then rotated, and the cloth 77 is co-wound onto the mandrel 70 with the first half of the tape 73 to Sform a single "pancake" coil. Thermocouple wire is wrapped around the first "pancake" coil in order to 35 secure it to the mandrel. The winding shaft 72 is then =Bosom= I. 17 Sremoved from the lathe chuck 71, and the storage spool 76 Scontaining the second half of the length of tape 73 is mounted on arm 75. A layer of insulating material is then placed against the first "pancake" coil, and the second half of the tape 73 and the cloth 77 are then cowound on the mandrel 70 using the process described above. This results in the formation of a second "pancake" coil adjacent to the "pancake" coil formed initially, with a layer of insulating material separating the two coils. Thermocouple wire is then wrapped around the second "pancake" coil to support the coil structure during the final heat treatment. Voltage taps and thermocouple wire can be attached at various points on the tape 73 of the double "pancake" coil in order to monitor the 15 temperature and electrical behavior of the coil. In addition, all coils can be impregnated with epoxy after heat treating in order to improve insulation properties and hold the various layers firmly in place. The double "pancake" coil allows one edge of the entire length of tape to be exposed directly to the oxidizing environment during the final heat treating step.

An explanation of a method for providing a graded °superconducting coil follows in conjunction with Fig. 6.

A graded superconducting magnetic coil similar to the one shown in Figs. 1 and 2 and having the characteristics shown below in Table I, is used to illustrate the method.

TABLE I Winding inner diameter (ID) 1.00 inch Winding outer diameter (OD) =3.50 inches Coil length 4.05 inches Number of double pancakes 9 Number of turns/double pancake 180 Conductor tape width .210 inches Conductor tape thickness .006 inches Critical current of the wire 82 A (4.2°K at 0 Tesla) Target center field 1 Tesla n °i '.V 11

I

18

Z,.

4 4 Referring to Fig. 6, in accordance with a particular embodiment of the invention, a first step in designing a graded superconducting coil is the design of a uniform current density (non-graded) coil in which 5 the conductor is evenly distributed along the axial length of the coil. The design of such a coil can be determined as described, for example, in D. Bruce Montgomery, Solenoid Magnet Design, pp 1-14 (Robert E.

Krieger Publishing Company 1969), which is hereby incorporated by reference. Taking into account certain geometrical constraints (for example, the size of the cryostat for providing the low temperature environment), current densities of the selected high temperature superconductor and the desired magnetic field required 15 from the coil, the following relationship can be used to determine the required geometry of the coil: os r ti r oiro aj =n alllXF (a, (1) n~ r

D

D *~f

O

L

0 where: Hcen is the field at the center of the coil; A (the winding density of the coil) equals the active section of the winding divided by the total winding section; and F is a geometric constant defined as: 4v1T a 1 F (Sinh 1 Sinh (2)

PB

where a and 6 -i

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ii I -fi aji ~i~iis~a~s ~B~s~l~se g~sl~lei SP.\OPERKAT\15614.-95.13 2 2,71 -19where a, and a, are the inner and outer radii of the coil and b is half of the total axial length of the coil (see Fig. 2).

To determine the critical current of the coil and its sections, it is necessary to know the critical current characteristic of the particular high temperature superconductor(s) used in the coil. This information (step 52) is often provided not only for the particular superconductor material, but because of changes in the manufacturing process, is generally provided for each manufacturing run of the superconductor. In one approach for providing I, as a function of magnetic field as shown in Fig. 3, a current is applied to a length of S the superconductor at a desired operating temperature, here 4.2°K, while monitoring the i 10 voltage across the length of superconductor. The current is increased until the superconductor resistivity approaches a certain value, thereby providing the critical current value at that field.

"The method of determining critical current for superconductors is described in D. Aized et al, Comparing the Accuracy of Critical-Current Measurements Using the Voltage-Current Simulator, Magnet Technology Conference (MT-13), to be published, and attached herewith 15 as Appendix II. An external magnet is used to provide a background magnetic field to the superconductor at various magnetic field intensities and orientations. Fig. 3, as discussed above, shows measured values of the critical current as a function of this applied magnetic Sfield for a background magnetic field oriented both parallel and perpendicular to the S" conductor plane.

S 20 Although it is desirable to characterize each superconductor at as many different field Sintensities and angles of orientation as possible, it is appreciated that such data collection can be voluminous and time 20

N

consuming, and thus extrapolation methods can be used to expand data measured at a limited number of points.

Thus, where measured data at different angles is not available, data measured with the magnetic field applied parallel and perpendicular to the conductor plane can be used with approximation models to generate critical current values for fields applied at different angles.

In one approximation model, called the minimum retention model, the critical current of the conductor is 10 determined for both parallel and perpendicular field I components with the lower value of critical current taken as the critical current at the point under consideration.

In another approximation model, called the gaussian distribution model, the effect of the 15 orientation of individual filaments of superconductor with respect to the plane of the tape (that is, the conductor plane) is considered. When the superconductor is formed as a multi-filament composite superconductor, as discussed above, the superconducting filaments and the 20 matrix-forming material are encased in an insulating ceramic layer to form the multi-filament composite S" conductor. Although the individual filaments are generally parallel to the plane of the composite conductor tape, some of the filaments may be offset from parallel and therefore have a perpendicular field component associated with them. The gaussian distribution model assumes that the orientation of the individual superconducting filaments with respect to the conductor plane follow a Gaussian distribution. The characteristic variance is varied to match the critical W current data measured in step 52 and once the variance is found, it can be used to determine the critical current at any given field and angle.

In still another model, called the superimposing model, a normalized critical current is determined for a~as~es~Ba~pper~8Be~, 21 both the perpendicular and parallel components of the magnetic field and then the product taken of the two values.

Curve-fitting based on the measured data can be advantageously used to derive a polynomial expression which provides a critical current value for any magnetic field intensity and orientation angle. The following polynomial expression having the constants as shown in Table II was used to generate the curves shown in Fig. 3: 10 I,(B)=l/(ao+aiB+a2 B2+a 3

B

3 +a 4

B

4 +aB+a6B 6 TABLE II Parallel Field Perpendicular Field Data S* Constants Data Fied 1a 0.995 1.032 a 1.650 18.550 a 1.096 -45.140 a 3 -3.335 51.967 0 a 3 2.344 -28.481 S 2 0 a 5 -0.659 7.817 1 a 5 0.0649 -0.669 Results from the minimum retention and gaussian A distribution models were generally found to be similar 25 and provided a better match to the measured data than the superimposing model with the minimum retention model preferred due to its ease of implementation.

Once a database of critical current as a function of magnetic field has been obtained for each superconductor material to be used in the graded superconducting coil, the magnetic field distribution for a predetermined number of points (for example, 1000 points) within the coil is determined (step 54). The field calculations for determining the field distribution 35 within the coil is dependent on the geometry of the coil

W"

W _W 22 (for example, inner and outer diameter, length of coil), the characteristics of the superconductor (for example, conductor width and thickness for tape, conductor radius for wire), as well as, the insulation thickness, and relative position of individual sections of the coil. A software program called MAG, (an in-house program used at American Superconductor Corporation, Westboro, MA), provided the total magnetic field, as well as the radial and axial components, as a function of radial and axial 0 position within the superconducting coil. Table III shows a small representative portion of the output data Sa' provided by MAG for the coil having the geometry and characteristics described above.

TABLE III Radial Axial Component of Field Position Position Position B, (Rad) Ba (Axi) B(tot) 1 0 0 4.82E-16 1.73E-02 1.73E-02 S. 2 0 0.12 -9.70E-17 1.73E-02 1.73E-02 3 0 0.24 2.24E-16 1.73E-02 1.73E-02 4 0 0.36 1.26E-16 1.73E-02 1.73E-02 0 0.48 2.55E-16 1.73E-02 1.73E-02 25 14 0 1.56 -7.80E-17 1.68E-02 1.68E-02 0 1.68 1.16E-15 1.68E-02 1.68E-02 16 0 1.80 9.69E-16 1.67E-02 1.67E-02 S17 0 1.92 -8.95E-16 1.66E-02 1.66E-02 Commercially available software, such as ANSYS, a product of Swanson Analysis Systems Inc., Houston, PA, or COSMOS, a product of Structural Research and Analysis Group, Santa Monica, CA, may also be used to generate the field distribution information.

Referring to Fig. 7, the total field distribution data for the coil defined in Table I is shown plotted in graphical form using any number of commercially available software programs, such as Stanford Graphics, a product S _A 23of 3-D Visions, Torrance, CA. In addition, as shown in Fig. 8, the magnetic field for the same coil when the field is oriented perpendicularly to the conductor plane is maximum at point 56, near the end regions of the coil (about 5.2 cm from the center along the longitudinal axis of the coil) and a little more than half of the radial distance to the outer diameter of the coil (about 2.7 cm).

The field distribution data generated in step 54 provides a magnetic field value at each of the predetermined number of points within the coil which can be used in conjunction with the I, versus B data provided in step 52 to derive a critical current distribution i within the coil (step 58). In other words, the magnetic field values from the field distribution data are used in the polynomial expression described above to determine critical current values for each point. In particular, critical current values are determined for both the S parallel field and perpendicular field orientations with the minimum value used to represent the critical current value for that point. The I c distribution data is shown plotted in Fig. 9 and indicates that, consistent with the field distribution data of Fig. 8, the minimum critical current retention values (that is, normalized critical {I ;Y 25 current) is found in shaded region 60 at end regions of 'the coil.

The next step of the method involves determining the contributions of each of the sections of coil that is pancakes 12a-12i, toward the center magnetic field of the coil (step 62). Contributions from each pancake 12a-12i are determined using the relationships described above in conjunction with determining the field distribution of the uniform density coil (step 54). To Sdetermine each contribution, the coil is assumed to be symmetrical about the mid-plane through axis 35 (Fig. 2)

U

iili~ 0 *I 0 01 00 24 with pancakes on either side of midplane 35 being symmetrically paired (for example, 12a and 12i, 12b and 12h, 12C and 12, etc.). The contribution of each pair of sections is then determined, using the field relationships described above, by 1) determining or evaluating the total field generated by a coil having length defined by the outermost length of the paired sections of interest, 2) determining or evaluating the total field enerated by a coil having a length defined 10 by the innermost length of the paired sections of interest, and then 3) subtracting the results of the two determinations or evaluations. Each of the paired sections can then be divided by one-half to determine the contribution for each pancake of the pair of sections.

15 For example, referring to Fig. 2 again, to determine the contribution of paired pancakes 12a and 12i, the field determined for a coil having length 2z is subtracted from the field of a coil having length 2b. The contribution toward the center field from each of pancakes 12a and 12i Sis then one-half of the contribution of the symmetric pair. imilarly, to determine the contribution of pancakes 12b and 12h, the field determined for a coil having length 2(b-d or 2z is subtracted from a coil having a length 2(b-2d). [Note that the inner and outer r pancakes.

25 radii al and a 2 are the same for all calculations-] The total field generated by the whole assembly of the coil is the sum of all the contributions from the different pancake I distribution data generated in step 58 is then used to optimize the distribution of superconductor for different regions of the coil. For a superconducting coil in which double pancake coils 12a-12i are used (like the one shown in Figs- 1 and 2) each position corresponds with an associated one of the individual pancakes and the *I 00

S

j 1* *i 4.0 ir I 0

WS

ik: z

T

i;

B

25

I

c value for positions along the longitudinal axis of the coil is determined (step 64).

In one approach, called the critical current averaging approach, a weighted average of all Ic values extending radially within the region for each axial position or pancake, is determined using the following relationship: Ic Ave(z)=(CI, x radius) S(radii).

10 Thus, for a given axial position of the coil, the average of all the critical current values corresponding to that axial position in that region is provided with the radius of each point being the averaging weight for that point.

In addition, the average critical current value for each radial position in the region associated with each section, with equal weight given for each point, is determined using the following relationship:

I

c Ave(r)=EIc/(number of points).

Fig. 10 shows the average I c for the superconducting coil of Table I having a uniform currert distribution as a function of the axial distance from the center of the coil. By estimating the average critical current for the different sections of a uniform current Sdistribution coil, and noting their relative differences, a determination can be made as to what degree of change in the cross-sectional area of the conductor or type of superconductor is needed to increase the critical current values for sections having low critical current values, so that the critical current values of all the sections i of ,he coil are relatively close in value to the critical cur.ent value associated with sections at the center of the coil.

ji, 26 SAs indicated in Fig. 10, the superconducting coil Swith the geometry described above in Table I, has an average normalized

I

c of approximately .68 (that is 68% of the critical current at zero field) for the region associated closest to the center of coil 10 and associated with pancake 12e. However, at the regions axially positioned approximately four centimeters from the center of coil (in the vicinity of pancakes 12a and 12i), the average normalized

I

c drops to about S 10 approximately one-half that associated with pancake 12e.

S' Thus, increasing the cross-sectional area of S superconductor for pancakes 12a and 12i by an order of S" two would provide critical current values closer in value.

1 5 For example, in one embodiment, the cross section is increased at regions of the coil by bundling two conductors at center pancake 12e and pancakes 12d and 12f, three conductors for 12b, 12c, 12g, 12h, and four o* conductors for pancakes 12a and 12h at the ends of coil 10 to provide a gradual increase in the cross section of superconductor from the center region 30 to the end regions 36 of the graded superconducting coil. As shown in Fig. 4, in one embodiment, bundling of the S. superconductor can be achieved by increasing the number S 25 of overlaying wraps of the conductor tape between wraps Sof insulating tape.

In addition, the average I, for the entire coil is ;i determined by averaging the I c over the individual pancakes and taking the length of the conductor used in that section as the averaging weight, expressed numerically as: ic(coil ave)=Z(I c of the pancake) x (conductor length for the section) total conductor length of the coil 'i g i -27- Alternatively, a critical current value which more accurately represents the value of the critical current of the entire coil can be provided by determining critical voltage values for different regions of the coil based on the following relationship: (v/vc) (i/ic) n where i c is the critical current at that region; v c is the critical current criterion which is dependent on the geometry of the conductor in that region; and n is the index value as described in detail in Aized's article, Comparing the Accuracy of Critical-Current Measurements Using the Voltage- Current Simulator, referenced above and incorporated herein by reference.

S. Voltages for each region are determined for each s current level and summed to provide a total voltage VT for that current level. Total voltages VT are then plotted as a function of current (line 62) and the above relationship is used to determine a total critical current criterion Ve for the coil. This plotted VS function, as shown in Fig. 11, is then used to provide the critical current I c of the entire coil that is associated with V c In another approach for optimizing the distribution of superconductor for different regions of the coil, referred to as the "minimum Ic" approach, the I c values for positions throughout the coil are determined on the basis of a minimum critical current value positioned closely to the center of the coil. In this approach, the coil is partitioned into a large number of I small regions each having an associated minimum I c value.

The region closest to the center of the coil, both -d a il.L .n ii-. l 28 axially and radially, establishes a reference level for grading the remaining regions of the coil.

For example, referring to Fig. 12, the same superconducting coil analyzed above in conjunction with Fig. 10, includes a region 111, positioned most closely, both axially and radially, to the center of the coil that includes a point within region 111 having a minimum normalized

I

c value of .44 (that is 44% of the critical current at zero field). This minimum normalized Ic value 10 establishes a reference to which all other minimum normalized values of the remaining regions are referenced. Thus, if the section of the coil associated o with region 111 includes two bundles of superconductor (like pancake 12c in Fig. regions 151-156, which are at the end regions of the coil and having minimum normalized

I

c values of .27, the degree of change needed to increase the critical current values for regions 151- 156 so that they are close in value to the critical current value associated with the section closest to 20 region 111 is about a three and one-third times the superconductor used at region 111 3.3].

In this situation, regions 151-156 may either be wound with three superconductor bundles having a proportionally higher I c retention value or with four superconductor 25 bundles having a proportionally lower I c retention value.

The minimum critical current at central region approach is generally considered to be a more conservative approach for determining the optimum distribution of conductor as compared to the critical current averaging approach because of its reliance on a minimum and not an average of critical current values.

Thus, the minimum

I

c at central region approach is generally more suitable in the design of high performance superconducting magnets which are more likely to be 3 35 operated very near the minimum critical current value of 77;

IF

a 29 0r o a ar ,J CS any part of the superconductor and are therefore, more susceptible to normal zone propagation.

Using the minimum I c at central region approach for the coil as defined in Table I resulted in a decrease in the G/A (gauss/ampere) rating of the entire coil from 172 G/A for a uniform current distribution coil (that is, a 22222 superconductor distribution) to 162 G/A for a graded coil having a 22234 superconductor distribution.

This is due to the decrease in winding turns associated with low critical current sections and is not representative of the magnitude of the magnetic field at the center of the coil which is usually increased.

Furthermore, the theoretical

I

c required to generate the desired one Tesla field at the center of the coil also decreased significantly from 215 A (10000/(172 0.27) to 140.3 A (10000/(172 0.44).

By using either the "critical current averaging" or "minimum Ic" approaches, the cross-sectional area of the conductor for each of the pancakes can be changed to 20 provide a higher average I c value for the coil and to provide I c values for all of the individual pancakes that are close in value (step 66). This objective can also be accomplished by changing the type of superconductor for each pancake proportionally to provide retention I, value closer to the maximum I c value.

Because the cross-sectional area or type of superconductor associated with the sections of the coil may be changed to increase the critical current at the regions of the coil in which that section is located, it is generally necessary to repeat steps 54-66 for the newly configured coil. Changing the distribution of conductor for the sections of the superconducting coil, requires that the field and critical current distributions, as well as field contributions of each of the sections of the new coil be redetermined (step 68).

rt I MEN 2' C a C C

I

a a

C,'

30 This is necessary because the change in the crosssectional area or type of superconductor associated with each section changes the field characteristics associated with that section, as well as the entire coil. For example, because it is generally desirable that the volume of the superconducting coil be substantially maintained, increasing the cross section of the superconductor for a section of the coil will generally decrease the number of turns or windings in that section, thereby changing the magnetic field characteristics and the contribution toward the center field of the coil.

However, because this change generally occurs at the end regions of the coil, where the critical current is lower (due to the substantially perpendicular orientation of the magnetic field), the lower magnetic field (due to the decrease in turns) does not significantly contribute to the magnitude of the center magnetic field. In other words, although there is generally a decrease in the magnitude of the magnetic field at the end regions of the coil, there is a relatively significant increase in the critical current and current carrying capacity of the coil.

The cross-sectional area of the superconductor or type of superconductor for each pancake, and thus their respective critical current values, can be iteratively adjusted until a desired average Ir for the entire coil is achieved (that is, the I c when all the sections of the coil have nearly same I c (step 70). Statistical analysis can be used to calculate the standard deviation for the coil sections and to minimize its value by adjusting the number of conductors in the different sections of the coil. It is important to note that providing a greater number of superconductor bundles at center region 30 of coil 10 provides a greater number of bundles which can be used for sections of the coil i ii i 1 ii ii 1-

I~

(-d J;r

B

I

a r I; 31 intermediate center region 30 and end regions 36, and thus a smoother grading of the coil.

For the superconducting coil having the geometry described in Table I, the cross sections of pancakes 12a- 12i were changed by varying the number of layers of superconductor as shown in Fig. 4 to provide a superconducting coil having an increased average critical current value, and hence an increase in the current carrying capacity and magnetic field for the coil. Table IV summarizes results after each iteration for the coil with the configuration arrangement (first column) describing the number of layers of conductor. For example, 22222 defines a uniform current density coil (that is, each pane .k having one layer of conductor) while 22334 describes a configuration where the three inner-most pancakes 12d-12f have two layers, pancakes .12b, 12c, 12g, and 12h have three layers, while outermost layers 12a and 12i have four layers. This configuration (22334) was selected as having the most optimal 20 arrangement because it provided a small variation (I c standard deviation 9.26) in the critical current over the coil volume while providing a large average I c St ;(89.41A) and high magnetic field (1.357 Although, S' configuration 22344 also provided a relatively low standard deviation and higher average I, and magnetic gb field, the field distribution provided by this configuration, as shown in Fig. 13, provided multiple areas (called "depressions") where the magnetic field intensity achieves a maxima for a field oriented perpendicularly to the conductor plane. Configurations having such field distributions degrade the overall Sperformance of the superconducting coil.

-32- TABLE IV onfiguratio G/A Ave.Ic(A) Field(T) I std.dev.(A) 22222 172.80 63.23 1.142 17.09 (25.8%) 22222 1.211 12.45 (17.4%) 169.34 71.50 S163.77 77.75 1.27 9.51 (12.2%) 22233 310.59 (13.0%) 161.99 81.28 1.316 22234 151.87 89.41 1.357 9.26 (10.3%) 22334 13.58 (14.4%) 22344 148.80 94.12 1.400 SIt is also important to note that the geometry of it is :rsr 1 the different sections of the coil can also be varied 4" along the radial axis of the coil, as opposed to along the longitudinal axis, as described above. For example, referring to Fig. 14, a cross-sectional view of a portion (one-half of one side) of an exemplary one of the double pancakes 12a-1 2 i of Figs. 1 and 2, shows that the number of bundled conductors 90 need not be the same throughout the pancake. fact, in much the same way a the crosssectional area of superconductor was varied along the Slongitudinal axis of the coil the crosssectional area of the superconductor, can be varied along the radial axis of each section or pancake of the coil. For exa- le, as is shownin Fig. 7, the total magnetic field for the unifo rm distribution coil decreases from the inner to the outer radius of the coil. Thus, it is desirable to decrease the cross-sectional area atthis region of the pancake, thereby allowing an increase in the number of turns of conductor, which increases the central magnetic field of the coil.ch using a critical current averaging approach, a 0 weighted average of all I values extending axially wighin theregion for each radial position of the pancake is determined in much the same way as was described above in conjunction with averaging for each axial position of in cn n i wit.-* ^y

A-

I:;

Ei:

-Y~

33 O~r.

*6 ae S.r

GUS.

.9 *r 9 @9 89 O 0 9 the coil. Referring to Fig. 15, the average normalized I. (line 98) for the middle pancake 12e of the superconducting coil of Table I having a uniform current distribution can be plotted as a function of the radial distance from the center of the coil. Note that the inner radius of the pancake is about 1.3 cm from the center of the coil. A determination can then be made as to what degree of change in the cross-sectional area of the conductor is needed to increase the critical current values for regions having low critical current values within the coil by observing the relative difference in average critical current between the different sections of the uniform current distribution coil. Similarly, the critical current distribution data, as shown in Fig. 12, 15 indicates regions along the radial axis of the coil having low I c values which should be increased when the "minimum critical current" approach is used.

Thus, either the "critical current averaging" or "minimum Ic" approaches, described above, can be used to change the cross-sectional area of superconductor within each of the pancakes to provide a higher average I value for the coil and to provide IC values for all of the individual pancakes that are substantially equivalent.

In general, the I, increases from the center to the outer windings of the coil and, therefore, it is generally desirable to provide superconductor of greater cross-sectional area at the regions closer to the center (that is, internal windings) than at regions radially outward. For example, referring again to Fig. 14, if three conductors are bundled at portion 94 (associated with, for example, regions 111-113), only two conductors would be required at portion 96 (associated with outermost radial regions 114-116) of the coil. During the fabrication of one embodiment of a pancake coil, the three conductors are wound around the coil until the i: r

.A

e 1 a 8.-i i, a ~s~g~gg~a~ -t 34 radial distance at which it is desired to reduce the number of conductors is reached. At this point, one of the conductors is cut leaving an end which is attached, for example, by soldering, to an adjacent one of the remaining conductors, and winding of the coil is continued. By decreasing the number of conductors of a coil at regions where the critical current has a sufficiently high value allows a greater number of turns to be wound on the coil at these regions, thereby 10 increasing the magnetic field provided by the coil.

What is claimed is: 9 a a* *4 o* 9 0 o a a sO 9 e I« 7 «e 5~5z5.5SS

S

I

APPENDIX I SUPERCONDUTNG MAGNETIC

COIL

DAWOOD AIZEDI ROBERT F- SCLWALL

TITLE:

-APPLICANT:

53 pages of SpecifiCiLO 19 pages oE Claims L Albstracr page 9 Sheets of Drawings 5 S 5S 0 0* S 0

C

CC

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71 5.525.5S3 23 24 ATTOuY OCT GS770/004C01 Superconducting Wind-and-Reac: Coils and Methods of Manufacture The invention relates generally to superconducting magnetic coils and methods for manufacturing them. In particular, the invention relates to a wind-and-react process used to produce mechanically robust, high SA: temperature superconducting ceils which have high winding densities and are capable of generating large magnetic i .'10 fields.

Background of the nvention The wind-and-react method involves winding the precursor to a superconducting material around a mandrel in order to form a coil, and then processing the coil with high 15 temperatures and an oxidizing environment. The processing method results in the conversion of the precursor material to a desired superconducting material, and in the healing of micro-cracks formed in the precursor during the winding process, thus optimizing the electrical properties of the coil.

Superconducting magnetic coils, like most magnetic coils, are formed by wrapping an insulated conducting Smaterial around a mandrel defining the shape of the coil.

SWhen the temperature of the coil is sufficiently low that the conductor can exist in a superconducting state, the current-carrying performance of the conductor is markedly

J

26 increased and large nagflet-c fields can te ge nerated by the coilcertain ceramic materials exhibit superconducting behavior at loW temperatures, such as the comPound Bi 2 5r 2 Ca,_ICU 0 2 n-4 where n can be either 1, 2, or 3- One matria, B2SrCalU3 10(BSCCo (2223)), performs particularlY well in device apnlicatiofl5 because *superconductivitY and correspondinlg high current densities .are achieved at relatively higqh temperatures 115 K).

S* 10 Other oxide superconducto:rs include general cu-:-based ceramic superconduCtOrSr such as members of the rare-earth- A copper-oxide tamily fe., YBCO), the thalliumbariumcalcium-copper-oxide family TBCCO), the mercurybarium-calciumcopper-oxide family (ie. KigBCCO), and BSCCO 15 compounds containingq lead (Bi,Pb) 2 Sr 2 Ca 2 CuI 3 O0).

insulatinlg materials surrounding the conductor are used to prevent electrical short circuits within the winding of a coil. From a design point of view, the insulation layer must be able to withstand large electric fielis (as high as 4 X 105 V/cm in some cases) without sufferinlg dielectric breakdown, a phenomenon that leads to electrical cross-talk between neighboring conductors. At the same time insulation layers must be as thin as possible (typically less than 50 150 gm) so that the amount of superconducting material in the coil czn be maximized.

'Y 5.525.583 ri c sr t o o io r~ r Using existing conducting and insulating materials, the maximum magnetic field generated by a superconducting coil is ultimately determined by the winding density (defined as the percentage of the volume of the coil occupied by the conductor) and the coil geometry. However, the large tensional forces necessary for high winding densities can leave conductors in highly stressed and/or strained states. The bend strain of a conductor, equal to half the thickness of the conductor divided by the radius of 10 the bend, is often used to quantify the amount of strain imposed on the conductor througn coil formation. Many superconducting magnet applications involving high-density conductor windings require conductor bend strains on the order of and in some cases much higher. The critical 15 strain of a conductor is defined as the amount of strain the material can support before experiencing a dramatic decrease in electrical performance. The critical strain value is highly dependent on the formation process used to fabricate the conductor, and is typically between 0.05% depending on the process used. If the bend strain exceeds the critical strain of a conductor, the current-carrying capability of the conductor, and hence the maximum magnetic field generated by a coil, will be decreased significantly.

One approach to manufacturing high-performance conductors having desirable mechanical properties involves 1 fb_ i_ r:l 1 Bi~~; r i 1 r ii; 5.525.583 29 starting with a precursor to a high temperature superconducting material, typically a ceramic oxide in a powder form. Despite relatively poor mechanical properties and more complex manufacturing processes which requires high temperatures and an oxidizing environment, high temperature superconducting materials are preferred to low temperature So:: superconducting materials for certain applications because they operate at higher ambient temperatures. Oxide powders are packed into a silver tube (chosen because of malleability, inertness, and high electrical conductivity) which is then deformed and reduced in size using standard metallurgical techniques: extrusion, swaging, and drawing are used for axisymmetric reductions resulting in the formation of rods and wires, while rolling and pressing are 15 used for aspected reductions resulting in the formation of tapes and sheets (Sandhage et al., "Critical Issues in the SOPIT Processing of High-Ic BSCCO Superconductors", Journal of Metals 3, 21, 1991).

a Following the deformation process, heating and cooling results in the growth and evolution of individual crystalline oxide superconductor grains in the conductor which typically take on a rectangular platelet shape.

Further deformation results.in a collective alignment of the crystallographic axes of the grains. An iterative heating/deforming schedule unique to the ceramic oxide forms L2s- 5.525.53 s 31 32 of superconductors is typically zarried out until the desired grain size, alignment, and density of the superconducting state are achieved.

Conductors having a single oxide core, classified as mono-filament composite conductors, result from the iterative schedule described above and can have critical S. strain values as high as Mono-filament composite S conductors can be transformed into multi-filament composite conductors using a rebundling fabrication operation involving further reduction in size of the mo.o-filament composite conductors, and finally concatenation of individual conductors to form a single conductor.

Typically, the evolution of cracks in response to bend strains is more likely in mono-filament composite conductors 15 than in multi-filament composite conductors, where critical I strain values increase with the number of filaments in the conductor, and can be greater than Other limitations of mono-filament composite conductors include decreases in o crack healing ability and oxygen access to the conductor during processing. Furthermore, because mono-filament composite conductors have only a single superconducting region, it is difficult to control the conductor size and shape, and mechanically robust conductors can not be easily fabricated Osamure, et al., Adv. Cryo. Eng., ICMC Supplemental, 38, 875, 1992). Thus, multi-filament composite 5.5S-1 *4 S S

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S :50<

CC

~1 conductors have desirable mechanizcal properties, and can be used in co ils requiringq high winding densities.

One method used to fabricate coils with multi- and mono-filament composite conductors is the react-afd-wind process. This method first involves the formation of an insulated composite conductor which is then wound into a coil. In this method, a precursor to a composite conductor is fabricated and placed in a linear geometry, or w-rapped loosely around a coil, and placed in a furnace for processing. The precursor can therefore be surrounded by an oxidizing environment during processing, which is necessary for conversion to the desired superconducting state. in the react-and-Wind processing method, insulation can be applied after the composite conductor is processed, and materials 15 issues such as the oxygen permeability and thermal decomposition of the insulatinlg layer do not need to be addressed.

In the react-and-wind process, the coil-formation step cam, however, result in straining composite conductors in exces -s of the critical strain value of the conductinlg filaments. Strain introduced to the conducting portion of the wire during- the deformation process can'result in microcrack formation in the ceramic grains, severely degrading the electrical properties of the composite conductor.

5.525.5S? Si 36 3 Another method used to fabricate magnetic coils with mono-filament composite conductors is the wind-and-react method. In this method, the eventual conducting material is typicallly considered to be a "precursor" until after the final heat treating and oxidation step. Unlike the reactand-wind process, the wind-and-react method as applied to high temperature superconductors requires that the precursor Sbe insulated before coil formation, and entails winding the coil immediately prior to a final heat treating and oxidation step in the fabrication process. This final step results in the repair of micro-cracks incurred during winding, and is used to optimize the superconducting properties of the conductor. However, these results are significantly more difficult to achieve for a coil geometry than for the individual wires which are heat treated and S: oxidized in the react-and-wind process.

Due to the mechanical properties of the conducting material, superconducting magnetic coils fabricated using S: the wind-and-react approach with mono-filamentary composite conductors have limitations related to winding density and current-carrying ability. Although the wind-and-react process may repair strain-induced damage to the superconducting material incurred during winding, the coils produced are not mechanically robust, and thermal strain ji.

38 37 resulting from cool down cycles can degrade the coil performance over time.

A feature of the invention is a wind-and-react process which is used to manufacture superconducting magnetic coils with multi-filament composite conductors.

This processing method can be used to manufacture several variations of coils types, all of which are discussed below.

An advantage of the invention is ability to produce mechanically robust coils requiring high winding densities, 10 without significantly degrading the superconducting properties of the multi-filament composite conductors used to form the coils.

*Summary of the Invention The present invention relates to a wind-and-react *a 15 processing method used to fabricate superconducting magnetic coils featuring strain-tolerant multi-filament composite C conductors. This invention has various aspects which individually contribute improvement over previous react-andwind coils, and wind-and-react coils made with mono-filament conductors. Specifically, materials and processing steps have been adapted in order to fabricate coils which allow adequate oxygen access to the precursor to the multifilament composite conductor in order to affect conversion to the desired superconducting state, while at the same time allowing preservation of the materials and geometrical

V

~1M 5.525.5S3 0*4* 4 04 *s 0 .c, *4 4

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e 0 o' r 0 4 o~ tolerances of the coil. Superconducting coils requiring high-density complex winding geometries can often only be fabricated with multi-filament composite conductors because mono-filament conductors are intrinsically less flexible and their electrical properties are more difficult to rehabilitate.

In one aspect, the invention relates to a method for producing a superconducting magnetic coil featuring the following steps: fabricating a precursor to a multi-filament 10 composite conductor from multiple high-temperature superconducting filaments enclosed in a matrix-forming material; surrounding the precursor to the multi-filament conductor with an insulating layer or a precursor to an insulating layer; forming the precursor to the multifilament composite conductor as a coil; heat treating the coil after the forming step by exposing the coil to high temperatures in an oxidizing environment, the superconductor precursor filaments being oxidized and the matrix-forming material reversibly weakening during the heat treating step, with the composition and thickness of the insulating layer or precursor to the insulating layer bein chosen to encase the matrix-forming material and the superconductor precursor filaments, and to permit exposure of the superconductor precursor filaments to oxygen during the heat treating step.

The heat treating step results in the improvement of the ;1 3 s .2.583 42 41 electrical and mechanical Froper:ies of the superconductor precursor filamenlts, and in the formation of a superconducting magnetic coil.

By "surrounlding the eventual mult-i-filament composite conductor with an insulating layer (or precursor &to aninsulating layer), direct contact betweenl adjacent Qconductors is prevented. By "encasing" the .arXfran a material and the superconlductinlg precursor filaments during the heat treating step, the insulation layer (or precursor to the insulation layer) Freserv.es the integrity of -he col during the heat treatment. By "reversibly weakceninlg" the matrjx-forming material is left essentially without mechanical strength during the heat treating step, with the 'material substantially regaining mechanical stability a. 15 following processing.

Preferably, the heat treating Step involves heating and-then cooling the coil in an environment comprising oxygen, and results in the conversion of the superconductor a C precursor filaments to a desired superconducting material, 2o and in the repair of micro-cracks formed in the filaments duriig the forming step.

In preferred embodiments, the heat treating step featur~es, heating the coil from room temperatu1re at a rate of" about 10 0 C/min. until a temperature between 765 Oc and 815 OC, and preferably 787 OC is 0 ob 5 jfed;.heating the coil at a oz 525.5SS 3

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t C r CC C t

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rate about I oC/flin. unti I a maxim±um temperature between 810 0 C and 860 0 C, and prefably 830 OC, is obtained; heating the coil at the maximum temperature for a time between 0.1. and 300 hours, and preferably for 40 hours; cooling the coil at a rate of about 1 OC/flif until a temperature between 780 OC and 845 OC, and preferably S1i. Oc, is obtained;~ heating the coil at this temperature for a time period in the range of 1 to 300 hours, and preferably for 120 hours; cooling the coil at-a rate of about 5 OC/rnin. to a temperature between 765 0

C

10 and 815 Oc, and-preferably 787 OC; '-eatinlg the coil at this temperature for a time period between 1 and 300 hours, and preferably for 30 hours; and, finally cooling the coil at a rate of about 5 0 C/min. until a temperature of 20 OC is reached, with the heat treating steps performed in an 15 atmosphere which consists primarily of gaseous oxygen at a pressure of about 0.001. to atm, and preferably at 0.075 atm.

In one preferred embodiment of the invention, the coil is formed by repeating the steps of first winding a layer of the precursor to the multi-filamenlt composite conductor around a mandrel, and then winding a layer of material comprising an insulating material or a precursor to An insulating material on top of the precursor to the multifilament composite conductor. In another preferred embodiment of the invention, the precursor to the insulating 4 7

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material is initially a liquid mixture of a solvent and dispersant, and a particulate material, With "he Mixture being applied by dipping the precursor to the mult;-filaelt composite conductor in the liquid mixture, followed by a heating step which results in the evaporation of the solvent and dispersant, and the formation of an insulating layer around the DrecurSOr to the multi-filament composite conductor. In a preferred embodiment of the inventonl, a heating step is used to remove impurities from the 2.0 insulating material, such as dirt_ or a binder material.

In another preferred embodiment of the invention, the coil forming step Features the step of concentrically winding the precursor to the multi-filament composite conductor to iform a multi-layer coil having a "pancake" 15 shape, with each of the layers wound to overlap the preceding layer. Each edge of the entire length of the precursor to the multi-filament composite conductor in this geometry is exposed to the oxidizing environment during a heat treating step. The heat treatment results in the oxidation and healing of micro-cracks in the superconductor filaments of the precursor to the multi-filament composite conductor, resulting in the formation of a multi-filament composite conductor The "pancakell coil can be wound around a mandrel having an arbitrary shape- In preferred embodiments, -the "panncake" coil is wound, around a mandrel

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-i 5.525.5832 47 18 having a circular cross secticn. In alternate embodiments, the mandrel cross section is pr-marily elliptical in shape.

In preferred embodiments, double "pancake" coils can be formed by winding a second "pancake" coil on the mandrel adjacent to the first "pancake" coil. In yet other preferred embodiments of the invention, multiple double "pancake" coils can be combined to form a single coil, and are preferably stacked in a coaxial manner.

a S. In one particular aspect of the invention, a methcd 10 for producing a superconducting magnetic coil, si-42-ar to the method described above, features sunjecting the precursor to the multi-filament composite conductor to a bend strain in excess of its critical strain. In a particular embodiment of the invention, the precursor to the multi-filament composite conductor is subjected to a bend strain in excess of 0.3%.

In another particular embodiment, each layer of the multi-filament composite conductor of the coil consists of multiple conductors, with all of the conductors surrounded by a single insulating layer. Preferably, the multi-filament composite conductor has multiple superconducting filaments enclosed in a matrix-forming material composed of a noble metal or an alloy to a noble metal, and is preferably made of silver. In a particular embodiment, the superconductingmaterial used for the filaments is selected from the oxide I: L' 49 superconductor familY, cOrorsing te Taal o~ eiter 1, ,o (Bi'Pb)2SrjCar._,Cn%.j1~4 where n sealteihr,2,o 3; members of the rare earih-coPperoaxide family, such a YBCO (123), YBCO (124), and YBCO 1-247); members Of the thailium-barium-calcium-copper-oxide family, such as TBCCO and TBCCO (12 23); and, members of the mercurybariumcalcium-copper-oxide family, such as HgBCCO (1212) and *HgBCCO (1223). Preferably, tlhree-1layer phase BSCCO ;s used for the superconducting filsmeflts.

i0 In preferred en-boditeflts o:t this aspect cf ':he invention, the nulti-Eilament czmcos-;te conductor Ls surrounded by an insulating layer which is permeable to 'S.gaseous oxygen and substanltially chemically inert relative to the multi-filameflt composite conductor. In a preferred embodiment, an insulating material selected from the group containing SiG.,, Ai.0 3 and zirconia fibers is used as the insulating layer. Preferably, the insulating material is cowound with the precursor to the multi-filament composite conductor. In alternate embodiments, the insulating material is wrapped around the precursor to the multi-filamenlt composite conductor. Preferably, the thickness of the insulating layer is between 10 and 150 gm. In other embodiments, the insulatinlg layer of the coil consists primarily of a particulate material selected from a group comprising A1 2 0 3 Mqor SiO., and zirconia.

.0 S 5.525,53 51 In particular aspects of the inventicn, a superconducting magnetic coil made with the method described above has an inner-ccil diameter no larger than about 1 cm, or alternatively, the coil is wound so that the berd strain of the multi-filament composite conductor is greater than In other aspects of the invention, the winding density of the coil is greater than about 60%, the fill factor of the multi-filament composite conductor is greater than about the minimum critical-current is about 1.2 Amperes, and the magnetic field prcduced by the coil is in excess of about 80 Gauss.

In one aspect of the invention, a "pancake" coil is *formed by the method described above. In a preferred embodiment, each layer of insulated multi-filament composite conductor of the "pancake" coil consists of "ultiple strands of multi-filament composite conductor, each having multiple superconducting filaments, with all strands surrounded by a single insulation layer. The conducting and insulating materials used in the "pancake" coil are the same as those described previously. In one embodiment of the invention, the coil is impregnated with a polymer. In a preferred embodiment, double "pancake" coils can be formed. Double "pancake" coils can be stacked coaxially and adjacent to each other. In certain preferred embodiments, the mandrel supporting the stacked coils is removed.

5 5~1 8 34 53 Brief Descr~If of 7he Dra1winas other objects, features and advantages of the invention will be apparent from the foll1cwIng description, taken together with the following drawings.

Figure 1 is a cross-SeCticflal view of a multi- 1511filament composite conductor.

Figure 2 is a graph comparing the eiectr -mecalia II ~~properties of mono- Pnd multi-filament CcpOT-t7 cndctrs Figure 3 is a graph comparing the electrical 1 properties cf colls inade withi nonc- multiflz-lnt composite conductors as a function of therm-al cycles.

Figure .4 is a block diagram of the wind-and-react coil formation process.

Figure 5 illustrates a coil winding deviceis Figure 6 is a graph illustrating the mechanical properties of superconlducting multi-filament composite conductor manufactured in accordance with the invention.

plotted against bend strain for a particular multi-filamlent composite conductor which was heat treated in accordance with the invention after being strained.

Figure 8 is a graph comparing the electro-mechanical properties of composite conductors treated with wind-andreact and react-and-wind processing methods.

~2~583

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Fig-ure 9 shows a superccnductilg coil made with a multi-filament composite conductor using the wind-and-react process in accordance with the invention.

Figure io shows a superconducting coil in the "Pancake!' geometry made in accordance with the invention.

Figure 10a shows a side view of the coil.

Figure 10b shows a side view of a primarily elliptical "racetrack" coil.

Figure 3.1 shows multiply stacked "pancake" coils.

Ficgure shows a cross-sectional view of Figure 311 taken along line la-ia.

fescriztion of the Preferred Ebodiments Insulated Composite Conductor Referring to Figure 1, a multi-filament composite conductor 11 manufactured in accordance with the invention and used in a superconducting coil has superconducting regions 12 which are approximately hexagonal in crosssectional shape and extend the length of the multi-Efilament composite conductor 11. Superconducting regions 12 fcrn the filaments of the conductor, and are surrounded by a matrixforming material 14, which is typically silver or another noble metal, which conducts electricity, but is not superconducting. Together, superconducting regions 12 and the matrix-forminlg material 14 form the multi-filament composite conductor.

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1 IMMUNE 5.55.583 57 58 In the Figure, the composite conductor is encased in an insulating ceramic layer 15. A standard "fill factor" describing the cross-sectional area encompassed by the superconducting regions 12 relative to the overall conductor cross-sectional area is 28%. The thickness of the ceramic insulation layer is typically on the order of 10 and 150 pm.

Multi-filament composite conductors offer many S: advantages over mono-filament composite conductors having similar fill factors. Referring now to Figure 2, the 10 electro-mechanical properties of multi- and mono-filament composite conductors are compared by plotting normalized critical-current density as a function of bend strain for different conductor samples having similar fill factors.

The critical-current density of the mono-filament composite conductor approaches zero for bend strains near while the multi-filament composite conductor samples show a much weaker dependence on the bend strain. Both composite conductor samples had a thickness of 2.4 mm and a rectangular-shaped cross section, and were 10 cm in length.

As the number of superconducting regions is increased from 7 to 2527, the conductive properties become less sensitive to bend strain, indicating the benefits of multi-filament composite conductors.

In the method of the present invention, the processing conditions used for the formation of the 3

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superconducting state have been inventively adapted to deal with problems unique to coils made with multi-filament composite conductors. In addition to the multi-filament composite conductor, materials used for insulation, mandrels, and other parts of the coil are subjected to the final heat treating process, and have been specifically chosen to adapt to the method of the present invention.

Wind-and-React Processing Method Precursor Formation The formation of the precursors to multi-filanent composite conductors has been described previously, and will be discussed only briefly here (Riley et al., supra, and Sandhage et al., supra, the contents of which are incorporated herein by reference).

Referring now to Figure 4, the steps of the windand-react manufacturing process for forming magnetic coils having strain-tolerant multi-filament composite conductors begins with the precursor to a multi-filament composite conductor 20 comprising filaments which consist of the ceramic precursor to the eventual superconducting material.

The precursor to the multi-filament composite conductor is processed with two distinct steps: 1) a deformation through a pressing and/or rolling step 21, resulting in an alignment of the ceramic material along the c axis of the single crystal grains; and 2) a sintering step 22 involving heating ;r i i g n B~i h-- 7- $-1 61 62 the precursor to the condu-tor to temperatures in excess of 800°C in an oxidizing environment, resulting in the formation of intergrannular connectivity. The precursor to the multi-filament composite conductor is returned to the 5 deformation step 21 after being cooled. This results in crystallization and evolution of the superconducting grains, which is necessary, but not sufficient, for superconductivity- The deformation and sintering schedule is repeated iteratively from step 1 to step n-1, where n is an integer. The number of steps is chosen to opimize the final conduction properties of the target superconductor.

For BSCCO (2223), the nunber of steps is typically 2 or 3 using the heat treatments described herein.

Both the material and number of filaments used in 15 superconducting regions can be changed to modify the electrical and mechanical properties of the eventual conductor. For example, in the BSCCO family, the number of layers of sheet-like CuO planes distinguish the different superconducting compounds. Along with BSCCO (2223), which has a three-layer phase, BSCCO (2201) (single-layer phase) ,i and BSCCO (2212) (two-layer phase) are compounds which also exhibit superconductivity. BSCCO compounds may also contain lead which can result in the improvement of the chemical stability of the materials at high temperatures. The critical temperature increases with increasing numbers 4-Is 63 64 of layers, with the single-layer phase having a of about K, the two-layer phase havinq aT. of about go K, and the three-layer phase having a T. of about 115 K. other desirabl~e oxide supercondulctors, such as 'iBCO (123),

TBCCO

5 (1212) and TBCCO (1223), have values of T. in excess of 77

K.

A reburndliflq process results in fabrication of the precursors to multi-filament composite conductors having a variable number of sections, with each sact~oll containing mltile ilaents (Sndhage e: supra). ypically, using the described process, multi-filaenlt comp~osites composed of two sections have 7 filaments, composites composed of 3 sections have 19 filaments, and composites composed of 4 sections have 37 filaments.

.15 Referring again to Figure the matrix-~forming material 14 is chosen to surround the superconducting regions 12 because of the malleability and nobility of the metal with respect to the superconducting material. The matrix-forming material 14 also protects the superconducting regions 12 from chemical corrosion and mechanical abrasion, and enhances the stability of the superconducting regions 12 at cryogenic temperatures. Although silver Is the preferred material, the mat~rix-forminlg material can also he made of

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4+ *c S electrical properties, such _s alloys of silver and other noble metals.

Insulation In the wind-and-react process, insulation (or a 5 precursor to an insulating material) is applied to the precursor to the composite conductor prior to the final heat treating step. A particular method for applying insulation to wires used in react-and-wind ccils has been described previously in Woolf, U.S. Patent 5,140,006. The insulating methods and material parameters described herein have been specifically adapted for the wind-and-react method used to fabricate coils with multi-filament composite ccnductors.

The coil geometry imposes constraints on the insulation that are not present for individual wires. In the method of the present invention, ceramic insulation is chosen to insulate the multi-filament composite conductor because certain ceramic materials are permeable to oxygen, which allows exposure of the precursor to the composite conductor to an oxidizing environment during processing.

Ceramic materials can also withstand the high temperatures an oxidizing environment of the processing conditions without suffering decomposition. Because insulation prevents electrical short circuits within the wound coil, ceramic materials are further desirable because they can withstand dielectric breakdown when exposed to electric fields as high :ui i,

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i .t r; i i.9 :w :s r in Irr 5.5255S3 H 67 68 as 4 x 105 V/cm. Other materials exhibiting electrical and mechanical properties similar to ceramic materials could also be used as insulation.

Wind-and-react coils formed with multi-filament 5 composite conductors have different insulation thickness requirements than wind-and-react coils formed with monofilament wires. It is well known in the art that thin superconducting regions are necessary to obtain high o critical-current densities for the BSCCO family of superconductors. The optinum current-carrying performance for mono-filament composite conductors is normally achieved when the thickness of the superconducting regions is on the order of 10 pm. In comparison, the thickness of multifilament composite conductors is a function of the number 15 and configuration of the superconducting regions, and can be flexibly controlled. Thus, the ratio of the thickness of the insulation layer relative to the conducting region can be decreased in multi-filament composite conductors. This also allows robust multi-filament composite conductors to be fabricated which can be made arbitrarily thick, and far less susceptible to damage during processing steps than their necessarily thinner mono-filament counterparts.

During the final heat treatment, the insulation also acts as a casing which holds the matrix-forming material (which is considerably weakened during heat 5 .54 69 treating) and the supercofducr preur o tthro an therefore must not be susceptible to0 decoW~ilf Furher~re jtis ndeirale for the inulating materi.al to react with the compositeprcro uig heea 5 reatng-Mateial s~c aschromium, which may be present in some ceramic materials, can diffuse thogh jvran may react with the supercornducn atia Qurz alumnazirofla, and magnesium are notabetdiUe ft through the silver mai-fomn materiala if I CO temperatures, and do not decompose when subjected to high temperatures, and thnus represent suitable mater~.als for insulation.

In some cases, the material used to insulate theV conductor is considered to be a precursor until a heating step isperformed, resultinlg in the formation of the ft insulating layer. Alternatively, the insulatinlg material May not exist in a precursor state. In this case 1 ahetngse may be used to remove dirt and other impurities. although such a heating step may not necesaiy ltrhececa composiino the insulating material. nadt~l heatingq step may_.improve the mechanical. properties of the insllat~flith~t haningthe actual insulation properties.

Ceramic materials used as the precursors to 5.525.583 72 71 such as a tape containing ceramic fibers, or a slurry, defined as a mixture of a solid particulate suspended by liquid. In a preferred embodiment, a cloth containing SiO, fibers is used as the insulating material. This material 5 does not exist in a precursor state, but a heating step may en result in the removal of dirt and other impurities, thus Ce improving the robustness of the cloth.

Sr Suitable solid-based materials should be flexible so cc/ that they can be formed into a coil with the precursor to the conductor, while liquid-hased materials should adhere to the precursor to the conductor, forming a continuous coating. Ceramic slurries and cloths both containing insulating materials may be used as the liquid-based and solid-based materials, respectively.

i In a preferred embodiment of the present invention, a solid-based insulating layer is formed by attaching a C cloth material composed of quartz fibers having a thickness between 10 250 pm and a width equal to the width of the precursor to the composite conductor. Quartz cloth is porous, and is chosen because of strength, flexibility, and its ability to resist degradation when exposed to high temperatures- In alternate embodiments, cloths woven from other ceramic fibers, such as zirconia and A1O0 3 are used.

Typically, a binder composed of an adhesive polymer is used to hold the fibers of the cloth together. The insulation -1 5.525.583 73 74 73 can be applied by co-winding a single layer of the cloth during the coil formation step, or braiding multiple layers of the cloth around the precursor to the conductor at any time prior to the coil formation step. The binder of the ceramic insulating cloth can be removed by subjecting the insulation to a heating step following coil winding. This "5 typically involves exposing the cloth to a temperature c *greater than about 450 cC for a time period of about 3 hours. Alternatively, the heat treating steps used to 10 optimize the electrical and mechanical properties of the composite conductor can be used to remove the binder.

In an alternate embodiment, a liquid-based insulation layer is formed around the precursor to the multi-filament composite conductor as described in U.S.

Patent 5,140,006, which is herein incorporated by reference.

The insulating layer is formed by first immersing the Sprecursor to the multi-filament composite conductor in the slurry, resulting in adhesion of the particulate to its outer surface. The precursor to the conductor is then removed from the slurry, and subjected to a processing step consisting of heating the particulate material to a temperature of greater than 600 oC for a time period of about 15 hours, resulting in the calcination of the particulate material and the formation of the insulation layer. The liquid-based insulation layer can also be ik 76 calcined during the heat t-eatin.: steps of the -orocessifg method used to opisize the electrical an mechanical prprisoftecnuctor. Both 11 ,ating processes result in the formation of the ceramic jmu.tiglyr n h 5 vaprati~ ad dc~mosit4cn of the solvent/dispersant, 9 ~leaving a thin ceramic i!m having athickness tYPicllly ro e between It and 150 Amcoil Formation oxidationl of the prec~Or to the nulti-filamant composite cond;uctor luring heat1 treatmnent iscuc to the overll erfr~2fl~ ofthe p Jerccnductzng material. Steps must therefore be taken. to insure that precursors to 0 0composite conductors wound into coils have adequate access to the oxidizing environment. One way t.o accomplish this is by forming a "pancake" coil in which the precursor is formed 9.049 Cinto- a tape and wrapped in concentric layers around a *mandrel to for-m a spiral pattern, with each layer woundZI aek 4 directly on top of the preceding inver layer. This allows the ouer edge of the precursor to be exposed to the oxygen atmosphere along its entire length during the final step of the wind-and-react processinq method.

Referring to T'igure 5, in a preferred embodiment of the invention, a mandrel 30 is held in place by a w~indinlg flange 32 mounted in a 1athe chuck 31, which can be rotated at various agirsed y device such as a lathe or 5.5255S3 @4 4 4* #4 t 4 ~4 U S 4 *4 *9S'.

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rotary motor. The precurScrtozt ,14Famn: nPse conucor ored in the shne of a tape 33 is ini.al wrapped around a conductor spool 2-4. and a cloth 37 comprising an insulatin~g material is wrapped around an 5 insulation spool 38, bcth of which are mounted on an arm The tension of the tape 33 and the cloth 37 are set by adjusting the tension brakes 39 to: the desired settings-

A

typical value for t:he tensionlal focrce is betweenl 5 lbS., although the amount can he adjusted for coil1s requtiring 13 different winding densities. Tne coil tcrn-ing procedure is accomplished by guiding the eventual ccnducting and insulatinlg materials onto the rotating material forrming the central axis of the coil. Additional storage spools 36 are also mounted on the winding shaft 32 in order to store portionls of tha tare 33 intended to be wound after the initial portions of =aterials stored on spool 34 on the arm 35 have been wound onto the mandrel.

In order to forM a coil 40, the mandrel 30 is placed on the winding shaft 32 next to storage spools 36 and the devices are rotated in a clockwise or caunter-clockwisa direction by the lathe chuck 31. In certain preferred embodiments of the invention, a "pancake" coil is formed by co-winding layers of the tape 33 and the cloth 37 onto the rotating mandrel 30. Subsequaent layers of the tape 33 and cloth 37 are then co-wournd directly on top of the preceding 11 -4 P~* -~1 11111 jljjgil l 5.525.583 eel.

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a layers, for-,ing a "pancake" ccil Iavin-.g a e t 41 equal the width of the tape 23- The "pancake" ccil allows both edges of the entire length cf tape to be expos, i to tne oxidizing environment during the heat treating step.

5 in other preferred embodiments of the invention, a double "pancake" coil nay be for-ed by first mounting the mandrel 30 on the windinq shaft 22 which is mounted in lathe chuck 31. A storage sDool 36 is ounted on the winding shaft 32, and half cf the total length of the tace 22 initially wrapped arcund spooi 24 is wcunad onto the stcrage spoc 36, resulting in the ienazh cf tape 33 being snared between the two spools. The spool 34 mounted to the arm 25 contains the first half of the length of tape 33, and the storage spool 36 containing the second half of the tape 33 is secured so 15 that it does not rotate relative to mandrel 30. The cloth 37 wound on the insulation spool 38 is then mounted on the arm 35. The mandrel is then rotated, and the cloth 37 is cowound onto the mandrel 30 with the first half of the tape 33 to form a single "pancake" coil. Thermocouple wire is wrapped around the first "pancake" coil in order to secure it to the mandrel. The winding shaft 32 is then removed from the lathe chuck 31, and the storage spool 36 containing the second half of the length of tape 33 is mounted on arrm 35. A layer of insulating material is then placed against the first "pancake" coil, and the second half of the tape 33 and 4 Bill

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tne cloth 37 are then c-votfld or. th de Quin g then f oracesS described above- This results -n t!7e EcratOf f a second "pancake"1 Coil adjacent 'o the ,-ancake Coil~ formed initially, with a layer of insulatinlg material separating the two coil.s- TherMocou-Ple wire is then wrapped around the second ,pancake" coil to suovort the Cciltrcredin the final heat treatment. :'oltage Zaos and htOoUi wire can be attachied at various mnts on the tape 33 of the double "Pancake" Coi.l in order to =Qnitor the temperature and electrical bena'.ior of the i> n addition, all cojls can be impregnated with epoxy after heatZ treating in order to improve insulationl prcperties and hold the various layers firmly in place. The double "pancake.. coil allowg one edge of the entire length of tape to be exposed directly to the oxjdizing environment during the final heat treating step.

In addition to providing oxygen access to the precursor to the superconducting mater4 al, the coil winding step can result in strengthening the =atrix-forming material. Straining of silver, as Well as other metals, during coil winding results in "strain hardening"l, a phenomenon which increases the ability of the metal to withstand an imparted stress. Because multi-filament composite conductors have metal regions5 surroundinlg the isolated superconlducting regions5, "strain hardenling" strengthens the metal uniformily across the conductor cross 5.525.583 00 C 0 o 0 section. This is not the case for mono-filament conductors, where the matrix-forming material surrounds the superconducting region in the core of the conductor, and "strain hardening" only strengthens the outer edges of the conductor.

Final Heat Treatment After winding, the coil wound with the precursor to the multi-filament composite conductor is subjected to a final heat treating process, the general parameters of which have been described in detail (Riley et 1a., American Superconductor Corporation, "Improved Processing for Oxide Superconductors", S/N 08041822, U.S. Patent Pending). The final heat treating process of the present invention has been adapted to treat precursors to composite conductors wound into coils, and detailed descriptions of several final heat treating steps are included in the Examples described hereinafter.

The purpose of the final heat treatment is to convert the precursor to the composite conductor to the desired superconducting material, whil at the same time heal micro-cracks and other defects incurred during winding.

Typically, the final heat treatment involves heating the coil to a temperature in the range of 780 860 °C for a neriod of time substantially in the range of 0.1 hr. to 300 d 1 i: ic.

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n 5.525.583 85 86 hr., typically in an oxidizing envi-onment having a pO 2 in the range of 0.001 1.0 atm.

During the final heat treating step of the present invention, two central processing problems specific to windand-react coils formed with the precursors to multi-filament composite conductors must be overcome: 1) proper oxygen access must be provided for the precursor; and 2) "sagging" of the precursor, induced by weakening of the matrix-forming material during heating, must be compensated for. Because of the strict geometric tolerances required for coils, the processing envircnment must not decompose the insulating material or cause detrimental "sagging" in the matrixforming material.

The oxygen-access requirements for the precursors to 15 multi- and mono-filament composite conductors differ because of the distribution of the superconducting precursor material in the composite. The increase in the relative surface area of the interfacial regions in the multifilament composite conductor allows for improved oxygen access to the oxide precursor during the heat treating step.

As discussed in Okada et al., U.S. Patent No. 5,063,200, the diffusivity of oxygen is much higher in a matrix-forming material made of silver than in the superconducting regions.

The increase in the surface area of interfacial regions in the multi-filament composite conductor results in better i y 17 5.525.533 e a *0 *0 p 0* 08**

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r a a 4 .40*4* 4* 4* exposure Of the supercoflductinq regions to Oxygefl, resulting in the optimization Of the electrical properties of the superconducting oxide.

As discussed herein, oxygen access can be increas ed 5 to the precursor of the superconducting material by using a ceramic insulation material having a suitable thickness.

oxygen access can also be increased by modifying the geometry of the coil in the furnace. To provide sufficient oxygen access, "pancake" or double "pancake" coils can be lo wound as described above. Durinq the heat treatlflg step, the coil can be placed on a oxygenl-porous, honeycomb mantle to provide increased oxygen access to the coil during processing.

The presence of the mandrel also has to he accounted for in the wind-afdreact process. The mandrel can become oxidized, and can also block oxygen access to the conductor. In a particular embodiment of the invention, the mandrel is made of silver, which is oxygen permeable at high temperatures, and thus allows increased exposure of the precursor to the multi-filament conductor to oxygen during processing. Furthermore, a mandrel composed of the same material as the matrix-forming material silver) will exhibit the same thermal expansion and contraction properties, thus reducing strain incurred during heating and cooling steps of the processing method., Al .4.

525.53 C

CC

C* CC( The abilitY of the precursor to the multi-fiament composite to undergo improved crack healing during the final heat treating step is also improved relative to monofilament composites due to the increase in the su per c onductor/matrix-forming material interfacial regions.

Because the surface-to-volume ratio of the superconducting region increases as the sizes of the individual regions are decreased, multi-fiiament composites will necessarily have an increased amount of interfacial regions when compared to mono-filament composites having the same fill factor.

Successful crack healing depends cn partial melting of the superconducting regions during processing, which leads to coexisting liquid and solid oxide phases of the superconducting material. Recrystallization back into the 15 superconducting oxide phase results in crack healing. It is well known in the art that the presence of silver lowers the melting point of the superconducting precursor material.

This effect will therefore be more prominent in multifilament composite conductors because of the increased surface area of interracial regions.

In addition, the thermal conductivity of the silver matrix-forminq material is significantly higher than that of the superconducting precursor material. The thermal gradient across the superconducting regions during processing will therefore be increased as the cross-

C,,

sectional size of the region is .ncreased. The decrease in size of the superconducting regions in the multi-filament composite conductors results in a more uniform heating field being applied to the superconducting material because of the 5* increased interfacial region. This results in partial ie melting of the superconducting region of the multi-filament composite conductor occurring at a lower temperature and being more uniform than for mono-filament composite conductors.

When heated to the high emper-tures of the final heat treating step, silver does not melt but is essentially

S.

**left without strength. A conductor wound in a coil geometry can therefore "sag", or deform under its own weight, r* tresulting in a decrease in the winding density. Furthermore, the complex winding densities used to provide the coil with sufficient oxygen access are more likely to expose the multi-filament composite conductor to non-uniform temperature distributions, resulting in unpredictable "sagging" of the composite conductor during heating- These problems are overcome by using a thermocouple wire, or other heat-resitant wire, to restrain the layers of insulated composite precursor during heat treatment. Coils can also be mounted with their central axis vertical in order to reduce the effects of "sagging".. 15 te cmplx wndig dnsiiesuse toproidethecoi wih 1 1« 1;: t c t c

ON

5.525.583 93 94 Once the superconducting state is achieved, critical-current densities in the conductor are strongly dependent on filament thickness, conductor thickness, and filament position within the conductor- Filament thickness 5 is typically on the order of 17 pm, and overall conductor thickness is typically 175 pm. Multi-filament composite conductors used in superconducting magnetic coils processed with the wind-and-react nethod can typically exhibit critical-current values between about 1 20 Amperes at 77 OK in self field, depending on the number cf conductors surrounded by a single insulating layer. The values of the critical-current is particularly sensitive to the magnetic field perpendicular to the wide portion of the conductor surface.

15 Electro-Mechanical Properties of Multi-filament Composite conductors Processed ith the Wind-and-React Method S Multi-filament composite conductors processed with the method of the present invention have higher strain tolerances than mono-filament composite conductors due to the strain-dependent properties of the superconducting regions and the matrix-forming material. For most superconducting materials, the critical current is independent of the amount of tensile strain (that is, strain associated with the tension of the conductor) unless the i 5525.583 a e a B o a a a critical strain of the material s exceeded. When this occurs, the thickness of the induced micro-cracks is proportional to the tensile strain, and the maximum critical-current value supported by the superconductor is 5 decreased significantly. This relationship between critical-current and tensile strain is illustrated in Figure 6 for a sample of multi-filament composite conductor 15 cm in length and cut from one end of a 70 m long conductor- The critical-strain for this particular sample is about 0.54%.

At strains exceeding the critical-strain value of the conductor, the critical-current decreases asymptotically towards about 2 kA/cm 2 If the local tensile strain is significantly greater than the critical strain value of the precursor to the conducting material, micro-crack formation 15 can occur to such an extent that crack healing becomes impossible. Because critical strain values are typically much greater for multi-filament composite conductors compared to mono-filament composite conductors, it is possible to subject the superconducting region to higher tensional strains during coil winding without the conductor incurring irreparable damage.

A decrease in critical-current density for both multi- and mono-filament composite conductors can also occur when the current generating the magnetic field rapidly increases or decreases, or otherwise oscillates with time.

~i 4:; 4B 98 5.525.583 a. *6

S

a S C Sa

S

*r In general, losses due to alternating currents in conductors can be reduced by subdivision of the superconducting regions, and will therefore be less severe for multifilament composite conductors. A detailed discussion of 5 this phenomenon can be found in M.N. Wilson, Suerconductinq Magnets, Monographs on Cryogenics, Clarendon Press, Oxford, 1983.

Referring now to Figure 7, another advantage of the processing method in accordance with the present invention is illustrated by the graph which plots critical-current densities measured in BSCCO (2223) composite conductors as a function of bend strain. The critical strain values of the conductors were in the range of 0.3 In the experiment, bend strain, normally incurred through winding, was simulated by bending composite conductors to various radii. After the bending, conductors were exposed to a sintering step. Following heating, the current density was measured across the bent section of the conductor.

The insensitivity and high value of the criticalcurrent density supported by the conductor in the presence of bend strains in excess of the critical strain of the conductor clearly demonstrates the crack healing ability of a multi-filament composite conductor. Although criticalcurrent density initially decreases by about 10% for small bend strains (from comparison with the critical-current t

:LF

i' r i! ii; ;ir at a

~B

Mr Si i.

i If 5.525.583 a, a *1 S a. a

A

La a value of about 11.2 x 10 3 A/cm 2 at zero bend strain), the critical-current density is relatively insensitive to values of bend strain up to nearly For a conductor thickness of 175 Am, a 5% bend strain corresponds bend radius of 5 about 1.6 mm.

Referring now to Figure 8, further benefits of windand-react processing of multi-filament composite conductors are illustrated by comparing the normalized critical-current density as a function of bend strain for multi-filament composite BSCCO (2223) ccnductcrs processed with different methods. Conductors processed with the wind-and-react processing method were first bent and then subjected to a final heat treating step, while the react-and-wind processing conditions comprised heat treating the conductor, 15 inducing the desired bend strain, and finally measuring the current density across the bent section of the conductors.

At 1% bend strain, the critical-current density supported by the conductor treated under the react-and-wind processing conditions is reduced to 43% of its maximum value (measured at 0% bend strain). In comparison, at 1% bend strain, the critical-current density supported by the conductor treated under the wind-and-react processing conditions is minimally reduced to 85% of its maximum value, indicating the advantage of the processing method of the present invention- I ;ilj C

IN

9 9999 a 9 99 9. 9 9 9 999 a .9 9 9* 9* 99 S C Variations of wind-and-Reac: Coils In commercial applicationls, the success of the windand-react processing method is dependent On the influence a' the processing environ~ment on the supe-,cornducting material.

5 Principally, two factors contribute to this inftluence: 1) the susceptibility of the precursor of the eventual superconducting material to te~raur during th~e sinteringy steps; and, 2) th ce~eability- of silver 17c exycen at temperatures :necs fSCO 0 C. Tthe F:rst factcr allows ic Successful r 1 1 ro-cracOt. hea Vr by -tlg anrecrystallizinq the soperconductzflg =raiins during the sintering land the subsecuen cocLi'nq) steps of the inventive method, and the second factor permits exposure of the precursor to the =ulti-filament composite conductor to 15 oxygen, which facilitates micro-structural gjrow.th of the superconducting grains. Both factors will be influenced by the design and physical dimensions of the various coil types- Because the coi;! is subjected to a final heat treating process, the design tolerances are of particular importance. The multi-filament composite conductors used to form the coils must have the length and width dimensions k~ept as uniform as possible. If multiple coils are to be stacked, it is important to fabricate coils having uniform geometric sizes, and to minimi.ze deformation during the heat :1 5.5255 S 3 4.

40 *00 4 O S S

S

a. C treating process. This ultinmately results in 'Inal Coil designs having high windinlg and packing densities, which are critical in determining thle resultant magnetic field.

Referring now to Figure 9, a layer-wounld solenoid superconducting coil 50 processed by the wind-arnd-react method of the present invention has a mandrel 53 wrapped by a multi-filament comosze :,onduc==r 551, %;hic has a ceramic insulation coverinq 52 wrapped arcund it. The designs and thermal prouerties of the superccroucting coil 50 and mandrel 53 have substanti'al influences on the heating and oxygenation of the superconducting material encased in the multi-filament composite conductor 51. For eXarMPle, if the heat capacity of the mandrel 53 is large, the temperature cooling rates of the heat treating steps of the present 15 processing method may have to be increased in order for the coil to thermally equailibrate. at low temperatures in the required amount of time. Similarly, the amount of heat transferred from mandrel 52 to the multi-filament composite conductor 51 will be dependent onl the size of the mandrel, with larger mandrels dissipating more heat to the surrounding conductor than smaller mandrels.

Referring now to Figures 10 and 10a, a preferred embodiment of the "pancake,, superconducting magnetic coil 67 wound with multi-filament composite conductor 66 is shown.

To ensure that -the multi-filament composite conductor 66 Ci 5.525.583 S 106 105 receives accepti-le exposure to oxygen during the final sintering step of the .ind-and-react process, the precursor to the multi-filaent composite conductor, which has a flattened ribbon or tape configuration, is wrapped in layers 5. concentrically around a mandrel 65 forming a spiral pattern.

n t h e p e e i g i n n e r Each layer is wound directly on top of the precedg inner ru lt h i layer, making the heigcht h of the coil 67 equal to the width of tape. Figure !Ca shows a top view of the illustrated *"embodiment of the conductor in ciqre 10, and illustrates how the outer edge cf the precursor to the co.mposite conductor is exposed to the oxvaygen atmosphere along .ts entire length during the heat zreating step of the wnd-and- Rreact processing method.

The "ancake" coil 67 is desirable because it 15 provides a configuration in which the ulti-filament composite conductor 66 has a high winding density, while maintaining suitable oxygen exposure for the multi-filament Scomposite conductor 66 during the final heat treatmert step.

In an embodiment of the present invention, approximately layers of the precursor to the multi-filament composite conductor are used to .rap the mandrel 65, with the total length used being about 00 cm.- Using a BSCCO (2223) conductor with 19 filaments, the illustrated embodiment of the invention is capable of supporting a current of about Amperes at 77 K, with an associated magnetic field being as 84 3^ 'I 107 108 larce as about 1.00 Gauss Th; c.Oil is expected to porforn at a higher level than a cnil having a layer-wound configuration (Fig'Jre 9) treated with the w-ind-and-react 19 processing method. For this latter case, only the outer surface of the winding is exposed to the oxidizing arnosphere during final processing, and the electricai a. properties of the conducting material are thus e-cpected to be inferior.

In an alternate emtcienLt of the present inetenlle-tr~i. cois can be fabrcated by removing the mandrel from tha cent-er Of the coil- -his embodiment can be desirable because elitinaticn of the mandrel results in reduced cycling stress which results from thermal expansion of the mandrel during heating and cooling steps.

Referrinq now to Figure l0b, in another alternate embodiment of the present invention, the "pancake" coil can be form-ed around a mandrel having a cross section with a primarily elliptical,. "racetrack" shape, rather than the circular cross section of the "pancake" coil illustrated in Figure 10a. In other alternate embodiments of the present invention, mandrels having arbitrary shapes and sizes can be used to support the multi-filanent composite conductorin another preferred embodiment of thie invention, double "pancake" coils having circular or pri".arily YI- ~LP~l~slLP---

-A

5.525.583S

IU

rJ A a. c elliptical ("racetrack") shaped cross secrions can be formed using the winding process descried herein. This coil geometry comprises two adjacent single -pancake" coils wound from a single tape comprising the precursor to a multifilament composite conductor, with the adjacent coils sharing the same central axis. In this geometry, each end of the tape forming the two coils is on the outer surface of the coil, thereby eliminating electrical connections inside the coils.

In another alternate embodiment of the invention, the winding density of the coil may be increased by cowinding two or more portions of tape comprising the precursor to the multi-filament composite conductors together with a single cloth comprising the precursor to an 15 insulating material, and then forming the cloth and tape into a single or double "pancake" (or "racetrack") coil. Cowinding multiple strands of conductor in this fashion is effectively the same as wiring multiple conductors in parallel, and coils formed in this manner can achieve even higher winding densities while minimizing the amount of insulation in the coil.

Referring now to Figure 11, which shows a side view of another preferred embodiment of the invention, and Figure lia, which shows a cross-sectional view of the same embodiment, a mechanically robust, high-performance k7_1 5.525.583 111 112 superconducting coil assembly 7C combines multiple double "pancake" coils 71 each having co-wound multi-filament composite conductors. In the coil assembly 70, double "pancake" coils 71 having four co-wound conductors wound in 5 parallel are stacked coaxially on top of each other, with adjacent coils separated by a layer of ceramic insulation 72. A tubular mandrel 74 supports the coils 71. End flange 77 is welded to the top of the tubular mandrel 74, and end flange 76 threads onto the opposite end of the tubular mandrel 74 in order to compress the double "pancake" coils 71. In an alternate embodiment, the tubular mandrel 74 and a the two end flanges can be removed to form a free-standing a coil assembly.

A segment of superconducting material 78 is used to 15 connect the double "pancake" coil adjacent to end flange 76 to termination post 79 located on end flange 77. Individual coils are connected in series with short segments of superconducting material, and an additional length of superconducting material 82 connects the double "pancake" coil adjacent to end flange 77 to termination post 81. These electrical connections allow current to flow from termination post 81, through the individual coils, to termination post 79. The current is assumed to flow in a counter-clockwise direction, and the magnetic field vector 1 11311 So is normal to the end flange 77 tCrMifl9 the top of coil assembly A particular advantage of coils featuring~ multifilament composite conductors is related to the thermal fatigue incurred through heating and cooling the coil, and is ilusrate bytheplot in Figure 3. The Frg'Jre plots the ,*retention of critical-curren7 for composite conductors '1(wound into coils) as a function cf thermal cycles, which are defined as the processesofcligtecldwno LO cryogenic teoperat-_zs and thien heazting t~ie coil back to room temperature. Due t-3 the Inherent l ack of fLexibility of ~the mono-filament copotecnccr the cc-; performance is decreased severely after 5 thermal cycles, with the critical-current retention dropping to 10% of its maximum~ value. ncotrast, the coil wound with multifaen composite conductor shows no significant decrease in coil performance after 5 thermal cycles, with the criticalcurrent density retaining qreater than 95% of its maximum value- Examples The following Examples are used to describe the wind-and-react processing method of the present invention.

Example 1 Layer-Wound Solenloid Coil The precursor to the superconducting phase of BSCCO (2223) was packed into a silver tube having an inn~er

P

_;7V a o rl o e r r r rr diameter of 1.59 cm, a length of 13.97 cm, and a wall thickness of 0.38 cm to form a billet. A wire was then formed by initially extruding the billet to a diameter of 0.63 cm, with subsequent drawing steps reducing the .ire 5 cross section to a hexagonal shape 0.18 cm in width.

Nineteen similar wires were -hen bundled together and drawn through a round die having a diameter of 0.18 ca to farm a precursor to a multi-filament ccr.osite conductor having a circular cross section. The precursor was then rolled to form a multi-filarent composite tape 30 in length having a rectangular (0.25 cm x 0.03 cm) cross section. A single layer of Nextel ceramic fiber having a thickness of 0.002 cm was braided around the multi-filament composite tape prior to the final sintering.

15 The layer-wound solenoid coil was formed by winding the insulated multi-filament composite tape around a cylindrical mandrel having height of 3.00 cm and a diameter of 1.27 cm. Two circular flanges, each having a diameter of 6.01 cm, were welded to each face of the mandrel. Both the mandrel and circular flanges were composed of Haynes 214, a nickel-based alloy. Radial slots were cut into each flange to promote oxygen access to the multi-filament composite tape during the final heat treating process.

A section of composite tape was then wound once around the perimeter of the mandrel, creating a bend strain 7 4,ir

I

S -'41" 117 118 about 6% ,ae condutOr PrecUar Layer o: thermocouple wire v-as wrapped aroun~d the Comlposite tape, *thus securin~g i -o the mandrel. TwJo silver Efoil electrical :terminations were connected to the initial segent~s of -he u .lti-filament CC: po 5 ite ape to form the current and *voltage leads. A single layer of tnemac -iamn composite tape was then ound helical.-Y al-ng thie length of the mandrel. The w,~adinq prccesl': Was repeated using te -remaining portions of tie coaooste -ape, resulting in layeP -beiflg wound cnto he inindrt- Trhe seg-meflt of the composite tape was secured to wair~ zth *thermocouple wire, and electrical leads were attached as described above.

The superconlducting phase of the multi-filament comsteap was formed by processing the solenoid coil a Cit af1fl-l heat treating step comprising the steps of: 1) heating tha coil from room temperature at a rate of i 0 Cfmrin to a temperature of 320 Oc in o.075 atm 02; 2) heating the coil at 820 0 C for 54 hours; 3) cooling the coil to 810 0

)C

and holding for 30 hours; and 4) allowing the coil to Cool to room temperature in I. atm 02.

Electrical properties of the coil were monitored using the voltage and current leads attached to the initial and final segments of the insulated multi-filament composite conductor. The critical current of the coil at 7 7 OK was 119 1203 measured to be A mperes, wit- te :nagne7ic feld in the center of the coil calculated to be 150 Sauss.

Example 2 "Pancake" Coil The precursor to the multi-iamelt comrosie conductor was Eor-med usin~g the defcrcation and rebundling processes described Ln £xni ,and then rolled toQ form a 2.7 m long multi-filament cczpos_ re tape havinq a thickness of 0.02 cm and a width of 2.25 cm. A .1exte1 cerami*c fiber having an adhesive binder -was braided arcund the ccrmpCsite -ape prior to coil form.ation.

A single layer of compcsL:o ape was then w'ound onto a candrel made from Raynes 114 alloy and hIaving a bore :diameter of 1.25 cm, creating a bend strain in the multifilament composite tape similar to the value described in the previous Example. Thermocouple wire and electrical *terminations (voltage and current leads) were attached to the initial layer of composite tape as described in Example 1. A 28-layer "pancake" coil having an outer diameter of 6.73 cm was formed by winding the remaining length of the multi-filament composite tape onto the mandrel, with each successive turn forming a layer of composite tape directly on top of the previous layer. Electrical terminations andk thermocouple wire were attached to the outer layer of the multi-filament composite tape as described in Example 1.

Following the winding process, the "pancake" coil 'was 121 subjected to ivo separate teat tr eating processes. The initial process was u sed to remove the adhesive binder from the Nextel ceramic fi-ber insulatinlg layer, and comprised the steps of: 1) heating the coil from room temperature to 550 O C at a rate of 5 OC/mifl; 2) fteating the coil at 550 0 C for 1L5 hours; and 2) allowing the coil to cool to room temperature- The forc-ation of the superconducting phase in the insulated conoosite tape was accorplished with a final heat treating step, coprislrg the steps c: 1,1 'eating the ccil from room temmeratura to ;90 -Cat a rate cf 10 OC/=4-n in 0.75 atm -'Mmediately ccolir.u tte coil at a rate of 0 C/mfifl to 810 0 C; 3) heating the Coil at 810 CC for !00 hours; 4) cooling the coil at a rate of 10 OC/min to 700 cC; and 5) allowing the coil to cool to room temperature.

Electrical properties of the "pancake" coil were monitored using the voltage and current leads attached to the initial and final layers of the coil. The critical current of the coil all 77 OK was measured to be 1.35 Amperes, with the magnetic field in the c-enter of the coil calculated to be 85 Gauss.

Example 3 Double "Pancake" Coil The multi-filament composite tape was formed using the deformation and rebundling processes described in Example 2. Four different sections of multi-filament composite tape and a section of quartz cloth were then wound 123 124 s r ;o a or rr a ~1 1 5 a a r r r r a onto five separate spoos, eac of which was =otnted on tho arm of the coil winding device shown in Figure Using the double "pancake" winding procedure described previously, portions of the four sections ot multi-filament composite tape were then co-wound with the quartz cloth onto a mandrel cade of silver and having an internal diameter of 2.86 cm. A singLe layer of the coil thus comprised four portios of cc:posite t'ape wound on Op of each other, with a sinlie certion of quart: cloth wournd on top of the forth :aver. The teno strain of te cmposte tape in the first layer of the cll -as eszimated to to 0.50%. The co-winding procedure for the double "pancake" coil was repeated to form two "pancake" coils, each having 55 layers, with the coils separated by a thin insulating sheet comprising quartz Eibers. The final outer diameter of the double "pancake" coil was approximately 10.8 cm.

The binder was removed from the insulation layer using the initial heat treating process described above. The formation of the superconducting phase in the insulated multi-filament composite tape sections of the double "pancake" coil was accomplished with a final heat treating step comprising the steps of: 1) heating the coil at a temperature of 20 OC for 1 hour; 2) increasing the temperature at a rate of 10 OC/min to 799 OC; 3) increasing the temperature at a rate of 1 oC/min to 830 OC; 4) heating s ia P; I-' w i-- 126 12 the at 83 c 0ht te a rate of 1. C/mir to 81' 6) '-eat nq te co-L! at S11L0 for 120 hours; 7) cCOLiflg the Coil at 3 rare Of 5 OCfo in to 787 OC; 8) fleatir.g the coil at 7a 7 1.0o hours; and, 9) cooling~ the coil at- rate of 5 OCJ/2ifl to cool tO rcom temperature. The atmosphere was conprisdct75 0- l steps of the final htoat treat _2n- steC. Fo11'n thne :processing steps, the manzireo was remc.'ed and !:he dcuble "Pancake" coil -Was ~pont~wt pX no~e ohl the layers cf insulat;.cn n -tOe:aCftI inla.

oElectrical properLe ct tl.e oube "pancake" Coil were monitored using the voltage an. c ernrt loads attached *to the ends of the supertonnd-_c~tinq compoSitQ tape located on the outside surface of each "pancake" co-i. The critical current of the coU at 77 OK was measured to be 18.9 Amperes, wiith the self field calculated to be 250 Gauss.

Example 4 Stacked Doublel "Pancake" Coils Eight double "pan~cake" coils were individually fabricated and hneat treated as described in Example 3. After removing each of the mandrels, the coils were then coaxially stacked on top of each other and supported by an aluminum tube having a height of 7.60 cm and a diameter of 2.86 cm which was placed through the center of the coils. Rn aluminum flange was welded to the tcp of the tube, and another flange was threaded to the bottom section of the 127 1 2S tube in order to corPr5ss the par._,ake cois tzce:ter.

Termination pcsts 'were attached to the toD cortcn of the end flange in order to nonitcr the current and voltage values of the coll.

n order to join -nii~ V oil toether n a series circuit, electr~cal connect~cns conti.:'.g of short lenaths oE .ult:-f-I5Oelt connosite topc cont3inn superconductinq SWCID fr::23 were sc ,4red to he ends of the composite tape iccazed cn the c-atsizO surface 0.t eacn' 0O double "pancak:e' lr composite ~tape were used to na.:e rrtLed rzt.

termination post to the coil. Resisztve !oss due to the soldered electrical ter-minaticfl5 used to connlect the coils *in series were measured to be n the Afl regimne. 'The critical current density of the stacked coils was similar to the value measured 4in Exar.Dle 3, cnd *:he calculated field in the center of the coil was approximately 4,000 Gauss at 77 The foregoing descriptions of preferred er-bodiments of the processing methods and related inventions have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limiit thie invention to the precise form disclosed. The embodiments chosen are described in order to best explain thprnies of the processing method and invention-

Claims

2. The =ethcd cf clai cefi 3a±este 2 treating comprises the stems Of: 3 heating and ccoling said Z::i er n r.Ze 4 comprj;sifg oxygen, said heat t-reating step res--;t:nc7 in th e czAersinn of said supeciZucr rcur filate0 tz deslred 7 superccnducun,-g =ater:3i. and Lntne repai- z: 8 Eurmed in sai.d flla ents Hurin sai z3: 1
3. The =etth-C C.f t7re'.za.Zc? 2 treating nmprlses tte stezs -f I heatin~g said c )il at a rate of abcut 'C :m in un t i 4 a temperature ter.;eel *65 OC and 315 OC is =btalned, heating said col at a rate cf abcu-: 1 zCnntflt *6 a maxiinu= tezmperature tt.ees. 31n CC and 86G CC is cbtained, *7 heating said col' at sald =aximuz te==eraztre 8 between 81.0 CC and a60 CC for a zimre period tet-ween and *9 300 hcurs, 101 coalna said at a rate of about 1- OC/i-n. until 11 a temperature tetween 780 OC and 245 c is obtained, 12 heatingq said ccil at said temperature between 730 =C 13 and 845 'C for a time period in th~e range of 1 to 300 hours, 14 cooling said ccii at i rate of about 5 Cfoifln. until a temperature betee 755 0 and a15 OC Is obtained, K l.j ULM.-MM-1 133 134 -6 heating said coil at said terperature between 765 C 17 and 815 0 C for a time period in the range of I to 300 hours, is cooling said coil at a rate cf about 5 oCrin. until 19 a temperature of about 20 0 C is obtained, said heat treating step being perfcred in said hhrinre ressre of said 21 environment comprising cxygen, wherein te pressure of said 22 oxygen is between about 0.001 and 1.0 atm. L 4. The -ethd of cin wherein said step of heat 2 treating comprises t=e steps cf. 3 heating said ci1 at a rate cf :0 -2/o0n. uti a 4 temperature of 789 'C i obtar.ea heating said coil at a rate of I 'C/in. until a 6 maximum temperature of 830 0 C is obtained, 7 heating said coil at said maximu temperature of 830 a 'C for a tire period of 40 hours, 9 cooling said coil at a rate of I OC/ain until a temperature of 811 00 is obtained, 11 heating said coil at said temperature of 811 OC for 12 a time period of 120 hours, 13 cooling said coil at a rate of 5 OC/min. until a 14 temperature of 787 OC is obtained, heating said coil at said temperature of 787 OC for 16 a time period of 30 hours, 136 135 coolinti said coil at a rate of 5 Cmf.until a 18 temperature of 2C cC is obtained, 19 said heat treating step being perfor--ed in said environment comrising cxygen, wherein the pressure of said 21 oxgnis 0.075 atm. The method of claim w;hereinl said surrounding *2 step comprises thze steps of: 3imerin sidprecursor to said inulti-f;Iamelt 4 composite conductor :n a licruid 7n~ture, of one cf a soivent n/o isesat adaparticulate mazerial ccnor~sing the 6 precursor to an insulating material, and then removinfg said *7 precursor to said multi-filament composite conductor from 8 said liquid mixture, heating said precursor to said multi-filament composIte conductor after said imersing and removing steps, 11 said heating resulting in the evaporation of said *12 solvent and/or dispersant, and resulting in "he formation of 13 an inuaiglyraround said precursor to said multi- 14 filament composite conductor. 1
6. The method of claim 1, wherein said 2 surrounding step comprises the steps of: 3 wrapping an insulatinlg material around said 4 precursor to said multi-filamelt composite conductor, 5.525.5S3 138 137 heating said precursor to said multi-filament 6 composite conductor after said wrapping step, 7 said heating resulting in the removal of impurities 8 from said insulating material. e 1
7. A method for producing a superconducting magnetic 2 coil, comprising the steps of: 3 winding one layer of a precursor to a multi-filament 4 composite conductor comprising multiple superconuctor precursor filaments enclosed in a =atrix-forming material 6 around a mandrel to form a precursor layer, 7 winding one layer of a material comprising one of an i 8 insulating material and a precursor to an insulating 9 material on top of said precursor layer to form a composite 10 layer, 11 forming a plurality of said composite layers by 12 repeating said winding steps, a'13 said plurality of composite layers forming .id 14 coil, heat treating said coil after said forming step by 16 exposing said coil to high temperatures and an environment 17 comprising oxygen, said superconductor precursor filaments 18 being oxidized and said matrix-forming material reversibly 19 weakening during said heat treating step, M R .i. Aso ~i~sa~rP;El~~O~- 5.525.583 o a s a a a a o a a a r a o s a 4* a the composition and thi.kness of one of said insulating material and said precursor to said insulating material chosen to encase said weakened matrix-forming material and said superconductor precursor filaments, and to permit exposure of said superconductor precursor filaments to oxygen during said heat treating step, said heat treating step resulting in the improvement of the electrical and mechanical properties of said superconductor precursor filaments, and in the formation of a superconducting magnetic coil.
8. The method of claim 7, wherein said coil forming step comprises the steps of: concentrically winding said composite layers around a mandrel, forming a multi-layer, insulated "pancake"-shaped coil, each of said layers of said coil being wound to directly overlap the preceding layer to form said "pancake"- shaped coil, said "pancake"-shaped coil -'rmitting exposure of an edge of the entire length of said precursor to said multi- filament composite conductor to a selected oxidizing atmosphere during a heat treating step, said heat treating step resulting in the conversion of said superconductor precursor filaments to a desired i ;Z 1. i~9 B ,:I L. i ii P ~-~S~Isre~pe~ 5S142 141 uprcndctngmaterial ai.d, in t .e repair of micro-cracks 16 for o d uin cid g ilaments during said forming step. 1
9. The method of claim 8, weenec opst 2 layer comprises multiple Precursor Lay'ers surrounded by a 3 si0gl layer ofoeofisulatingq material and a C 4 precursor to an insulating material. The method of claim 8, further Imrigthe 2 Step of -winding a second "pancake-shaued coil on said 3 mandrel adjacent to said "pancake-shaped coil, oiga 4 double .Ipancake"-shaped coil. 2.
11. The method of clai.. 10, wherein said double 2 pancake"-shaped coil is formed by the steps of: 3 windingq said precursor to said muiti-filanent 4 coposteconlductor around a supply spool, 5 winding a portion of the length of said precursor to 6 said multi-filament. composite conductor wound around said 7 Supply spool onto a storage spool, 8 concentrically winding the remainling portionl of said 9 precu-rsor to said multi-filament composite conductor wound around said supply spool and one of said insulating material 11 and said precursor to said insulatinlg material onto a 12 mandrel to form a composite layer, I .s -i ci~Di~. 1~ 5.525.583 0s* a a c r os r se s as a r a a Ir rrr r forming a plurality of said composite layers, each of said composite layers being wound to directly overlap the preceding layer to form a first "pancake"-shaped coil, winding a second "pancake"-shaped coil onto said mandrel using said portion of the length of said precursor to said multi-filament composite conductor wound onto said storage spool and one of an insulating material and a precursor to said insulating material.
12. The method of claim 8, wherein said cross section of said mandrel has an arbitrary shape.
13. The method of claim 8, wherein said cross section of said mandrel is primarily circular in shape.
14. The method of claim 8, wherein said cross section of said mandrel is primarily elliptical in shape. The method of claim 10 further comprising the step of combining multiple double "pancake"-shaped coils.
16. The method of claim 15 further comprising the step of coaxially stacking said multiple double "pancake"- shaped coils. i- i I :li i 1 -i~I I ~se~o~eP~ ;m I~i~a~8~ss~a~t~ I. 5.5:5.583 145 146 1
17. A method for producing a superconducting 2 magnetic coil comprising the steps of: 3 fabricating the precursor to a multi-filament 4 composite conductor comprising multiple superconductor 5 precursor filaments enclosed in a matrix-foring material, "to sid multi-filament 6 surrounding said precursr to said ultifil i- 7 conductor with one of an insulating layer and a precursor to a a, a an insulating layer, 9 forming said precursor to said =mlti-filacent composite conductor as a coil, 1i heat treating said coil after said forming step by 12 exposing said coil to high temperatures and an environment 13 comprising oxygen, said superconductor precursor filaments 14 being oxidized and said matrix-forming material reversibly 15 weakening during said heat treating step, 16 the composition and thickness of one of said 17 insulating layer and said precursor to said insulating layer 18 ciisen to encase said weakened matrix-forning material and 19 said superconductor precursor filaments and to permit exposure of said superconductor precursor filaments to 21 oxygen during said heat treating step, 22 said heat treating step resulting in the formation 23 of a multi-filament composite conductor from said precursor 24 to said multi-filament composite conductor, ,t =111 I ii i LLi '-7*1 i, ,r! 5.525.583 6@Cr I *4 C.i e C. said multi-filament composite conductor formed into said coil being subjected to a bend strain in excess of the critical strain of said multi-filament composite conductor, said coil, after said heat treating step, having a minimum critical-current of about 1.2 Amperes.
18. The method of claim 17, wherein said forming step comprises the step of subjecting said =ulti-filament composite conductor to a bend strain in excess of about 0.3%.
19. A superconducting magnetic coil made by the method of claim 1.
20. A superconducting magnetic coil comprising: a multi-layer coil of a multi-filament composite conductor comprising multiple superconducting filaments enclosed in a matrix-forming material, said multi-filament composite conductor formed into said multi-layer coil being subjected to a bend strain in excess of the critical strain of said multi-filament composite conductor, said multi-filament composite conductor having a minimum critical-current of 1.2 Amperes. 1 ~ei j" B 5 3 -5.5 8 149 150 1
21. The supercornd=:ting magnetic Ccoil of claim 2 wherein said mult_-fiia5Oflt composite conductor is 3 surrounded by an insulating layer. 1
22. The superconducting magnetic coil Of claim 21, 2 wherein said 4nsulatiflg layer comprises an insulating 3material permeable to gaseous oxygen and substanltially 4 chemically inert relative to said multi-filiaCflt composite conductor. 1
23. The superconducting maqoetic coil of claim 22, 2 wherein said insulating material. comprises ceramic fibers, 1
24. The superconductinqm~ag-ietic coil of claim 23, 2 wherein said ceramic fibers are composed primarily of 3 ceramic m-aterials selected from the group comprising Si0 2 4 A1 2 0 3 and zirconia. 1
25. The Superconducting magnetic coil of claim 24, 2 wherein said insulating material has a thickness between 3 and 150 m 1
26. The superconducting magnetic coil of claim 22, 2 wherein said insulating material comprises a particulate 3 material. 151 15 2 1
27. The superconducting magnetic coil of claim 26, 2 wherein said particulate material i.s a ceramic material 3 selected from a group comprising Al 2 0 3 1 MgO, Si 2 and 4 zircaflia. ,1
28. The superconducting magnetic coil of claim 27, S2 wherein the thick:ness of said insulating layer Is between 3 and 150 pm. 1
29. The superconducting magnetic coil of claim 2 wherein each layer of said coil comprises multiple multi- 3 filament composite conductors, S 4 said multiple muiti-filament composite conductors surrounded by a single insulatinlg layer. 1
30. The superconducting magnetic coil of claim 2 wherein said matrix-fformiing material is selected from a 3 group comprising a noble metal and an alloy of a noble 4 metal. 1
31. The superconducting magnetic coil of claim 2 wherein said noble metal is silver. nor, I jg-il 5.525.583 r t I
32. The superconducting magnetic =oil cf claim wherein said superconducting filaments are comprised of materials selected from the oxide superconducting family.
33. The superconducting magnetic coil of claim 32, wherein said superconducting filaments are composed primarily of the three-layer phase of BSCCO.
34. The superconducting -agnetic coil of claim wherein said coil is impregnated with a polymer.
35. A superconducting magnetic coil, comprising: a coil cf multi-filament composite conductors comprising multiple superconducting filaments enclosed in a matrix-forming material, the bend strain of said multi-filament composite conductors being greater than about 0.3%.
36. A superconducting magnetic coil, comprising: a coil of multi-filament composite conductors comprising multiple superconducting filaments enclosed in a matrix-forming material, said multi-filament composite conductors wound to form said coil, the inner-diameter of said coil being no larger than about 1 cm. i i/i. B a! r r r- d i~ r- i- 9 i 5.52&5S3
37. A 5 s 0 oecnduct_4, t-a_ netC citrpiil a coil of 5 ulti-filament composite conductors comprising multiple superconducting Eilazenzs enclosed in a matrix-forminlg material, said multi-filament ccMposite conductors wounld to form said coil, the win~dinlg densitY of said coil being greater than about
38. A SU 0 ercondUCtiflg m-4fletIC coil, ccmprisiflg: a coil of Multi.-fila~eflt composite zOnductCrs 'naving mul~tiple superccnduc::ing filaments enclosed in a matr~x- forming material, the bend strain of said multi-filamelt composite conductors being greater than about 0.3%, said multi-filament composite conductors having a f ill factor greater than about S o 5* 2 3 4 6 7 8
39. A superconducting magnetic coil, comprising: a coil of multi-filament composite conductors having multiple superconducting filaments enclosed in a Matrix- forming material, the bend strain of said muiti-filament composite conductors being greater than about 0.,31, said coil producing a magnetic field in excess of about 80 Gauss. rk
525.58 3 p a Se A A superconducting magn-etic col, acoprising: a coil of Multi-flaet Composite conductors naving multiple superconducting filamenzs enclosed in a matrix- formeing material, the bend strain of said mualti-filatent composite conductors being greater than about 0.31, the winding density of said coil teing greater than about said mu~ti-filament composite conductors havint_ a fill factor greater than about said coill producinlg a magnetic field in excess of about 90 Gauss. 41. A superconducting magnetic coil, comprising: a coil of multi filament composite conductors having multiple superconducting filaments enclosed in a matri.x- forming material and surrounded with an insulating layer to form an insulated multi-filamelt composite conductor, said multi-fiiamelt composite conductors and said insulation layers being located concentrically around a mandrel to form a single-width, multiple-layer coil of a continuous length of said insulated multi-filamenlt composite conductor, T"t Ali 159 160 each of said layers off said lnstlated lifl~n 12 Composite conductor being located directly over tte 13 preceding layer, forming a "pancakel'-shaiped coil. 14 said I"mancake -shaped coil permitting e.-vosure of an edge of the entire Ler'ath of sait woun~d insulated inulti- 16 filament composite conrductzr tn a selected cxidiJ_-zi"ng 17 atmosphere durirng a heat treating steD. 142. The superccnducting nanmi coilc claim 41, 2 wherein each layer cf said insulated multi-filament 3 composite conductor forming said coil consists of multiple 4 multi-filament composite said multiple multi -fil ament composite conductors 6 being surrounded by a single insulating layer. 1 43. The superconducting magnetic coil of claim 41, *2 wherein said matrix-forming material is selected from a 3 group comprising a noble metal and an alloy of a noble 4 metal. 1 44. The superconducting magnetic coil of claim 43, 2 wherein said noble metal is silver. 161 162 45. The supercornduztrng ganetic coil of claim 41, 2 wherein said superconducting filaments are comprised of 3 materials selected from the oxide superconductor family. 1 46. The superconducting =agnetic uo- of claim j2 wherein said sucerconducting filaments are composed *3 primarily of the three-layer phase of ESCCO. 1 47. The supercooducting magnetic coil of claiM 41, 2 wherein said insuiating layer corprises an insulating 3 material perm-eable to graseous o%-ygen and sutstafltially 4 chemically inert relative to said mult--fiament composite conductor. 48. The superconducting magnetic coil of claim 47, C 2 wherein said insulating material comprises ceramic fibers. 1 49. The superconducting magn.- tic coil of claim 48, 2 wherein said ceramic fibers are composed primarily of a 3 ceramic material selected from the group comprising S'O 2 4 A1 2 0 3 and zirconia. 150. The sup erconduCting magnetic coil of claim 49, 2 wherein said insulating material has a thickness bet~deen 3 and 150 gm. 163 16.4 a, a4 44 44 94 *4 CC 52.. The superconductin~g magneticCcoil of claim 47, wherein said insulatin~g layer comprises a Dart-'culate material. 52. The superconducting magnetic coji Of claim 51., wherein said particulate material is a ceramic material selected from a group comprlIslfg AbCO,, Kiq0, Si10, and z irconia. 53. The superconducting =agneti4c coil of claim 52, wherein the thickness or said insulating layer is betw~een and 150 tim. 54. The supercoflductiflg-ragfletic coil of claim= 41, wherein said cross section of said mandrel has an arbitrary shape- The superconducting magnetic coil of claim 41, wherein said cross section of said mandrel is primarily circular in shape. 56. The superconducting magnetic coil of claim 41, wherein said cross section of said mandrel is primarily elliptical in shape. 4 3. ~1 1~ 525. 5 83 165 166 1 57. The superconducting magnetic coil of claim 41, 2 wherein said mandrel is composed primarily of silver. 1 58. The superconducting magnetic coil of claim 41, 2 wherein said coil is impregnated with a polymer. S1 59. The superconducting magnetic coil of claim 41, 2 wherein a seccnd 1'r-ancakell-shaped coil is wound on said *3 mandrel adjacent to said I"pancaker,-staped coil, forming a 4 double "parncake"-shaped coil. 1 60. The superconducting magnetic coil of claim 59, 2 further comprising a plurality of said double "pancake"- 3 shaped coils stacked coaxially-and adjacent to each other 4 with groups off at least one of said double "pancake"-shaped coils being separated from each other by an insulating *6 material. 1 61. The superconducting magnetic coil of claim 2 wherein said mandrel is removed. I- :1 II P:OPER\KATll5614-95.ES 3f1129 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: e..t I, .7 te C' 0* *9~ 'C 1. A magnetic coil comprising sections positioned axially along a longitudinal axis of the coil, each section including a high temperature superconductor wound about the longitudinal axis of the coil, each section having regions with critical current values, measured at a zero magnetic field, increasing in value from a central portion of the coil to end portions of the coil. 2. The magnetic coil of claim 1 wherein the critical current value of each region is 10 dependent on the type of superconductor and the angular orientation of a magnetic field of the coil. 3. The magnetic coil of claim I wherein the critical current values of the regions of the sections decrease in value from an inner radial portion of the coil, proximate to the longitudinal axis of the coil, to an outer radial portion of the coil. 4. The magnetic coil of claim 1 wherein the critical current values of the regions are varied by varying the cross-sectional area of the superconductor of the regions of each section. The magnetic coil of claim 4 wherein the superconductor is formed as a superconductor tape comprising a multi-filament composite superconductor including individual superconducting filaments which extend the length of the multi-filament composite conductor and are surrounded by a matrix-forming material. 6. The magnetic coil of claim 5 wherein the cross-sectional area of the superconductor of the regions is varied in a direction parallel to the longitudinal axis of the coil. 7. The magnetic coil of claim 6 wherein the cross-sectional area of the superconductor increases for the sections positioned at the central portion of the coil to the sections positioned jl' ;i_ st:: B-ie i 1i n-::t;i ~e~gi~B~B~g~B~g C 1-; ,II- 'r ?ii' :B P;\OPERUCAT1 5614-95.RES 31128 -36- at the end portions of the coil. a a a. a as Lit 8. The magnetic coil of claim 5 wherein the cross-sectional area of the superconductor of the regions is varied in a direction transverse to the longitudinal axis of the coil. 9. The magnetic coil of claim 8 wherein the cross-sectional area of the superconductor for each section decreases from regions proximate to the inner radial portion of the coil to the outer radial portion of the coil. 10. The magnetic coil of claim 5 wherein a number of individual superconducting filaments associated with a first one of the plurality of sections is different than a number of individual superconducting filaments associated with a second one of the plurality of sections. 11. The magnetic coil of claim 5 wherein the orientation of the individual superconducting 15 filaments is other than parallel with respect to a conductor plane defined by a broad surface of the tape. 12. The magnetic coil of claim 1 wherein the critical current value of each region is selected by changing the type of superconductor of at least one section. 13. The magnetic coil of claim 4 wherein the sections of the superconductor are formed of pancake coils and the cross-sectional area of the superconductor is varied by increasing the number of strands of superconductor in parallel. 14. The magnetic coil of claim 1 wherein the sections of the superconductor are formed of double pancake coils. The magnetic coil of claim 1 wherein the critical current values of the regions of each section are varied to provide a desired magnetic field profile for the coil. Slogan= I I p\OPaRKAT15614-95-RES -3112/985 -37- 16. The magnetic coil of claim 1 wherein the high temperature superconductor comprises Bi 2 Sr 2 Ca 2 Cu 3 0. 17. A magnetic coil comprising sections positioned axially along a longitudinal axis of the coil, each section including a high temperature superconductor wound about the longitudinal axis of the coil, each section having regions with critical current values, the critical current values being substantially equal when a preselected operating current is provided through the superconducting coil. S DATED this 3rd day of December, 1998 15 o 15 i AMERICAN SUPERCONDUCTOR CORPORATION S, By its Patent Attorneys DAVIES COLLISON CAVE C CSI