US3336549A

US3336549A - Superconducting magnet coil

Info

Publication number: US3336549A
Application number: US428676A
Authority: US
Inventors: Kafka Wilhelm; Depping Friedhelm
Original assignee: Siemens AG
Current assignee: Siemens Schuckertwerke AG; Siemens AG
Priority date: 1964-01-31
Filing date: 1965-01-28
Publication date: 1967-08-15
Anticipated expiration: 1984-08-15

Description

I) l l 6 S R F2 '1 3 1 x. 14?? q 3359 54 Aug. 15, 1967 w. KAFKA ETAL 3,336,549

' 1 SUPERCONDUCTI'NG MAGNET COIL Filed Jan. 28, 1965 NORMAL METAL COATING United States Patent Filed Jan. 28, 1965, Ser. No. 428,676 Claims priority, applicatiognzgermany, Jan. 30, 1964,

7 Claims. (ci. 335-216) Our invention relates to superconducting magnet coils.

With such coils, particularly those of large dimensions, an inadvertently or intentionally effected transition from superconductance to normal conductance may result in high voltages and heat concentrations at individual parts or localities of the coil winding. These occurrences tend to damage or destroy the coil insulation or to melt the superconductor at individual points, thus rendering the coil inoperative.

It is an object of our invention to device a superconducting magnet coil in which excessive voltages and excessive heat concentrations in individual parts of the winding, due to occurrence of transition, are effectively prevented.

According to the invention, we provide a plurality of ohmic resistance bridges between adjacent windings or turns of the superconducting coil so as to secure a rapid propagation of the transition; the resistance of these bridges being sufiiciently large to prevent the current flowing through the bridges during the build-up period of the coil excitation and then heating the coil, from reaching a value detrimental to the superconducting property of the coil. On the other hand, the absolute resistance values of the ohmic bridges are kept lower than the resistance of an individual portion of coil wire as may initially convert to normal conductance, such a wire portion being short in comparison with the length of a winding turn.

That is the material of the resistance bridges has a higher specific resistance than the coil material when the latter is in the condition of normal conductance, but the resistance bridges nevertheless have a smaller absolute resistance value than a given short length of such a coil-wire in normal conductance, for example a wire piece of 1 centimeter length. The resistance bridges which thus may form a current path from winding to winding or turn to turn in the vicinity of such a short coil-wire piece in transition to normal conductance, are preferably about 5 to 50 micron long and are interposed between the individual Winding turns.

The ohmic resistance bridges according to the invention may be located at several individual points of each winding turn or they may extend over the entire periphery of the turn. It will be understood that the resistance of the bridges must be at least so large that, when the current intensity gradually increases while the excitation of the coil is being built up, the current flowing through the bridges produces within the coil only a small amount of heat which can readily be dissipated by the surrounding cryogenic medium, such as a bath of liquid helium, and hence without appreciably increasing the temperature of the superconducting coil material. For this reason, massive resistance bridges of silver, copper and the like good conducting materials are not suitable for the resistance bridges. These bridges rather consist of material having a much higher specific resistance such as a coating or mixture containing carbon granules, as will be more fully described hereinafter.

The superconducting material of the magnet coil may consist of any metal or alloy known for such purposes. For example, when forming the superconducting coil of niobium-zirconium or niobium-tin alloy, a temperature increase from 4.2 to 5.2 K. is still permissible because it decreases the loadability of the winding only by a slight percentage, namely by less than 15% in the example just mentioned. It is therefore of advantage in some cases to reduce the rate of current increase, during starting-up of the excitation of a superconducting magnet coil equipped with resistance bridges according to the invention, down to a fraction of the initial rate of current increase, as soon as the current approaches the critical intensity value at which the transition is about to occur.

As explained, the absolute resistance of the ohmic bridges according to the invention is considerably smaller than that of a short coil-wire piece, for example in the dimensional order of 1 centimeter, which during transition has converted to normal conductance. If under these conditions the transition occurs in a piece of a few centimeters, for example, the absolute resistance of this wire piece increases to a multiple of the resistance value of the resistance bridges between the windings. Thus, the shunt path now formed by the ohmic resistance bridges and the still superconducting adjacent winding turns, with respect to the coil piece already converted to normal conductance, has a lower resistance than the now normalconducting coil piece. The conductor current therefore commutates onto the adjacent winding turns with a time constant determined by the inductivity and the resistance difference of the two parallel current paths. With the usual wire dimensions, this time constant is in the order of magnitude of l0 second. As soon as an adjacent winding turn is loaded in this manner with commutated current in addition to the current already traversing this winding, the condition for transition in the neighboring winding turn is established or promoted. As a result, the phenomenon of transition is rapidly propagated through the entire coil layer within a few microseconds.

In this manner, the magnetically active current flow or flux in the windings of one layer is substituted by a transverse current passing through the ohmic resistance bridges. Since this transverse current flows parallel to the coil axis, it does not by itself produce a magnetically active fiow (flux). For that reason, the current in the other layers of coil windings increases. Consequently, at those localities where the current is closest to the critical current value, new transitions are released. These again propagate within a few microseconds throughout the particular coil layer. Thus an entire multi-layer coil converts to normal condition within fractions of milliseconds. When this condition is reached, the resistance bridges are relieved of the transverse current because the voltage induced in each winding is now consumed by its own ohmic resistance. Due to the short duration of the total transition period, the heat concentration in the locality first convered to normal conductance is still harmless.

As to the magnitude of the resistance value to be given to the ohmic bridges according to the invention, the following condition is significant: The resistance value is preferably made so small that the product of the number of winding turns, the resistance value of the bridge between two winding turns, and the rated current of the coil, is smaller than the breakdown voltage of the winding insulation at the most unfavorable spot. Depending upon the rate of current increase, the size of the coil, the number of windings, the insulation and the cooling conditions, the resistance value of the bridges is approximately between 0.1 milliohm and 1 ohm. Only under extreme con ditions is it advisable to select a resistance value outside of this range.

As explained, the resistance between two mutually adjacent windings is formed by bridges of material which has a relatively poor conductivity, This material consists for example, of a conducting varnish with which the superconducting wire of the coil is coated, and/or of a carbon-containing paste interposed between winding turns wound in spaced relation to each other. The layer of conducting varnish with which the wire is coated may have a thickness of 10 micron, for example. Preferably, the materials for the resistance bridges consist of fine granular graphite or other carbon or also of granular metal, or mixtures of such substances distributed. colloidally in insulating varnish or synthetic resinous plastic of the kind conventionally employed in electrical equipment for insulating purposes. Such resistance bridges in dry condition and at the cryogenic temperatures of the superconductor, for example at 4.2 K., exhibit specific resistances of 0.1 to 10 ohm-cm, depending upon the dimensions of the coil and the wire.

Another way of providing for the ohmic resistance bridges is to simultaneously wind a thread of glass fiber or glass wool, particularly a glass-fiber material impregnated with conducting varnish or conducting casting resin or potting resin, together with the superconducting wire so that the glass-fiber thread is firmly located between adjacent winding turns on the spool or carrier of the winding. The glass-fiber thread is preferably made slightly thicker than the wire. When the layers of coil windings are being wound upon each other, the thread is firmly pressed against the wire thus providing for a sufiicient contact engagement between wire and thread. If it is desired to produce the ohmic resistance bridges not uniformly along the entire periphery of the winding turns, but only at individual localities, the impregnation of the thread is to be effected only at these particular localities.

The invention will be further explained with reference to embodiments of superconducting magnet coils according to the invention illustrated by way of example on the accompanying drawing in which:

FIG. la shows schematically and in section a portion of a first coil, and FIG. 1b is a corresponding cross-sectional view.

FIGS. 2, 3 and 4 show respectively. three other embodiments also by partial views and in section.

In all embodiments, the spool or carrier of the coil is denoted by 1, insulating intermediate layers between the individual winding layers are denoted by 2, the coil axis by 3, the cross section of the bare superconducting coil wire by 4. The coating of the superconducting coil wire is denoted by 5, this coating having only poor conductance in comparison with the wire itself. Disposed between the individual layers of windings are copper foils 6.

In the embodiment according to FIGS. la and lb, the layers of the coil winding are individually insulated within the winding space of the spool 1. The spool structure may consist of copper, for example, and be lined with an insulating foil 2a. Placed between the winding layers are insulating intermediate layers 2. These consist of a material of good mechanical stability, name-1y with respect to pressure, tearing and bending forces, at all occurring operating temperatures including those in the vicinity of absolute zero. Particularly well suitable as material for the intermediate layer 2 is polyethylene terephthalate. In distinction from conventional magnet coils, the superconducting wires of niobium-zirconium alloy are not insulated but are provided with a coating 5 of relatively poor conductance, which consists of the abovementioned conductive varnish or similar material, to secure the desired transfer resistance from turn to turn.

It is further preferable to have the superconducting wire piece which interconnects two adjacent winding layers of the coil, contacted by a metallic conductor, particularly a copper foil. Such a foil is shown wound onto the wire piece at 11 in FIG. lb. The metallic conductor then commences to conduct the current from the wire piece which interconnects the two layers, as soon as these layers have converted to the condition of normal conductance. This is advantageous because the mentioned transfer localities between two winding layers are not tion and therefore are particularly jeopardized in the event of transition occurring at these localities.

In the embodiment of FIG. 2, each winding layer is separately insulated with respect to the spool structure 1 and relative to the copper foils 6 placed between the winding layers. The copper foils may be short-circuited by being electrically connected with each other, or they may also be left unconnected. The heat generated in the coil is conducted by the copper foils to the spool structure 1 and thence to a surrounding bath of liquid helium (not illustrated). The thickness of the copper foils is chosen in dependence upon the axial length of the coil and is about 5 to 50% of the coil-wire diameter, the thicker foils being used with the longer coils.

To prevent the copper foils from being electrically in conducting connection with the spool structure, an insulating lining 2b is preferably provided between the spool structure and the foils, the edges of the foils being bent upwardly as illustrated. For the same reason, namely for preventing short-circuit currents, the copper foils may be slitted along their edge in parallel relation to the coil axis up to the middle of the coil, such slits being shown at 12 in FIG. 2.

The use of thermally and electrically good conducting foils, such as the above-described copper foils, is also of advantage without the conjoint provision of the abovementioned ohmic resistance bridges. That is, such foils are also applicable to advantage in other superconducting coils for the dissipation of heat and/ or the rapid propagation of a transition once it has occurred. In a coil whose conductors are coated with a good heat-conducting material, the foils are preferably employed for heat dissipation by placing them electrically insulated between the layers of coil windings. Furthermore, such coils, if placed in contact directly with the coil wires which are coated with conducting varnish or similar material, may contribute to rapid propagation of a transition as will be further explained below with reference to FIG. 3.

In the embodiment of FIG. 4, the resistance bridges between the winding turns 4 of bare superconducting wire, are formed by a paste 7. After winding of each individual layer of turns, the paste is inserted into the spaces between the turns which are kept suitably spaced from each other. The resistance paste may contain colloidally distributed graphite or other carbon material and/ or metal as described above.

In the embodiment of FIG. 3, several axially short component coils 9 are placed about a cylindrical spool or coil carrier 8. Each component coil 9 has only a relatively small number of winding turns 4 per layer, for example a few hundred turns, the wire being coated with conducting varnish 5. The resistance bridges are formed not only between each turn and the adjacent turn, but are effective through intermediate layers 6 of copper between each turn and all other turns of the same layer. Although this results in a larger amount of parallel current when the coil is being charged up, the heat dissipation is particularly well effective by virtue of the axially short intermediate layers of copper which directly contact a bath of liquid helium (not illustrated). In this manner, the large heat losses occurring during the build-up period of the coil excitation can be dissipated without appreciable increase in temperature. The free space-between the intermediate layer 6 of copper and the wires 4 coated with varnish 5 may be filled with a resistance paste as mentioned above with reference to FIG. 2.

In the embodiment of FIG. 4, the resistance bridges are formed with the aid of threads 10 which are wound between the wire turns 4 and have a relatively poor conductance. The threads consist of glass wool or similar glass-fiber material impregnated with the above-mentioned conducting varnish or conducting synthetic resinous plastic. If desired, the glass-fiber threads may be impregnated in this manner only locally along a length of about one centimeter, each impregnated locality being spaced about ten centimeter from the next impregnated locality. The glass-fiber threads have a somewhat larger diameter than the wires 4 and, during winding operation, are firmly pressed between the wire turns and the insulating intermediate 'layers 2, so that a sufiicient electrical contact between the threads and the superconducting wire is secured. The remaining free space between the insulating layers 2, the wires 4 and the threads 5, may be filled with a resistance paste as described above with reference to FIG. 2.

It may happen that the surface of the bare superconducting wire suffers corrosion, for example by oxidation or the elfect of nitrogen. As a result, the surface resistance of the wire may have an order of magnitude equal to, or larger than, that of the resistance bridges between the individual winding turns. Under conditions where such corrosion is to be expected, it is advisable to provide the superconducting wire with a protective coating, preferably of silver, gold or copper. The thickness of such coating should be approximatley 0.5 to 5% of the wire cross section. The coating may be deposited during drawing of the wire or by electrolytic deposition. Aside from the improvement with respect to the transverse resistance between the individual windings thus obtained, the metallic coating on the super-conducting wires also improves the heat condutance and thus promotes a more rapid propagation of the transition along the superconducting.

As mentioned, the resistance bridges provided according to the invention between the windings or turns of a superconducting magnet coil, not only reduce voltage stresses during transition, but also prevent excessive heat concentration at individual localities of the coil. The magnetic energy of a large coil, if uniformly distributed over the entire coil, may increase the temperature of the superconductor by several hundred degrees Kelvin. The temperature increase, however, can be limited to a maximum of about 40 K. by increasing the coil mass, for example by interposed copper inserts or by providing heat-conductively inter-communication hollow spaces filled with evaporating liquids such as nitrogen or water. If no local overheating occurs, a limit of about 400 K. is not detrimental to the insulation of the windings. Without special expedients, however, the transition, commencing at one locality, propagates only over a small fraction of the entire wire length, and the temperature increase in this considerably much smaller volume is much higher than may have been computed for the entire coil.

Consequently, without the benefit of the present invention, not only the insulation of the superconducting coil is endangered, but the wire itself may melt locally despite the fact that this require temperatures of 2000" K.

or more.

The resistancebridges provided according to the invention between adjacent windings take care of having the transition rapidly propagated over large portions of the coil, particularly rapidly throughout each individual layer of winding turns. This requires that a bridge to the adjacent winding is present not only at a single locality of the winding, but at least at several localities of the periphery. It is most favorable if the ohmic resistance bridges between the windings virtually extend over the entire periphery. For very large windings, for example with more than 1 meter wire length per turn, it is advisable, however, to provide the resistance bridges only at individual localities because otherwise the total resistance of the ohmic bridge, if continuously extending between the windings over the entire periphery, would be too small.

To those skilled in the art it will be obvious upon a study of this disclosure that our invention permits of various modifications and may be given embodiments other than particularly illustrated and described herein, within departing from the essential features of our invention and within the scope of the claims annexed hereto.

We claim:

1. A superconducting magnet coil comprising layers of windings of superconducting material mutually insulated from each other and having axially sequential winding turns, and a plurality of ohmic resistance bridges interposed between adjacent turns and resistively interconnecting said turns, said bridges having a high resistance relative to that of a respective winding when said winding is in superconducting condition so as to limit the bridge current to a permissible value during build-up periods of coil excitation, and said bridge resistance being lower than that of a given fractional length of a single turn when in condition of normal conductance, whereby excessive voltage and heat concentration due to local initiation of transition in the winding is prevented, said resistance bridges being formed of composition material containing embedded fine-granular substance from the group consisting of carbon and metal.

2. A superconducting magnet coil comprising layers of windings of superconducting material mutually insulated from each other and having axially sequential winding turns, and a plurality of ohmic resistance bridges interposed between adjacent turns and resistively interconnecting said turns, said bridges having a high resistance relative to that of a respective winding when said winding is in superconducting condition so as to limit the bridge current to a permissible value during build-up periods of coil excitation, and said bridge resistance being lower than that of a given fractional length of a single turn when in condition of normal conductance, whereby excessive voltage and heat concentration due to local initiation of transition in the winding is prevented, said resistance bridges comprising a body of synthetic plastic with embedded and colloidally dispersed fine-granular substance from the group consisting of carbon and metal.

3. A superconducting magnet coil comprising layers of windings of superconducting wire mutually insulated from each other and having axially sequential and mutua-lly spaced winding turns, and a plurality of ohmic resistance bridges interposed between adjacent turns and resistively interconnecting said turns, said bridges comprising a body of synthetic plastic with embedded and dispersed fine-granular substance from the group consisting of carbon and metal, the resistance of said bridges being higher than that of a respective winding when said winding is in superconducting condition so as to limit the bridge current to a permssible value during build-up periods of coil excitation, and said bridge resistance being lower than that of a given fractional length of a single turn when in condition of normal conductance, whereby excessive voltage and heat concentration due to local initiation of transition in the winding is prevented.

4. A superconducting magnet coil comprising a winding of superconducting material having axially sequential winding turns, and ohmic resistance bridges interposed between adjacent turns and resistively interconnecting said turns, said bridges having a high resistance relative to that of said winding when said winding is in superconducting condition so as to limit the bridge current to a permissible value during build-up periods of coil excitation, and said bridge resistance being lower than that of a, given fractional length of a single turn when in condition of normal conductance, whereby excessive voltage and heat concentration due to local initiation of transition in the winding is prevented, said resistance bridges comprising glass-fiber threads wound between said turns and in contact therewith, said threads containing a resistively conducting impregnation to provide for said bridge resistance.

5. In a superconducting magnet coil according to claim 4, said impregnation being located at mutually spaced localities of said thread.

6. A superconducting magnet coil comprising layers of windings of superconducting material mutually insulated from each other and having axially sequential winding turns, and a plurality of ohmic resistance bridges interposed between adjacent turns and resistively interconnecting said turns, said bridges having a high resistance relative to that of a respective winding when said winding is in superconducting condition so as to limit the bridge current to a permissible value during build-up periods of coil excitation, and said bridge resistance being lower than that of a given fractional length of a single turn when in condition of riorrnal conductance, whereby excessive voltage and heat concentration due to local initiation of transition in the winding is prevented, said resistance bridges being formed of a layer of material consisting of conducting varnish.

7. A superconducting magnet coil comprising layers of windings of superconducting material mutually insulated from each other and having axially sequential winding turns, and a plurality of ohmic resistance bridges interposed between adjacent turns and resistively interconnecting said turns, said bridges having a high resistance relative to that of a respective winding when said winding is in superconducting condition so as to limit the bridge current to a permissible value during build-up periods of coil excitation, and said bridge resistance being lower than that of a given fractional length of a single turn when in condition of normal conductance, whereby excessive voltage and heat concentration due to local initiation of transition in the winding is prevented, said resistance bridges being formed of a layer of material consisting of insulating varnish containing embeddmi fine-granular substance selected from the group of substances consisting of carbon and metal.

References Cited UNITED STATES PATENTS 3,187,235 6/1965 Berlincourt et a1. 3352l6 3,255,335 6/1966 Kortelink 3352l6 X 3,263,133 7/1966 Stekly 335-2l6 X

Claims

1. A SUPERCONDUCTING MAGNET COIL COMPRISING LAYERS OF WINDINGS OF SUPERCONDUCTING MATERIAL MUTUALLY INSULATED FROM EACH OTHER AND HAVING AXIALLY SEQUENTIAL WINDING TURNS, AND A PLURALITY OF OHMIC RESISTANCE BRIDGES INTERPOSED BETWEEN ADJACENT TURNS AND RESISTIVELY INTERCONNECTING SAID TURNS, SAID BRIDGES HAVING A HIGH RESISTANCE RELATIVE TO THAT OF A RESPECTIVE WINGING WHEN SAID WINDING IS IN SUPERCONDUCTING CONDITION SO AS TO LIMIT THE BRIDGE CURRENT TO A PERMISSIBLE VALUE DURING BUILD-UP PERIODS OF COIL EXCITATION, AND SAID BRIDGE RESISTANCE BEING LOWER THAN THAT OF A GIVEN FRACTIONAL LENGTH OF A SINGLE TURN WHEN IN CONDITION OF NORMAL CONDUCTANCE, WHEREBY EXCESSIVE VOLTAGE AND HEAT CONCENTRATION DUE TO LOCAL INITIATION OF TRANSITION IN THE WINDING IS PREVENTED, SAID RESISTANCE BRIDGES BEING FORMED OF COMPOSITION MATERIAL CONTAINING EMBEDDED FINE-GRANULAR SUBSTANCE FROM THE GROUP CONSISTING OF CARBON AND METAL.