EP1970921B1 - Power supply line with high-temperature superconductors for superconductive magnets in a cryostat - Google Patents

Description

The invention relates to a cryostat arrangement for a superconducting magnet arrangement,
with a helium tank for liquid helium, wherein in the helium tank the superconducting magnet arrangement is arranged,
with a nitrogen tank for liquid nitrogen, where the nitrogen tank surrounds the helium tank,
and having at least one access tube in which a power supply arrangement is mounted, can be passed through the current of space temperature-warm area of the cryostat in the superconducting magnet assembly,
wherein the power supply arrangement comprises at least one power line with a normal-conducting line part and a superconducting line part of high-temperature superconductor (= HTSC) material.

Power supply arrangements with normal-conducting line parts and superconducting line parts made of HTSC material for charging superconducting magnet arrangements in cryostats have become known, for example, by US Pat JR Hull, IEEE Transactions on Applied Superconductivity, Vol. 1, March 1993, pp. 869-875 ,

Cryostat arrangements of the aforementioned type are used, for example, for measurements with nuclear magnetic resonance (NMR) or electron spin resonance (EPR) or laboratory magnets. The contained superconducting magnet arrangements are used to generate strong magnetic fields, whereby stable, low temperatures must prevail in order to achieve the superconducting state, as can be ensured in such a cryostat arrangement.

The superconducting magnet assembly (magnet coil system) is disposed in a cryogenic liquid cryogenic container, usually liquid helium, which is surrounded by radiation shields, superinsulation sheets, and another cryogenic container, typically liquid nitrogen. The liquid containers, radiation shields and superinsulation films are accommodated in an outer container which limits a vacuum space (= outer vacuum envelope, outer jacket). The superconducting magnet is cooled by the vaporizing surrounding helium and kept at a constant temperature. The elements surrounding the helium container serve to heat-insulate the helium container so that the heat input to the helium container is minimized and the evaporation rate of the helium remains small.

The helium container is usually connected to at least two thin-walled hanger ears with the outer vacuum envelope. On the one hand, the container is thus mechanically fixed, on the other hand, the suspension tubes provide access to the magnet, as z. B. when loading is necessary. In addition, that will Leakage gas is discharged through the suspension tubes, whereby the suspension tubes are cooled again and ideally the heat input through the tube wall is completely compensated. Such a system is used for example in DE 29 06 060 A1 and in the Reference, Superconducting NMR Magnet Design (Concepts in Magnetic Resonance, 1993, 6, 255-273 ).

The charging of the superconducting magnet is usually carried out via a power supply, which is installed stationary or introduced via a plug connection in one of the suspension tubes. The power supply is the connection between the magnet at cryogenic temperature and the power supply at room temperature.

One type of such leads is copper or brass leads whose cross-section is optimized for the given length and magnet current and which are cooled by the effluent helium gas. Such a supply line achieves a minimum heat input of the order of magnitude of 1 mW / A of the design current without current flow. In the case of a frequent charging and discharging operation of the superconducting magnet, the heat input also increases due to the Joule heat generated in the conductor.

One way of substantially reducing the heat input to the helium vessel is to use a two-part power supply, the lower part being formed by the temperature of the helium bath (about 4K) to a temperature between 20K and 90K by a HTSC and the upper part to room temperature through an exhaust-cooled copper conductor. The use of HTSC makes it possible to carry high electrical currents at, compared to copper / brass, lower thermal conductivities and cross-sectional areas. Since the current in the superconductor can flow without loss, the heat input to the helium bath is practically independent of the current and only determined by the heat conduction in the superconductor. The The ability to transport high electrical currents lossless, combined with a low thermal conductivity, causes a reduction in He losses and thus the operating costs. There are many articles regarding the design of HTSC power supplies, in particular JR Hull, IEEE Transactions on Applied Superconductivity, 1993, 3, 869-875 ; R. Wesche and AM Fuchs, Cryogenics, 1994, 34, 145-154 ,

HTSC and copper conductors are connected to a transition piece by soldering. In particular, when charging the superconducting magnet much Joule heat is generated in the metallic part of the power supply, which would heat the upper part of the HTSC above its critical temperature. The transition from the HTSC to the normal conductor is therefore additionally cooled in order to keep the temperature below the critical temperature of the HTSC. This can be done by a heat exchanger through which helium gas or liquid nitrogen is pumped. Such an arrangement is for example in US 5,563,369 or US 5,166,776 described. Another possibility is to actively cool the transition by coupling to a cold stage of a cryocooler. Such an arrangement is for example in EP 870,307 or in US 4,895,831 or in US 5,991,647 described.

The arrangements described have the disadvantage that the cooling must be carried out by additional components or cooling units, which are not required for the normal operation of the cryostat or even interfere with normal operation. The additional components or cooling units, the structure of the arrangements is complicated and expensive.

The FR 2 637 728 A1 describes a cryostat assembly according to the preamble of claim 1, with a helium tank containing a superconducting coil and in turn is arranged in a nitrogen tank. From the superconducting coil lead to a voltage feedthrough power lines in an insulating tube. This tube is fitted with a clamping ring on a lid of a nitrogen tank seated, held. The tube ends at an upper end of the voltage feedthrough, which is also seated on the lid and filled with an oil. The power line in the pipe contains a transition from a superconducting line section to a normal line section.

Object of the invention

Object of the present invention is to ensure a simple and inexpensive way efficient cooling of the transition from HTSL to the normal conductor in the power supply.

Brief description of the invention

This object is achieved by a cryostat arrangement of the aforementioned type, which is characterized in that a terminal pole of the at least one power line through which the normally conductive line part is electrically connected to the superconducting line part is thermally coupled to a wall of the nitrogen tank. The thermal coupling is produced by a solid-state connection of the connection pole to the nitrogen tank via contact elements, contacting elements and the same made of highly thermally conductive material, preferably copper, aluminum and / or aluminum nitride. A material is considered to have good thermal conductivity if the thermal conductivity is at least 20 W / (K * m), preferably at least 100 W / (K * m), in each case measured at room temperature.

According to the invention, the cooling of the at least one terminal pole in which the normal conductor part and the HTSC part of the current line are connected (for example by soft soldering) takes place by means of a thermal coupling to the nitrogen container present in the cryostat. This container is independent of the power supply part of the cryostat arrangement.

The transition from the metallic conductor to the HTSC conductor is coupled with a preferably detachable and highly thermally conductive compound to the nitrogen container, and the compound simultaneously ensures a galvanic separation. The temperature of the nitrogen tank of about 77 K allows the transition from the normal conductor to the HTSC conductor in the temperature range between 81 and 90 K to operate. The advantage of the arrangement is also that a simple suspension tube of the helium container can be used with few modifications for receiving the power supply.

Preferred embodiments of the invention

Preferred embodiments of the invention will become apparent from the dependent claims.

A variant of the cryostat arrangement according to the invention provides that a good heat-conducting element, such as a Wärmeleitblech made of aluminum, in the N ₂ container is mounted. If this element connects the lid and bottom of the container, the temperature gradient from the bottom to the lid remains small even at a low LN ₂ level. This thermal short circuit thus makes it possible to keep the temperature of the junction between the normal conductor and the HTSC conductor at a lower level, independently of the LN ₂ level, even if the contact element is thermally coupled to the cover of the nitrogen tank (ie the connection of the terminal pole to the Cover of the nitrogen tank is set up). In the context of the invention, however, it is sufficient if the heat conducting plate dips into the liquid nitrogen.

The HTSC line part preferably consists of strip conductors. At the temperature of the transition from the normal conductor to the HTSC conductor, the critical current I _{c of} a single HTSC band conductor is very low. The number of bands is chosen according to the magnetic current and the current carrying capacity of the band at the maximum operating temperature so that the current flows lossless.

Bands of the same pole are soldered together in a stack to obtain a compact conductor.

In a preferred embodiment, a multifilament ribbon conductor with Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{x is used} with a critical temperature T _c of 110 K.

In a further advantageous variant, another HTSC with a critical temperature> 90 K, such as YBa ₂ Cu ₃ O _x (T _c = 93 K) or Bi ₂ Sr ₂ CaCu ₂ O _x (T _c = 95 K) is used ,

A preferred embodiment of the invention provides for integrating a plurality of individual and galvanically isolated supply lines (current lines) for different current loads (such as coil sections of the magnet arrangement) in a single power supply in order to charge different superconducting coils of the magnet arrangement separately. For this purpose, the warm end of the HTS part and the cold end of the metallic part of a lead to a terminal pole made of a highly conductive metal such. Pure copper, soldered. The terminal poles of the different leads are electrically isolated from each other and against the cryostat assembly and connected to a metallic contact piece (inner contact element) by an electrically insulating and good heat conducting material. This material is preferably made of aluminum nitride. In a preferred arrangement, the aluminum nitride is coated with a solderable metal film and connected by soldering to the terminal pole and the contact piece.

As a further preferred variant, each terminal pole has an electrically insulating and highly thermally conductive coating. This coating preferably consists of a diamond-like carbon layer (DLC). The pole of this variant is preferably made conical and pressed into a conical bore of the contact piece (inner contact element), whereby an electrically insulated and good heat-conducting connection is formed.

In an advantageous embodiment of this invention, the contact piece (inner contact element) is made conical and pressed during assembly of the cryostat assembly in an outer copper part (outer contact element). This compound ensures excellent heat transfer due to the high surface pressure, can be easily released and is very compact. The contact piece (inner contact element) allows through openings the flow of vaporizing helium gas from the helium container and thus a helium gas cooling of the power supply over its entire length. The outer copper part (outer contact element) is connected in vacuum via a good thermal conductivity metal with the nitrogen tank. With this arrangement according to the invention results in a thermal resistance of less than 0.5 K / W between the terminal poles and the nitrogen tank.

It is also conceivable to use such a compact contact connection when the connection poles are not cooled via a coupling to the nitrogen tank, but with another cooling source, such as a cryocooler. This would be interesting for cryogen-free magnet systems, but especially for magnet systems in which the magnet is still cooled with LHe (such as in US 2002 002 830 described) and the power supply is mounted in a suspension tube of the helium container.

The power supply according to the invention achieves a minimum heat input without current flow, which is 3 to 4 times smaller than the heat input of a metallic power supply.

The inventive arrangement allows the operation of the power supply line up to a current of about 150 A with respect to the currentless state comparable helium losses. Due to the good thermal contact with the nitrogen tank, only the nitrogen losses increase steadily with rising Current in the power supply line. The maximum current in the power supply is limited by the critical current of the HTSC conductor at the temperature setting and the magnetic field at the location of the terminal poles.

In particular for charging currents above 150A in the power supply line, the cooling at the connecting poles can be increased. In a further advantageous arrangement, the temperature of the transition metal - HTSL is monitored, for example with a temperature sensor. The monitoring or a control can be implemented in the power supply unit. If the temperature exceeds an upper threshold, heating in the helium vessel is activated to produce additional low helium loss. The additional helium loss leads to improved cooling of the power supply by passing, cold helium vapor. The heating is switched off as soon as the temperature falls below a lower threshold.

Further advantages of the invention will become apparent from the description and the drawings.

The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

Drawing and detailed description of the invention

Fig.1

einen schematischen Querschnitt durch eine erfindungsgemäße Kryostatenanordnung;

Fig.2

eine schematische Darstellung des Zugangsrohrs der erfindungsgemäßen Kryostatenanordnung;

Fig.3

eine schematische Darstellung des Bereichs um die Anschlusspole der erfindungsgemäßen Kryostatenanordnung;

Fig.4

einen schematischen Querschnitt durch das innere Kontaktelement der erfindungsgemäßen Kryostatenanordnung.

The invention is illustrated in the drawing and will be explained in more detail with reference to embodiments. Show it:

Fig.1

a schematic cross section through a cryostat arrangement according to the invention;

Fig.2

a schematic representation of the access tube of the cryostat arrangement according to the invention;

Figure 3

a schematic representation of the area around the terminal poles of the cryostat arrangement according to the invention;

Figure 4

a schematic cross section through the inner contact element of the cryostat arrangement according to the invention.

Fig.1 shows a Kryostatanordnung with an inner and outer liquid tank and a superconducting magnet coil and two suspension tubes.

Fig. 1 shows a schematic representation of a cryostat 1 with a magnet assembly 6. The cryostat 1 comprises a filled with helium liquid tank (helium tank) 2, which is connected to suspension tubes 4 with an outer jacket 9 of the cryostat 1 and in which a superconducting magnet assembly 6 is housed. The suspension tubes 4 are simultaneously access tubes 4 for power supply assemblies (see Fig. 2 ) to the magnet assembly 6. To the liquid tank 2, a further liquid tank (nitrogen tank) 3 is arranged, which contains nitrogen at about 77 K and is connected to suspension tubes 5 with the outer jacket 9 of the cryostat 1. The liquid tank 3 with nitrogen is thermal with the hanger ears (Access pipes) 4 contacted. Between the two liquid tanks 2 and 3, a radiation shield 7 is arranged, which in turn is thermally contacted with the suspension tubes 4. In the liquid tank 2, a heating resistor (heater) 11 is mounted. The liquid tank (nitrogen tank) 3 contains a heat conducting element 10, which is welded to the lid of the nitrogen tank.

Fig.2 shows a representation of the power supply tower with inserted power supply.

In at least one of the suspension pipes (access pipes) 4, a power supply is used according to the invention. The power supply comprises a metallic part 13 with a plurality of (shown here two) electrically isolated lines (from room temperature to Anschlußpol 12) and a HTSC part 14 (from Anschlußpol 12 to the liquid tank 2) with galvanically separated strip conductors or strip conductor stacks. The connection of the metallic part 13 to the HTSC part 14 is made by connecting poles 12, which are soldered to the two conductors 13, 14. The connection poles 12 are arranged within an inner contact cone (inner contact element) 16. The inner contact cone 16 is positively within an outer contact cone (outer contact element) 15, which in turn with a good heat conducting element 8, such as (here) a contacting tube or highly conductive strands of pure copper, screwed or flanged. The contacting element 8 is connected to the cold surface 17 of the liquid tank (nitrogen tank) 3.

Figure 3 shows a preferred arrangement of the inner 16 and outer 15 contact cone. When mounting the power supply, the inner cone 16 is pressed with high surface pressure against the outer cone 15. The cone angle is between 1 and 5 °. By this arrangement according to the invention can with low mounting force a detachable connection with high Surface pressure and good heat transfer are produced. The detachable connection facilitates access, for example for repair purposes.

Fig. 4 shows an illustration of a contact arrangement according to the invention.

Fig. 4 For example, Figure 6 shows a preferred arrangement of (shown here) six separate terminal poles 12 within the inner contact cone (inner contact element) 16. Between terminal poles 12 and cone 16, thin aluminum nitride (contact) plates 18 are soldered. The aluminum nitride ensures galvanic separation with high thermal conductivity. The metallic conductors are soldered into corresponding holes of the connection poles 12. In the embodiment shown, the connection poles 12 are each connected to the inner contact element 16 with two adjacent side surfaces in a corner of the opening 19 via contact elements 18 made of AIN.

This embodiment allows the compact arrangement of different and galvanically isolated lines within a standard suspension tube with an inner diameter of (here), for example, 29 mm. The open arrangement of the connection poles leaves a sufficient opening (opening) 19 for the passage of the evaporating from the liquid tank 2 helium. This allows helium gas cooling over the entire length of the power supply.

In summary, the invention describes a power supply arrangement within a cryostat arrangement, with which electrical current can be conducted from room temperature into a superconducting magnet arrangement. The power supply consists of a metallic part and a part with HTSL, which are mounted inside a suspension tube. The arrangement provides that the galvanically isolated terminal poles between the metallic part and the HTSL bands of the power supply via a removable conical positive connection via an inner and outer contact cone are connected to the nitrogen tank.

LIST OF REFERENCE NUMBERS

1

cryostat

2

helium tank

3

nitrogen tank

4.5

suspension tubes

4a, 4b

Sections hanger ear 4

6

Superconducting magnet arrangement

7

radiation shield

8th

contacting

9

Outer jacket of the cryostat

10

thermally conductive element

11

heater

12

connecting pole

13

Metallic power lines

14

HTSL tapes or stacks

15

Outer contact cone / outer contact element

16

Inner contact cone / inner contact element

17

Wall of nitrogen tank

18

insulation plates

19

Through opening

Claims (16)

1. Cryostat assembly (1) for a superconductive magnet assembly,
   with a helium tank (2) for liquid helium, wherein the superconductive magnet assembly (6) is located in the helium tank (2),
   with a nitrogen tank (3) for liquid nitrogen, wherein the nitrogen tank (3) encloses the helium tank (2),
   and with at least one access tube (4) in which a power supply assembly of the cryostat assembly is mounted through which current can flow from the room-temperature warm area of the cryostat (1) into the superconductive magnet assembly (6),
   wherein the power supply assembly comprises at least one power supply line with a normal conductor part (13) and a superconductor part (14) made of HTSL material,
   characterized in that
   a terminal (12) of the at least one power supply line, through which the normal conductor part (13) is electrically connected to the superconductor part (14), is thermally coupled with a wall of the nitrogen tank (3), wherein the thermal coupling is produced through a solid state connection of the terminal (12) to the nitrogen tank (3) via contact elements and contacting elements (15, 16, 8) of a material with good thermal conductivity having a thermal conductivity of at least 20 W/(K*m) measured at room temperature.
2. Cryostat assembly according to claim 1, characterized in that the terminal (12) is fixed to a radially inner contact element (16),
   that the inner contact element (16) is detachably fixed to a radially outer contact element (15),
   and that the outer contact element (15) is thermally coupled with the wall of the nitrogen tank (3).
3. Cryostat assembly according to claim 2, characterized in that the inner contact element (16) has a cone-shaped outer surface,
   that the outer contact element (15) has a cone-shaped inner surface, and that the cone-shaped outer surface and the cone-shaped inner surface abut in a form-fit way,
   in particular the inner contact element (16) being pressed into the outer contact element (15).
4. Cryostat assembly according to claim 2 or 3, characterized in that the outer contact element (15) is fixed to the nitrogen tank (3) with a contacting element (8) with good thermal conductivity having a thermal conductivity of at least 20W/(K*m) measured at room temperature, in particular with a contacting tube or a highly conductive strand assembly made of pure copper, in particular via a flange connection.
5. Cryostat assembly according to one of the claims 2 to 4, characterized in that two opposing sections of the access tube (4a and 4b) are fixed to the outer contact element (15).
6. Cryostat assembly according to one of the claims 2 to 5, characterized in that the inner contact element (16) has a breakthrough opening (19),
   in particular the breakthrough opening (19) being formed in such a way that helium gas evaporating from the helium tank (2) flows through the breakthrough opening (19), thus cooling the power supply assembly over its entire length.
7. Cryostat assembly according to one of the claims 2 to 6, characterized in that the terminal (12) is fixed to the inner contact element (16) via at least one contact shim (18), in particular by soldering, the at least one contact shim (18) consisting of an electrically insulating material with a high thermal conductivity of at least 20 W/(K*m) measured at room temperature.
8. Cryostat assembly according to claim 7, characterized in that the at least one contact shim (18) consists of aluminium nitride, in particular the contact shim (18) having a metal coating on both sides so that it can be soldered with a soft solder.
9. Cryostat assembly according to claim 7 or 8, characterized in that the terminal (12) has a rectangular, in particular square, cross-section, that one contact shim (18) each is fixed to two adjacent side surfaces of the terminal (12),
   that two adjacent contact shims (18) of the terminal (12) are fixed in a corner of a breakthrough opening (19) of the inner contact element (16), in particular by soldering,
   and that the side surfaces of the terminal (12) not provided with contact shims (18) are distanced from the inner contact element (16).
10. Cryostat assembly according to one of the claims 2 to 6, characterized in that the terminal (12) is cone-shaped and pressed in a corresponding cone-shaped bore of the inner contact element (16).
11. Cryostat assembly according to one of the claims 2 to 6 or 10, characterized in that the terminal (12) has an electrically insulating coating with good thermal conductivity having a thermal conductivity of at least 20 W/(K*m) measured at room temperature, in particular a coating consisting of a diamond-like carbon coating (=DLC).
12. Cryostat assembly according to one of the claims 2 to 11, characterized in that the power supply assembly has several galvanically isolated power supply lines and therefore several terminals (12) being fixed to the inner contact element (16).
13. Cryostat assembly according to one of the preceding claims, characterized in that the superconductor part (14) is formed from Bi₂Sr₂Ca₂Cu₃Ox strip conductor.
14. Cryostat assembly according to one of the claims 1 to 12, characterized in that the superconductor part (14) has a critical temperature T_c > 90 K, in particular the superconductor part (14) being formed from YBa₂Cu₃O_x or Bi₂Sr₂CaCu₂O_x.
15. Cryostat assembly according to one of the preceding claims, characterized in that the assembly comprises an element (10) with good thermal conductivity having a thermal conductivity of at least 20 W/(K*m) measured at room temperature, in the nitrogen tank (3), the element (10) with good thermal conductivity being a heat-conducting plate which connects a top cover and a bottom of the nitrogen tank (3) and is preferably made of aluminium.
16. Cryostat assembly according to one of the preceding claims, characterized in that the assembly comprises a temperature sensor for monitoring the temperature of the terminal (12), heat equipment (11) in the helium tank (2) and control equipment, the control equipment being designed to activate the heat equipment (11) when a limit temperature at the terminal (12) is exceeded.

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