Magnet assembly with cryostat and magnet coil system, with cold reservoirs on the current leads

Abstract

A magnet assembly (1) with a cryostat (2) has a superconducting magnet coil system (3), an active cooling device (4) for the coil system, and current leads (5a, 5b) for charging the coil system. The current leads have at least one normal-conducting region (15a, 15b), wherein multiple cold reservoirs (20) are thermally coupled to the current leads along the normal-conducting region thereof, in order to absorb heat the normal-conducting region during charging of the magnet coil system. The current leads have a variable cross-sectional area B in the normal-conducting region along the extension direction thereof, wherein at least over a predominant fraction of their overall length in the normal-conducting region, the cross-sectional area B decreases from a cold end (18a, 18b) toward a warm end (19a, 19b). This provides a magnet assembly requiring reduced cooling power during charging, with less heat introduced into the magnet coil system in normal operation.

Claims

1. A magnet assembly comprising: a cryostat with a superconducting magnet coil system, an active cooling device for the magnet coil system, and current leads configured to charge the magnet coil system in the cryostat, wherein: the current leads comprise at least one normal-conducting region, multiple cold reservoirs are thermally coupled to the current leads along the normal-conducting region of the current leads, in order to absorb the heat arising in the normal-conducting region during the charging of the magnet coil system, the current leads have a variable cross-sectional area B in the normal-conducting region along an extension direction of the current leads, and at least over a predominant fraction of an overall length of the current leads in the normal-conducting region, the cross-sectional area B decreases from a cold end toward a warm end in the normal-conducting region.

2. The magnet assembly as claimed in claim 1, wherein the current leads in the normal-conducting region each have N successive subsections, with N2, and wherein the subsections each have a constant cross-sectional area Bi within a subsection, and the cross-sectional areas Bi decrease from the cold end toward the warm end.

3. The magnet assembly as claimed in claim 2, wherein different ones of the subsections are thermally coupled to different cold reservoirs.

4. The magnet assembly as claimed in claim 2, wherein pairs of the subsections comprise respective transitions and each transition of two subsections is thermally coupled to at least one respective cold reservoir.

5. The magnet assembly as claimed in claim 4, wherein the at least one cold reservoir is also thermally coupled onto the cold end of the current lead in the normal-conducting region.

6. The magnet assembly as claimed in claim 2, wherein 3N7.

7. The magnet assembly as claimed in claim 1, wherein K stages of the thermal coupling are configured along each of the current leads in the normal-conducting region, and wherein at least one cold reservoir is thermally coupled to the current leads at each stage, with K2.

8. The magnet assembly as claimed in claim 7, wherein a heavy mass Mi of cold-storing material in the at least one cold reservoir of a respective stage of the thermal coupling decreases over the stages from the cold end toward the warm end.

9. The magnet assembly as claimed in claim 7, wherein 3K7.

10. The magnet assembly as claimed in claim 1, wherein the cryostat is configured as a cryogen-free cryostat.

11. The magnet assembly as claimed in claim 1, wherein at least some of the cold reservoirs are formed as gas-tight containers, and wherein a part of the volumes of the gas-tight containers are filled with an evaporable substance.

12. The magnet assembly as claimed in claim 11, wherein the current leads extend at least partially inside the containers in the normal-conducting region.

13. The magnet assembly as claimed in claim 8, wherein at least some of the containers are thermally coupled, respectively, with a lower end via a heat conduction element to a heat sink of the active cooling device, and the boiling point of the substance contained in the containers is greater than the temperature of the heat sink.

14. The magnet assembly as claimed in claim 1, wherein at least some of the cold reservoirs are formed as metallic bodies.

15. The magnet assembly as claimed in claim 14, wherein a plurality of the cold reservoirs formed as metallic bodies are arranged spaced apart from one another in a vacuum region of the cryostat.

16. The magnet assembly as claimed in claim 1, further comprising an active auxiliary cooling device, which is thermally coupled to respective sections of the current leads in the normal-conducting region.

17. The magnet assembly as claimed in claim 16, wherein the auxiliary cooling device is furthermore thermally coupled to a radiation shield of the cryostat and/or to a vacuum container of the cryostat and/or to a temperature control device for a sample under study.

18. The magnet assembly as claimed in claim 16, wherein a lowest working temperature AT.sub.hilf of the auxiliary cooling device is higher than a lowest working temperature AT.sub.mss of the active cooling device for the magnet coil system.

19. The magnet assembly as claimed in claim 1, wherein the cross-sectional area B changes from the cold end toward the warm end by at least a factor of 3.

20. A method for operating a magnet assembly as claimed in claim 1, comprising: charging the magnet coil system via the current leads, selecting a charging current, and configuring the variable cross-sectional area B and/or the cold reservoirs such that: (i) for a thermal load WL.sub.load, which acts maximally on a coldest stage of the current leads in the normal-conducting region during the charging, and (ii) for a thermal load WL.sub.es on the coldest stage in an equilibrium state with charged magnet coil system, the following applies:
WL.sub.load5*WL.sub.es.

21. The magnet assembly as claimed in claim 20, wherein WL.sub.load2*WL.sub.es.

22. The magnet assembly as claimed in claim 1, wherein the current leads further comprise a high-temperature superconductor (HTS) region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is illustrated in the drawing and will be explained in greater detail on the basis of exemplary embodiments. In the figures:

(2) FIG. 1 shows a schematic illustration of a first embodiment of a magnet assembly according to the invention, with metallic bodies as cold reservoirs;

(3) FIG. 2 shows a schematic illustration of a second embodiment of a magnet assembly according to the invention, with containers filled with evaporable substance as cold reservoirs;

(4) FIG. 3 shows a schematic illustration of a current lead in the normal-conducting region for the invention, with subsections of constant cross-sectional area, with cold reservoirs contacting in the middle;

(5) FIG. 4 shows a schematic illustration of a current lead in the normal-conducting region for the invention, with subsections of constant cross-sectional area, with cold reservoirs at the transition of subsections;

(6) FIG. 5 shows a schematic illustration of a third embodiment of a magnet assembly according to the invention, with an auxiliary cooling device for cooling the outer radiation shield;

(7) FIG. 6 shows a schematic illustration of a fourth embodiment of a magnet assembly according to the invention, with an auxiliary cooling device for cooling the outer radiation shield and a temperature control device of a sample to be studied.

DETAILED DESCRIPTION

(8) FIG. 1 schematically shows a first embodiment of a magnet assembly 1 according to the invention. It comprises a cryostat 2, a magnet coil system 3, an active cooling device 4, and two current leads 5a, 5b here for charging the magnet coil system 3.

(9) The cryostat 3 is formed here with a vacuum container 11, an outer radiation shield 6, a middle radiation shield 7, and an inner radiation shield 8. The vacuum container 11, which simultaneously forms the outer wall of the cryostat 2, is at room temperature (approximately 20 C.). The outer radiation shield 6 is at approximately 213 K (approximately 60 C.). The middle radiation shield 7 couples to an upper cooling stage 9 of the active cooling device 4 at approximately 50 K, and the inner radiation shield 8 couples to a lower cooling stage 10 of the active cooling device at approximately 3.5 K; the latter also represents the lowest working temperature AT.sub.mss of the active cooling device 4.

(10) The magnet coil system 3, which can be short-circuited to superconduct via a switch 12 of a charging and short-circuit electric circuit 12a, is arranged in the interior of the inner radiation shield 8 in vacuum. The magnetic field generated by the magnet coil system 3 can be used, for example, for an NMR measurement in normal operation. The inner radiation shield 8 can also be formed gas-tight, and therefore to improve the thermal conductivity, for example, gaseous helium can be provided and/or contained, which does not have to be filled in the scope of operation (including charging and normal operation) and also cannot escape (cryogen-free cryostat).

(11) Alternatively to the cryogen-free cryostat, the cryostat 2 can also be designed as a cryogen-containing cryostat (not shown in greater detail in FIG. 1). In this case, a cryogenic container is provided instead of the inner radiation shield 8, which typically contains liquid cryogen (such as helium), in which the magnet coil system 3 is entirely or partially immersed. The cryogen in the cryogenic container can be refilled as needed in operation in the case of cryogen-containing cryostats, possibly even during charging.

(12) The current leads 5a, 5b lead from connections 13a, 13b on the vacuum container 11 through the cryostat 3 up to connections 14a, 14b on the charging and short-circuit electric circuit 12a. The current leads 5a, 5b each comprise for this purpose, in the embodiment shown, a normal-conducting region 15a, 15b (between vacuum container 11 and middle radiation shield 7), an HTS region 16a, 16b (between middle radiation shield 7 and inner radiation shield 8), and an LTS region (inside the inner radiation shield 8).

(13) The current leads 5a, 5b in the normal-conducting region 15a, 15b each have a cross-sectional area B which continuously decreases from the cold end 18a, 18b (close to the magnet coil system) to the warm end 19a, 19b (close to the room temperature connection), recognizable from a diameter decreasing in size upward; the cross-sectional area B is shown by way of example here approximately in the middle (along the longitudinal direction) of the current leads 5a, 5b in the normal-conducting region 15a, 15b. The cross-sectional area B decreases in the exemplary embodiment shown by a factor of approximately 3 (it can be seen that the square of the diameter is incorporated into the cross-sectional area B, wherein the diameter ratio of cold to warm is approximately 1.75 here). The cross-sectional reduction is configured here over the entire (vertical) length of the current leads 5a, 5b in the normal-conducting region 15a, 15b.

(14) Along the current leads 5a, 5b in the normal-conducting region 15a, 15b, cold reservoirs 20 are coupled thereon. The cold reservoirs 20 are formed here as metallic masses 20a. In the example shown, three stages 21, 22, 23 of the thermal coupling are configured in each case, wherein two cold reservoirs 20 (left and right) are coupled on at the same longitudinal position (the longitudinal direction extends vertically in FIG. 1) at each of the stages 21, 22, 23. The cold reservoirs 20 of the coldest stage 21 in total have a heavy mass M1 which is greater than the total heavy mass M2 of the cold reservoirs 20 of the middle stage 22, and the total heavy mass M2 of the cold reservoirs 20 of the middle stage 22 is in turn greater than the total heavy mass M3 of the cold reservoirs 20 of the warmest stage 23. The cold reservoirs 20 of the different stages 21-23, and also within the stages 21-23 here, are arranged spaced apart from one another in the vacuum region 11a of the vacuum container 11, to avoid a thermal short-circuit.

(15) At the lower, cold end 18a, 18b, the current leads 5a, 5b are coupled to the middle radiation shield 7, and therefore a certain cooling power of the upper cold stage 9 of the active cooling device 4 can be used. Moreover, the outer radiation shield 6 also contacts the current leads 5a, 5b in the normal-conducting region 15a, 15b here, between the stages 22 and 23 here; alternatively, a non-coupling feedthrough can also be provided on the outer radiation shield 6.

(16) During the charging (or discharging) of the magnet coil system 3 via the current lead lines 5a, 5b, heat arises in the current leads 5a, 5b in the normal-conducting region 15a, 15b, which the cold reservoirs 20 at least partially compensate for by heating the metallic masses 20a, whereby a heat introduction into the HTS region 16a, 16b of the current lines 5a, 5b or even into the magnet coil system 3 is reduced. The geometry of the current leads 5a, 5b expanding toward the cold end 18a, 18b in the normal-conducting region 15a, 15b reduces the ohmic heat development close to the cold end 18a, 18b, in this case and reduces a heat introduction from the room-temperature warm end 19a, 19b. The thermal load (heat flow downward) in the region of the lowermost stage 21 during the charging WL.sub.load can be limited in this case in comparison to the thermal load in the equilibrium state in normal operation WL.sub.es, and therefore WL.sub.load2*WL.sub.es. The remaining thermal load WL.sub.load can be compensated for by the active cooling device 4, and therefore the superconducting magnet coil system 3 and also the HTS region 16a, 16b of the current leads 5a, 5b do not heat up impermissibly (above the respective transition temperature).

(17) FIG. 2 shows a second embodiment of a magnet assembly 1 according to the invention, which substantially corresponds to the structural form of FIG. 1; only the essential differences will be explained hereafter.

(18) The cryostat 2 only has an outer radiation shield 6, which is coupled on the upper cooling stage 9 of the active cooling device 4, and also an inner radiation shield 8, which is coupled on the lower cooling stage 10, but not a middle radiation shield.

(19) The current leads 5a, 5b in the normal-conducting region 15a, 15b each extend here with two cylindrical subsections 25, 26, wherein the colder subsection 25 has a significantly larger cross-sectional area Bi in comparison to the cross-sectional area B.sub.2 of the warmer subsection 26.

(20) The lower subsection 25 substantially extends in a cold reservoir 20, which is formed with a gas-tight container 27 and an evaporable substance 28 contained therein. The evaporable substance 28 is provided in liquid form; some evaporable substance 28 is already evaporated in the container 27. The lower end of the container 27 is coupled via a heat conduction element 29 to the lower cooling stage 10 of the active cooling device 4.

(21) The upper subsection 26 extends substantially in a cold reservoir 20, which is formed with a gas-tight container 30 and an evaporable substance 28 contained therein. The lower end of the container 30 is coupled via a heat conduction element 29 to the upper cooling stage 9 of the active cooling device 4.

(22) The lower container 27 is significantly larger than the upper container 30, and the lower container 27 contains significantly more evaporable substance 28 (with respect to the heavy mass) than the upper container 30.

(23) During the charging (or discharging) of the magnet coil system 3 via the current lead lines 5a, 5b, heat arises in the containers 27, 30, which is at least partially compensated for by evaporating the evaporable substance 28 (which elevates the gas pressure in the containers 27, 30), whereby a heat introduction into the HTS region 16a, 16b of the current leads 5a, 5b or even into the magnet coil system 3 in the inner radiation shield 8 is reduced. In normal operation, stored heat energy can be gradually dissipated again via the heat conduction elements 29 to the cooling stages 9, 10, which act as heat sinks, and therefore the evaporated substance can recondense again. It is to be ensured in the design of the containers 27, 30 that the evaporation and re-condensing are isochoric processes, since no substance can escape from the containers 27, 30 in operation. The change of the latent heat in the event of rising pressure and rising temperature in the respective container 27, 30 has to be taken into consideration accordingly.

(24) FIG. 3 shows a current lead 5a in the normal-conducting region 15a for the invention. It comprises N=4 successive subsections 41, 42, 43, 44 here, wherein each subsection 41-44 has a separate, uniform cross-sectional area B1-B4. The cross-sectional areas B1-B4 decrease from the cold end 18a toward the warm end 19a.

(25) The different subsections 41-44 are coupled to different cold reservoirs 20, in the form of metallic bodies 20a here. The respective two coupled cold reservoirs 20 of a subsection 41-44 each contact their subsection 41-44 here approximately in the middle in relation to the vertical longitudinal extension of the current lead 5a via a short bridge element 45. The number K of the stages of thermal coupling, each formed here by the contacting of two cold reservoirs 20 at a common longitudinal position, is also 4 here, and therefore K=N=4 here. The total heavy masses Mi of the cold reservoirs 20 of the four stages of the thermal coupling decrease from the cold end 18a toward the warm end 19a.

(26) It is to be noted that to set a certain heat flow or temperature profile, the ratio Bi/Hi in the various subsections 41-44 can also be varied, with Hi: length of the subsection i, with i=1 to 4 for the subsections 41-44. The ratio Bi/Hi typically decreases from the cold end 18a toward the warm end 19a.

(27) In FIG. 4, a further current lead 5a is shown in the normal-conducting region 15a, which substantially corresponds to the structural form of FIG. 3, and therefore only the essential differences will be explained.

(28) The cold reservoirs 20 are each coupled on here at the transitions between the subsections 41-44 with short bridge elements 45, and in addition a pair of cold reservoirs 20 is coupled on at the lower, cold end 18a of the current lead 5a in the normal-conducting region 15a via bridge elements 45.

(29) The current lead 5a is integrally manufactured here from a single part, for example, as a metal plate cut to size in the corresponding shape.

(30) FIG. 5 shows a third embodiment of a magnet assembly 1 according to the invention, which substantially corresponds to the structural form of FIG. 1; only the essential differences will be explained hereafter.

(31) In addition to the active cooling device 4, an active auxiliary cooling device 50 is also provided here, which is coupled via a heat exchanger 51 on the outer radiation shield 6. The outer radiation shield 6 in turn contacts a part (a piece) of the current leads 5a, 5b in the normal-conducting region 15a, 15b, here between the stages 22, 23 of the thermal coupling. The auxiliary cooling device 50 can reach a lowest working temperature AT.sub.hilf of approximately 60 C. here.

(32) Via the auxiliary cooling device 50, a part of the thermal load arising during charging can be discharged from the current leads 5a, 5b in the normal-conducting region 15a, 15b, and therefore the active cooling device 4 is relieved. It is also possible to assist the cooling in normal operation with the auxiliary cooling device 50.

(33) FIG. 6 shows a fourth embodiment of a magnet assembly 1 according to the invention, which substantially corresponds to the structural form of FIG. 5, and therefore only the essential differences will be explained hereafter.

(34) The active auxiliary cooling device 50 not only cools the heat exchanger 51 to the outer radiation shield 6 here, but rather also a heat exchanger 52, which in turn cools a heat exchanger 53 of a temperature control device 54 for a sample 55 to be studied. The sample 55 to be studied is kept at a constant temperature during its measurement by NMR spectroscopy in a room-temperature borehole (not shown in greater detail) of the cryostat 2 by the temperature control device 54, wherein the magnetic field generated in normal operation by the magnet coil system 3 of the magnet assembly 1 is used.

LIST OF REFERENCE SIGNS

(35) 1 magnet assembly 2 cryostat 3 superconducting magnet coil system 4 active cooling device 5a, 5b current leads 6 outer radiation shield 7 middle radiation shield 8 inner radiation shield 9 upper cooling stage (heat sink) 10 lower cooling stage (heat sink) 11 vacuum container 11a vacuum region 12 superconducting switch 12a superconducting charging and short-circuit electric circuit 13a, 13b connection (on the vacuum container) 14a, 14b connection (on the charging and short-circuit electric circuit) 15a, 15b normal-conducting region 16a, 16b HTS region 17a, 17b LT S region 18a, 18b cold end 19a, 19b warm end 20 cold reservoir 20a metallic body 21 coldest stage of the thermal coupling 22 middle stage of the thermal coupling 23 warmest stage of the thermal coupling 25, 26 subsection 27 container 28 evaporable substance 29 heat conduction element 30 container 41-44 subsection 45 bridge element 50 active auxiliary cooling device 51-53 heat exchanger 54 temperature control device 55 sample B cross-sectional area B1-B4 cross-sectional area (subsection) H1-H4 length (subsection) M1-M3 heavy masses