US20100236260A1

US20100236260A1 - Undercooled horizontal cryostat configuration

Info

Publication number: US20100236260A1
Application number: US11/476,713
Authority: US
Inventors: Gerhard Roth; Marco Strobel
Original assignee: Bruker Biospin GmbH
Current assignee: Bruker Biospin GmbH
Priority date: 2005-07-08
Filing date: 2006-06-29
Publication date: 2010-09-23
Also published as: KR100843389B1; EP1742234A1; KR20070006590A; DE502005005693D1; EP1742234B1

Abstract

A cryostat configuration has a magnet coil system (2) disposed in a helium tank (1), and a horizontal room temperature bore (3) which provides access to a volume under investigation in the center of the magnet coil system (2). The helium tank (1) contains undercooled liquid helium at a temperature of less than 3.5 K, in particular of approximately 2 K, and the cryostat configuration has at least one vertical tower structure (4) on its upper side for filling in and evaporating helium. The tower structure (4) contains a container (5) with liquid helium of 4.2 K which is separated from the helium tank (1) by a thermal barrier (7), and the helium tank (1) contains an undercooling unit (9). This yields a compact cryostat configuration which achieves continuous, stable long-term operation with an undercooled high-field magnet coil.

Description

This application claims Paris Convention priority of EP 05 014 826.1 filed Jul. 8, 2005 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a cryostat configuration with a magnet coil system which is disposed in a helium tank, and a horizontal room temperature bore which provides access to a volume under investigation in the center of the magnet coil system, wherein the helium tank contains undercooled liquid helium of a temperature of less than 3.5 K, in particular approximately 2K, and wherein the cryostat configuration has at least one vertical tower structure on its upper side for filling-in and evaporation of helium.
A configuration of this type is disclosed in “Specification for an 11.74 Tesla/310 mm room temperature bore magnet system” of September 2001 by the company Magnex.
The horizontal cryostat configuration disclosed in “Specification for an 11.74 Tesla/310 mm room temperature bore magnet system” comprises one single helium tank. The helium is directly pumped therefrom in order to undercool the helium located therein. The resulting pressure, reduction in the helium tank cools the helium. Refilling of the pumped helium is realized using a two-part helium inlet valve which permits filling the helium directly into the helium tank, which is at underpressure. Cryostat configurations of this type comprising undercooled helium are required to generate high magnetic fields and improve the efficiency of the configuration.
One disadvantage of direct pumping of the helium tank is that the helium tank is permanently operated at an underpressure of approximately 30 mbar. This permanent underpressure represents a considerable risk for the system which is intended for continuous operation over many years. Air can get into the system even through the smallest of leaks, and form ice in the helium tank (water ice, N₂ice, CO₂ice etc.). The ice may deposit on the coil and impair cooling thereof, which can cause a quench.
A further risk is that helium must be filled into a system at underpressure. Helium must thereby be introduced into the helium tank via a safety valve and at the same time be cooled from 4.2 K down to the operating temperature of approximately 2 K. Operating errors can easily result in disturbances causing a magnet quench. One further disadvantage is the fact that replacement of faulty components that ensure tightness of the system (valves, sealing rings etc.) is essentially impossible during operation, since the magnet coil can be operated only at a very low temperature.
A further disadvantage is that the current must be supplied from the outside into the underpressure region for charging and discharging the magnet. This can easily lead again to operating errors having serious consequences.
Configurations which eliminate these disadvantages are disclosed in DE 40 39 332 A1 and DE 40 39 365 A1 for vertical magnets with undercooled helium, wherein two helium tanks are disposed on top of each other along the axis of the room temperature bore. The helium tanks are in contact with each other and are separated by a thermal barrier. In such a system, the upper helium tank is at normal pressure at 4.2 K. This eliminates the above-described disadvantages of vertical magnets. The magnet coil is in the lower tank surrounded by helium of approximately 2 K which is also at normal pressure, since it is hydrostatically connected to the upper tank via narrow gaps.
It is the underlying purpose of the invention to propose a horizontal cryostat configuration comprising a magnet coil system which eliminates the above-described disadvantages, has a compact construction, and can generate high magnetic fields to obtain continuous, stable, long-term operation with an undercooled high-field magnet coil.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention in that the tower structure holds a container with liquid helium at 4.2 K, which is separated from the helium tank via a thermal barrier, and an undercooling unit is provided in the helium tank.
The container located in the tower structure contains liquid helium at a temperature of 4.2 K which can be guided into the helium tank, when required. In contrast to prior art, the helium gas is not directly pumped above the helium bath in order to generate an underpressure in the helium tank, rather the helium in the helium tank is undercooled using an undercooling unit. This may e.g. be a Joule Thomson valve which undercools the helium in the helium tank through expansion of the helium.
Liquid helium at a temperature of approximately 4.2 K is located in the container of the tower structure. Transition of the cryogenic liquids is basically possible by the thermal barrier between the helium tank and the container in the tower but the heat exchange between the undercooled helium and the helium in the container is thereby minimized as is the loss of undercooled helium.
This construction completely eliminates the above-described disadvantages of direct pumping on the helium bath. This is possible through integration of the container in the tower region, such that all advantages which, up to now, were exclusively available in cryostats for vertical magnets can now also be utilized for horizontal magnets.
In a preferred embodiment of the inventive cryostat configuration, at least two radiation shields are provided between the helium tank and the room temperature region. The cryostat configuration may then be used as a high-performance cryostat.
In order to provide the system with maximum efficiency, the tower structure is advantageously formed like a dome with at least one further tower being disposed on its upper side, in which the helium evaporating from the cryostat configuration discharges its enthalpy to the radiation shields provided in the cryostat configuration.
At least two, preferably three annularly disposed additional towers are advantageously provided, with, in particular, throttles of predetermined flow cross-section for uniformly distributing the pumped helium to the towers.
Flow detectors for measuring the flow rate of the evaporating helium through the additional towers and preferably a flow device may also be provided, which automatically controls the flow rate of the evaporating helium through the additional towers.
In one particularly advantageous embodiment of the inventive cryostat configuration, an annular heat exchanger having the shape of a hollow tube is disposed in the further towers, through which the helium evaporating and/or being pumped from the cryostat configuration is guided to the outside, and to the outer side of which the radiation shields are coupled in a thermally conducting fashion. Heat input into the cryostat is thereby minimized, since the shielding system is cooled with particular efficiency due to the annular heat exchanger and the pumped helium.
In one particularly preferred embodiment of the invention, a refrigerator, in particular, a pulse tube cooler projects into the container for re-liquefying the helium. The helium evaporating from the helium bath must no longer be pumped out of the container and no fresh helium must be supplied. It may be re-liquefied within the container without losing helium. The container may be correspondingly small, since the required helium supply is smaller due to the reduced loss.
The refrigerator is preferably a two-stage refrigerator and cools at least one of the radiation shields.
A further advantage is that the helium pumped from the undercooling unit cools at least one of the radiation shields.
In a special embodiment of the inventive cryostat configuration, helium is removed from the helium lank or the container via the undercooling unit.
With particular advantage, the container is additionally connected to an external reservoir with gaseous helium, the reservoir preferably having a slight overpressure compared to atmospheric pressure. The refrigerator may suction helium from the reservoir which is again liquefied in the container, and can be further guided from that location into the helium tank to be undercooled. The slight overpressure of the reservoir compared to atmosphere prevents impurities from getting into the container.
The helium pumped via the undercooling unit is preferably pumped into the reservoir. The reservoir is thereby constantly filled. The cryostat configuration thereby forms a closed system.
In an advantageous embodiment, the external reservoir is connected to the refrigerator, such that at least part of the gas of the reservoir is directly re-liquefied by the refrigerator. The reservoir may also be connected to the upper part of the container.
The external reservoir may also exclusively be connected to the refrigerator. The reservoir may also be exclusively connected to the container.
A heating element may be provided in the container which controls the pressure in the container to prevent an excessive pressure drop in the container e.g. due to excessive liquefaction of the helium by the pulse tube cooler.
In a particular embodiment of the inventive cryostat configuration, the helium tank and the container form a divided tank, wherein the helium tank with the undercooled liquid helium is disposed below the container. The tank is thereby divided by the thermal barrier.
The barrier separating the container from the helium tank is advantageously produced from a material having poor heat conducting properties to largely prevent heat transfer from the helium in the container to the undercooled helium in the helium tank.
One particularly advantageous embodiment is characterized in that the thermal barrier consists of at least two plates which are substantially separated by a vacuum, the vacuum separating the plates preferably being part of a uniform vacuum within the cryostat configuration. Vacuum insulation prevents heat exchange between the container and the helium tank in a particularly effective manner.
In case of a quench of the magnet coil system, a large amount of energy in the form of heat is discharged from the magnet coil system to the undercooled helium bath, such that the helium in the helium tank suddenly heats up and expands. For this reason, a pressure control valve is preferably provided in the barrier which opens an increased pressure compensation cross-section in the barrier when a certain pressure difference between the helium tank and the container has been exceeded, and/or at least one wall of the container, which does not border the helium tank, has at least one rupture disc which opens a large cross-section to the outside of the cryostat configuration when a maximum container pressure has been exceeded.
In a preferred embodiment, a limited flow cross-section, in particular, a pressure compensation gap, preferably an annular gap, is provided between the helium tank and the container, through which liquid helium can flow from the container into the helium tank.
In a particularly simple embodiment, the pressure control valve consists of a preferably conical stopper having heat exchanging surfaces directed into the container and helium tank and is inserted into a likewise preferably conical seat in the barrier, which narrows towards the helium tank. During normal operation, the stopper is held in its position by its weight which is selected such that it corresponds to the maximum admissible pressure acting on the stopper.
With particular advantage, the electric feed lines required for charging a superconducting magnet coil of the magnet coil system are guided through the container before entering the helium tank, and devices are preferably provided which permit short-circuit operation of the magnet coil, wherein the electric feed lines to the magnet coil are removed after short-circuiting. The feed lines are thereby cooled by the warmer helium in the container in the tower structure before entering the helium tank containing the undercooled helium, thereby reducing the heat input via the feed lines.
In a preferred embodiment of the inventive cryostat configuration, the center of the magnet coil system, in a radial direction, does not coincide with the center of the containers surrounding the magnet coil system. The magnetic center may therefore be disposed closer to a container end. This facilitates access to the magnetic center.
In a further preferred embodiment of the inventive cryostat configuration, the center of the magnet coil system and the center of the container are disposed in different planes perpendicularly to the axis of the room temperature bore. In this case, the longitudinal axis of the coil does not coincide with the longitudinal axis of the container. This provides a larger helium supply volume over the magnet coil, while maintaining the cylindrical construction of various containers. The containers need, of course, not be circular, but may have any other arbitrary shapes.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic vertical section along the axis of the room temperature bore of an inventive cryostat configuration;

FIG. 2 shows a schematic vertical section through an inventive cryostat configuration with asymmetrically arranged magnet coil system; and

FIG. 3 shows a schematic vertical section through an inventive cryostat configuration with asymmetrically arranged magnet coil system and refrigerator without nitrogen tank.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The figures show different embodiments of an inventive cryostat configuration. A tower structure 4 with a helium container 5 is provided above a helium tank 1 in which a magnet coil system 2 is disposed about a horizontal room temperature bore 3. The container 5 (FIG. 3) is provided with a refrigerator 6, preferably a multi-stage pulse tube cooler whose coldest cold stage 10 liquefies the helium in the container 5. The container 5 of the tower structure 4 thus contains pre-cooled liquid helium of a temperature of approximately 4.2 K. In case of heat input into the container 5, the helium which is thereby evaporated may be re-liquefied using the refrigerator 6 such that evaporation of helium from the container 5 is largely prevented. In contrast to conventional devices, a large supply of liquid helium is therefore not necessary and the container 5 can be relatively small.
The tower structure 4 with container 5 is disposed radially outside the magnet coil system 2 relative to the axis of the room temperature bore 3. The container is also usually disposed proximate the edge of the cryostat configuration in an axial direction to permit easy access e.g. for maintenance work. The center of the magnet coil system and the center of the container 5 are therefore generally disposed in different planes, perpendicular to the axis of the room temperature bore. The generally central longitudinal axis of the magnet coil and the central longitudinal axis of the different containers and shields also do not coincide, but are radially offset.
The container 5 is separated from the helium tank 1 by a thermal barrier 7. The liquid helium can flow from the container 5 into the helium tank 1 via an annular gap 8, if required, where it is further cooled to less than 3.5 K using an undercooling unit 9. The undercooling unit 9 may be realized in the form of a closed cooling cycle with a separate coolant, or can pump the helium to be expanded for undercooling from the helium tank 1 or container. In order to minimize the dimensions of the cryostat configuration, the container 5 is advantageously fed via an external reservoir (not shown).
In a particularly advantageous embodiment of the invention, the helium which is pumped from the undercooling unit 9 may be guided to the reservoir. The pressure in the reservoir will thereby increase. At the same time, the helium in the container 5 of the tower structure 3 will be liquefied by the refrigerator 6, which reduces the pressure in the container 5. When the reservoir is connected to the container 5, helium gas is suctioned from the reservoir into the container 5 due to the pressure difference between the reservoir and the container 5, which is again liquefied by the refrigerator 6. This yields a closed coolant circle which ensures that the helium loss is minimized and the system is not soiled.
If the undercooling unit pumps more helium into the reservoir than is transferred from the reservoir into the container 5, an overpressure may build up in the reservoir. For this reason, the reservoir is advantageously provided with a pressure control valve. On the other hand, excessive cooling performance of the refrigerator 6 can cause a pressure drop in the reservoir. This can be counteracted either through throttling the refrigerator 6 or through heating the helium in the container 5 using a heating element disposed in the container 5.
In order to reduce the radiation energy incident on the helium tank 1, the embodiments of the inventive cryostat configurations of FIGS. 1 and 2 have radiation shields 12 a, 12 b, 12 c between the helium tank 1 and an outer shell 11, wherein the radiation shields 12 b and 12 c can be cooled by the helium pumped by the undercooling unit 9. Towards this end, further towers 14 are provided on the upper side of the tower structure 3, containing annular heat exchangers 15 in the form of hollow tubes, through which the helium evaporating from the container 5 and being pumped by the undercooling unit 9 is guided to the outside, and on the outer sides of which the radiation shields 12 b, 12 c are coupled in a thermally conducting manner. It is, however, also possible that at least one of the radiation shields 12 b, 12 c contacts the first cold stage 13 of the refrigerator 6.
The outermost radiation shield 12 c is designed as a nitrogen tank 16 for shielding against thermal radiation (FIGS. 1 and 2). The nitrogen in the nitrogen tank 16 may additionally be cooled by the first cold stage 13 of the refrigerator 6.
The thermal barrier 7 which separates the container 5 from the helium tank 1 comprises two plates 17 made from a material having poor heat conducting properties. The space between the plates 17 is evacuated to largely prevent heat transfer from the container 5 into the helium tank 1. The thermal barrier 7 contains a pressure control valve in the form of a conical stopper 18 which opens an increased pressure compensation cross-section in the thermal barrier 7 in case of a quench, such that the expanding helium can escape from the helium tank 1.
In the embodiments shown, the thermal barrier 7 is disposed in such a manner that the container 5 terminates exactly with the tower structure 3. Other arrangements are also feasible. The thermal barrier 7 may e.g. be disposed at a larger outer radial distance such that the helium tank 1 projects into the tower structure 3. The volume of the helium tank 1 is then enlarged compared to that of FIG. 1. It may, however, also be advantageous to provide the thermal barrier radially inside the tower structure 3 such that the container 5 is only partially positioned in the tower structure 3. In order to minimize the dimensions of the cryostat configuration, the magnet coil system 2 is advantageously asymmetrical relative to the outer shell 11 and the radiation shields 12 a, 12 b, 12 c of the cryostat configuration.
FIGS. 2 and 3 show cryostat configurations with asymmetrically disposed magnet coil system 2. The thermal barrier 7 is disposed on the border of the tower structure 3 such that the magnet coil system 2 of the cryostat configuration is also disposed asymmetrically relative to the helium tank 1.
The cryostat configuration of FIG. 3 comprises an additional pulse tube cooler whose first stage cools the outermost radiation shield which is designed only as a metallic radiation shield 19 and not as a nitrogen tank.
In total, one obtains a compact cryostat configuration for minimizing the helium consumption required for operating a high-performance cryostat.

LIST OF REFERENCE NUMERALS

1 helium tank
2 magnet coil system
3 room temperature bore
4 tower structure
5 container
6 refrigerator
7 thermal barrier
8 annular gap
9 undercooling unit
10 coldest cold stage of the refrigerator
11 outer shell
12 a,b,c radiation shield
13 first cold stage of the refrigerator
14 further tower
15 annular heat exchanger
16 nitrogen tank
17 plate
18 stopper
19 metallic radiation shield

Claims

1. A cryostat configuration for a magnet coil system, the cryostat configuration comprising:

a helium tank in which the magnet coil system is disposed, said helium tank having a horizontal room temperature bore providing access to a volume under investigation in a center of the magnet system, said helium tank structured to contain undercooled liquid helium at a temperature of less than 3.5 K or of approximately 2 K;

a vertical tower structure disposed on an upper side of the cryostat configuration for filling-in and for evaporation of helium;

a container structured for holding liquid helium at 4.2 K, said container disposed in said vertical tower structure;

a thermal barrier disposed to thermally separate said container from said helium tank;

an undercooling unit disposed in said helium tank; and

a radiation shield, said radiation shield having a horizontal component surrounding said helium tank and said room temperature bore, said horizontal component extending substantially cylindrically about a horizontal axis, said radiation shield also having a vertical component cooperating with an upper side of said horizontal component and extending in an upward direction to surround and enclose said container, said vertical component extending substantially cylindrically about a vertical axis.

2. The cryostat configuration of claim 1, further comprising at least two radiation shields disposed outside of said helium tank.

3. The cryostat configuration of claim 2, wherein said tower structure has at least one additional tower on an upper side thereof in which helium evaporating from the cryostat configuration discharges enthalpy to said radiation shields.

4. The cryostat configuration of claim 3, further comprising at least two or three annularly disposed additional towers and throttles with predetermined flow cross-section for uniform distribution of pumped helium to said additional towers.

5. The cryostat configuration of claim 4, further comprising flow detectors to measure a flow rate of evaporating helium through said additional towers, and a flow device to automatically control a flow rate of evaporating helium through said additional towers.

6. The cryostat configuration of claim 3, further comprising a hollow tube annular heat exchanger disposed in said additional tower, through which helium evaporating and/or being pumped out of the cryostat configuration is guided to an outside, wherein said radiation shields are thermally coupled to an outer side of said heat exchanger.

7. The cryostat configuration of claim 2, further comprising a refrigerator or a pulse tube cooler that projects into said container to re-liquefy helium.

8. The cryostat configuration of claim 7, wherein said refrigerator has two stages and cools at least one of said radiation shields.

9. The cryostat configuration of claim 2, wherein liquid helium pumped by said undercooling unit cools at least one of said radiation shields.

10. The cryostat configuration of claim 1, wherein helium is removed from said helium tank or said container via said undercooling unit.

11. The cryostat configuration of claim 7, wherein said container is connected to an external reservoir with gaseous helium, said reservoir being slightly overpressurized relative to atmospheric pressure.

12. The cryostat configuration of claim 11, wherein liquid helium pumped by said undercooling unit is pumped into said reservoir.

13. The cryostat configuration of claim 12, wherein said external reservoir is connected to said refrigerator.

14. The cryostat configuration of claim 13, wherein said external reservoir is exclusively connected to said refrigerator.

15. The cryostat configuration of claim 1, further comprising a heating element disposed in said container.

16. The cryostat configuration of claim 1, wherein said helium tank and said container define a separated tank, wherein said helium tank is disposed below said container.

17. The cryostat configuration of claim 1, wherein said thermal barrier separating said container from said helium tank consists essentially of a material having poor heat conducting properties.

18. The cryostat configuration of claim 1, wherein said thermal barrier comprises of at least two plates which are substantially separated by a vacuum, said vacuum being part of a uniform vacuum within the cryostat configuration.

19. The cryostat configuration of claim 1, further comprising a pressure control means disposed in said thermal barrier to open an increased pressure compensation cross-section in said thermal barrier when a certain pressure difference between said helium tank and said container has been exceeded and/or further comprising at least one rupture disc disposed in at least one wall of said container which does not border the helium tank, said disc opening a large cross-section to an outside of the cryostat configuration when a maximum pressure in said container has been exceeded.

20. The cryostat configuration of claim 1, wherein a limited flow cross-section, a pressure compensation gap, or an annular gap is defined between said helium tank and said container, through which liquid helium can flow from said container into said helium tank.

21. The cryostat configuration of claim 19, wherein said pressure control valve comprises a stopper or a conical stopper having heat exchanging surfaces directed into said container and said helium tank, wherein said stopper is inserted into a seat in said thermal barrier, said seat having a shape congruent to said stopper.

22. The cryostat configuration of claim 1, wherein electric feed lines required for charging a superconducting magnet coil of the magnet coil system are initially guided through said container before entering said helium tank, and further comprising devices for short-circuit operation of the magnet coil, wherein said electric feed lines to the magnet coil are removed after short-circuiting.

23. The cryostat configuration of claim 1, wherein a center of the magnet coil system, in a radial direction, does not coincide with a center of said container surrounding the magnet coil system.

24. The cryostat configuration of claim 1, wherein a center of the magnet coil system and a center of said container are disposed in different planes, perpendicular to an axis of the room temperature bore.