Cryostat configuration

Abstract

A cryostat configuration for keeping liquid helium comprising an outer jacket (3) surrounding a helium container (1) connected at at least two suspension tubes (2) to the outer jacket (3), wherein the helium container (1) also comprises a neck tube (5) whose upper warm end is connected to the outer jacket (3) and whose lower cold end is connected to the helium container (1) and into which a multi-stage cold head (6) of a cryocooler is installed, wherein the outer jacket (3), the helium container (1), the suspension tubes (2) and the neck tube (5) delimit an evacuated space (7), and the helium container (1) is surrounded by at least one radiation shield (4) which is connected in a heat-conducting fashion to the suspension tubes (2) and also to the neck tube (5) of the helium container (1) is characterized in that there is a direct connection (8) between the warm ends of the suspension tubes (2) and the neck tube (5) through which helium gas can flow. A cryostat configuration of this type considerably reduces or completely eliminates the heat input via the suspension tubes of an actively cryocooler-cooled NMR magnet system, as a result of which a less powerful cryocooler can be used.

Description

This application claims Paris Convention priority of DE 10 2004 037 172.5 filed Jul. 30, 2004 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a cryostat configuration for keeping liquid helium, comprising an outer jacket and a helium container installed therein, wherein the helium container is connected to the outer jacket via at least two suspension tubes, wherein the helium container also contains a neck tube whose upper warm end is connected to the jacket and whose lower cold end is connected to the helium container, the neck tube containing a multi-stage cold head of a cryocooler, wherein the outer jacket, the helium container, the suspension tubes and the neck tube delimit an evacuated space, and wherein the helium container is surrounded by at least one radiation shield which is connected in a heat-conducting fashion to the suspension tubes and to the neck tube of the helium container.

EP 0905436, EP 0905524, WO 03036207, WO 03036190, U.S. Pat. No. 5,966,944, U.S. Pat. No. 5,563,566, U.S. Pat. No. 5,613,367, U.S. Pat. No. 5,782,095, US2002/0002830, US2003/230089 e.g. describe possibilities of cooling of a superconducting magnet system having a cryocooler with no or little loss of cryogens.

The e.g. two-stage cryocooler cold head is usually installed into a separate vacuum chamber (as described e.g. in U.S. Pat. No. 5,613,367) or directly in the vacuum chamber of the cryostat (as described e.g. in U.S. Pat. No. 5,563,566) such that the first cold stage of the cold head is rigidly connected to a radiation shield and the second cold stage is connected to the helium container in a heat-conducting fashion, either directly or via a fixed, rigid or flexible thermal bridge. The overall heat input into the helium container can be compensated for by re-condensation of the helium, which is evaporated due to external heat input, on the cold contact surface in the helium container to obtain loss-free operation of the system. Disadvantageously, the connection between the second cold stage and the helium container has a thermal resistance.

One possibility to avoid this thermal resistance is to insert the cold head into a neck tube which connects the external vacuum shell (outer jacket) of the cryostat to the helium container and is correspondingly filled with helium gas, as is described e.g. in US2002/0002830. The first cold stage of the two-stage cold head is in fixed heat-conducting contact with a radiation shield and the second cold stage is freely suspended in the helium atmosphere to directly liquefy evaporated helium.

Since the cold head is surrounded by helium gas and since there is a temperature difference between the cold head and the neck tube wall or further structural elements of the neck tube, a considerable amount of heat may be transferred between the tube wall and the cold head due to thermal conduction in the gas as well as convection currents.

WO03036207 and WO03036190 therefore propose insulating the cold head tubes. Heat conduction in the helium gas column and in the neck tube wall from the top to the bottom produces further heat input into the helium container.

US2002/0002830 therefore proposes installation of a separating sleeve around the cold head, which is open at the top and at the bottom to guide a gas flow such that the gas rises on the neck tube wall thereby absorbing the heat conducted in the tube and being heated. The gas is deflected on the upper warm end and flows downwardly along the tubes of the cold head, thereby being cooled and finally being re-liquefied at the cold end of the cold head. The cryocooler thereby loses some cooling power as is e.g. disclosed in the publication “Helium liquefaction with a 4 K pulse tube cryocooler” (Cryogenics 41 (2001) 491-496).

In an arrangement of a magnet system for high-resolution nuclear magnetic resonance spectroscopy (NMR), the helium container is usually connected to the outer vacuum jacket at at least two thin-walled suspension tubes. The helium container housing the superconducting magnet is thereby mechanically fixed and the suspension tubes simultaneously provide access to the magnet as is required e.g. for charging and also for refilling liquid helium. In conventional systems without cryocooler cooling, the boil-off gas is additionally discharged via the suspension tubes thereby cooling the suspension tubes and, in the ideal case, completely compensating the heat input via the tube wall.

In contrast thereto, in a system without any loss of cryogens (i.e. which is actively cooled by a cryocooler), the entire heat conducted through the suspension tubes enters the helium container, since the tubes are not cooled due to lack of a gas flow. This amount of heat represents in many cases the main contribution of the overall heat input—depending on the tube wall thickness, number of suspension tubes, size of helium container etc.—and can require use of a more powerful cryocooler. In addition, heat enters the helium container via the neck tube which accommodates the cold head of the cryocooler.

It is therefore the object of the present invention to reduce or completely eliminate the heat input via the suspension tubes of an actively cryocooler-cooled cryostat configuration, especially of a cryostat configuration containing a superconducting magnet arrangement, and thereby permit use of a less powerful cryocooler.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention in that there is a direct connection between the warm ends of the suspension tubes and the neck tube through which the helium gas can flow.

Through the direct connection between the warm ends of the suspension tubes and the neck tube a gas flow occurs automatically which is excited and maintained by a suction-effect at the cold end of the cold head. The evaporating gas thereby cools the wall of the suspension tubes, in the ideal case, such that the heat input into the helium container through the suspension tubes is eliminated. The gas is thereby heated, exits the suspension tubes at approximately room temperature, and re-enters into the neck tube at the room temperature flange of the cold head. The gas from the different suspension tubes is advantageously combined in one line and then guided to the neck tube. Due to the downward flow in the neck tube, the gas is cooled on the tubes of the cold head or on the neck tube, and is finally liquefied at the second cold stage of the cold head, thereby closing the cycle. The suction, which maintains the flow, is generated i.a. through the phase transition from gaseous to liquid in the region of the second cold stage. The overall power of the cryocooler slightly decreases but the gain due to the reduced heat input is larger than the cooling power loss. Especially for systems with more massive suspension tubes, use of a less powerful cryocooler is therefore possible compared to the case without a circulating flow.

In a preferred embodiment, the cold head of the cryocooler has several cooling stages. Therefore very low temperatures, in particular, temperatures in a range of 4K or less are possible.

In particular, for high-resolution NMR methods, the cryocooler advantageously is a pulse tube cooler, since pulse tube coolers can be operated with extremely low vibration. Pulse tube coolers are also very reliable and require little maintenance. In principle, other cryocoolers such as e.g. Gifford-McMahon coolers can also be used.

In a particularly advantageous manner, helium can be liquefied at a temperature of 4.2 K or less at the coldest cold stage of the cold head to provide a plurality of different applications in the region of very low temperatures. The helium which is evaporated within the cryostat is liquefied at the cold stage which is freely suspended in the neck tube, and drips back into the helium container. This reduces helium loss and the number of refilling processes or permits no-loss operation if the cooling power of the cooler is sufficiently large.

In a preferred embodiment of the invention, the tubes of the cold head above the first cold stage and possibly also in the region of further cold stages are surrounded by a thermal insulation to eliminate or at least reduce undesired heat input from the neck tube into the tubes of the cold head. The tubes above the first cold stage of the cold head have temperatures between room temperature and the temperature of the first cold stage.

In a preferred embodiment of the cryostat configuration, a gap or channel is provided between the heat insulation and the neck tube wall, through which the gas can flow to provide sufficiently good thermal contact between the gas and the tube wall.

The neck tube has no mechanical support function. For this reason, the neck tube may have a thin wall and/or be designed like a bellows and be made from a material having poor thermal conductivity. In this manner, the heat input into the helium container is only small and at the same time, the transmission of vibrations via the neck tube is minimized.

In a further embodiment, a preferably electric heater is provided in the helium container or in contact therewith to keep the pressure in the helium container at a constant value above the surrounding pressure in case of surplus cooling capacity of the cryocooler. It is, however, also feasible to control the power of the cooler via its operating frequency and/or the amount of the working gas (i.e. the gas pressure) in the cooler.

In a preferred embodiment, one or more cold stages of the cold head (except for the coldest cold stage) are connected to one or more radiation shield(s) in a heat-conducting manner. The radiation shield(s) can then be directly cooled by the cold head.

In a further embodiment of the inventive cryostat configuration, the or one of the radiation shield(s) include(s) a container with liquid nitrogen to which the cold head is connected in a heat-conducting manner, wherein the cold head of the cryocooler at least partially re-liquefies the nitrogen after evaporation. The nitrogen is liquefied through thermal connection between the radiation shield and the cold head of the cryocooler. In this case, the radiation shield is not cooled directly by the cooler but indirectly via the evaporating nitrogen.

In a further development of this embodiment, a preferably electric heater is provided in the nitrogen container or in contact therewith to keep the pressure in the nitrogen container at a constant level above the surrounding pressure in case of surplus cooling capacity of the cryocooler.

In an advantageous embodiment, the connecting line between suspension tubes and neck tube has a valve to control the gas flow. The gas flow can be reduced if required, e.g. if the suction effect on the cold head is so large that the gas flow exceeds that which would be sufficient for optimum cooling of the suspension tubes.

In a further advantageous aspect, a controllable circulating pump is provided in the connecting line between the suspension tubes and the neck tube for actively controlling the cooling flow.

The advantages of the inventive cryostat configuration are utilized in a particularly favorable manner if the cryostat configuration contains a superconducting magnet arrangement, in particular, if the superconducting magnet arrangement is part of a magnetic resonance apparatus, in particular, magnetic resonance imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR).

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 view of an inventive cryostat configuration;

FIG. 2 shows a schematic view of an inventive cryostat configuration with insulated cold head tubes;

FIG. 3 shows a schematic view of an inventive cryostat configuration with a nitrogen tank;

FIG. 4 shows a schematic section of an inventive cryostat configuration with a valve which is integrated in the connecting line; and

FIG. 5 shows a schematic section of an inventive cryostat configuration with a pump which is integrated in the connecting line.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a schematic view of an inventive cryostat configuration with a helium container 1 which is connected to the outer jacket 3 via at least two suspension tubes 2. The helium container 1 is surrounded by a radiation shield 4 and moreover comprises a neck tube 5 which contains the cold head 6 of a cryocooler. The neck tube 5 only serves as a separating wall for an evacuated region 7 of the outer jacket 3 and need not bear the weight of the helium container 1. For this reason, it may be designed such that the heat input and the vibration transmission can be minimized, which is advantageously realized using a bellows. The weight of the helium container 1 and of a superconducting magnet arrangement 26 disposed in the helium container is borne by the suspension tubes 2 which are connected to the warm end 9 of the neck tube 5 via a line 8. A gas flow 10 occurs automatically and is excited and maintained by the suction effect at the cold end 11 of the cold head 6. The evaporated helium thereby cools the wall 12 of the suspension tubes 2 (ideally to that extent that the heat input into the helium container 1 via the suspension tubes 2 is eliminated), is thereby heated, exits the suspension tubes 2 at approximately room temperature, and re-enters the neck tube 5 at a room temperature flange 13 of the cold head 6. Due to the downward gas flow 10, the gas is cooled on the tubes 14 of the cold head 6 or on the neck tube 5 and is subsequently liquefied at the second cold stage 15 of the cold head 6. The cycle is thereby closed. The power of the cryocooler thereby slightly decreases, but the gain due to the reduced heat input is larger than the cooling power loss. Especially for systems with more massive suspension tubes 2, a less powerful cryocooler can be used than without circulating flow. The partial gas flows through the various suspension tubes 2 are advantageously combined in one line 8.

Since it is not important whether the returned gas flows and is cooled along the neck tube wall 18 or the tubes 14 of the cold head 6, the cold head 6 may also be provided with thermal insulation 16 to reduce heat exchange between the neck tube 5 and the tubes 14 of the cold head 6. FIG. 2 shows thermal insulation 16 between the room temperature flange 13 and the first cold stage 17 of the two-stage cold head 6. In case of cold heads with several cold stages, the tubes of further cold stages may also be thermally insulated by insulation 16. It is only important to provide a sufficiently large gap 19 between the thermal insulation 16 and the neck tube wall 18 to permit sufficiently good thermal contact between the gas and the neck tube wall 18. The neck tube wall 18 of the present invention is not cooled by a gas flowing towards the warm end. As already mentioned above, the contribution of the heat input via the neck tube wall 18 is rather small in the present case compared to the overall heat input.

Indirect cooling of the radiation shield 4—similar to a non-actively cooled system (i.e. without cryocooler)—using evaporating nitrogen is also possible (FIG. 3). In this case, the first cold stage 17 of the cold head 6 of the cryocooler must be connected to a nitrogen container 20 in a heat-conducting manner such that nitrogen which evaporates can be re-liquefied on the cold contact surface 21.

A flow impedance (e.g. a valve 22) may be integrated in the connecting line 8 (FIG. 4) to control the gas flow. The cooling flow can be actively controlled using a pump 23 (FIG. 5). The valve 22 or pump 23 may also be installed together in the connecting line 8. The partial gas-flows of the different suspension tubes 2 are preferably initially combined in a connecting line 8 before integrating a valve 22 or a pump 23.

In any case, the pressure in the helium container 1 (and possibly also in the nitrogen container 20) is advantageously kept at a constant level above the surrounding pressure. This can be realized using a heater 24 in the liquid helium (FIG. 1, FIG. 2 and FIG. 3) or using a heater in the liquid nitrogen 25 (FIG. 3).

The inventive cryostat configuration is particularly suited for cooling a magnet arrangement 26 which is part of an apparatus for magnetic resonance, in particular, magnetic resonance imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR).

The inventive cryostat configuration considerably reduces, in particular, the heat input via the suspension tubes of an actively cryocooler-cooled high-resolution NMR magnet system thereby permitting use of a less powerful cryocooler.

LIST OF REFERENCE NUMERALS

• 1 helium container
• 2 suspension tubes
• 3 outer jacket
• 4 radiation shield
• 5 neck tube
• 6 cold head of a cryocooler
• 7 evacuated chamber
• 8 line
• 9 warm end of the neck tube
• 10 gas flow
• 11 cold end of the cold head
• 12 wall of the suspension tubes
• 13 room temperature flange
• 14 tubes of the cold head
• 15 second cold stage of the cold head
• 16 thermal insulation
• 17 first cold stage of the cold head
• 18 neck tube wall
• 19 gap
• 20 nitrogen container
• 21 cold contact surface
• 22 valve
• 23 pump
• 24 heater in liquid helium
• 25 heater in liquid nitrogen
• 26 magnet arrangement

Claims

1. A cryostat configuration for keeping liquid helium, the cryostat configuration comprising:

an outer jacket;

a helium container disposed within said outer jacket;

a first suspension tube connected between said helium container and said outer jacket;

a second suspension tube connected between said helium container and said outer jacket;

a neck tube, said neck tube having a warm upper end connected to said outer jacket and a cold lower end connected to said helium container, wherein said outer jacket, said helium container, said first suspension tube, said second suspension tube, and said neck tube delimit an evacuated space;

a multi-stage cryocooler cold head disposed within said neck tube;

a radiation shield surrounding said helium container, said radiation shield being connected in a heat-conducting fashion to said first and second suspension tubes and to said neck tube; and

direct connection means disposed between a warm end of said neck tube and warm ends of said first and said second suspension tubes, said direct connection means structured and dimensioned for helium gas flow therein.

2. The cryostat configuration of claim 1, wherein said cold head of said cryocooler has several stages.

3. The cryostat configuration of claim 1, wherein said cryocooler is a pulse tube cooler.

4. The cryostat configuration of claim 1, wherein helium can be liquefied at a temperature of 4.2K or less at a coldest cold stage of said cryocooler cold head.

5. The cryostat configuration of claim 1, wherein said cold head comprises tubes surrounded with thermal insulation and disposed above a first cold stage.

6. The cryostat configuration of claim 5, wherein said tubes are disposed in a region of further cold stages.

7. The cryostat configuration of claim 5, wherein said thermal insulation and a neck tube wall define a gap or a channel through which gas can flow.

8. The cryostat configuration of claim 1, wherein said neck tube has a thin wall and is made from a material having poor thermal conductivity.

9. The cryostat configuration of claim 1, wherein said neck tube is designed like a bellows and is made from a material having poor thermal conductivity.

10. The cryostat configuration of claim 1, further comprising a heater disposed or in contact with said helium container.

11. The cryostat configuration of claim 10, wherein said heater is an electric heater.

12. The cryostat configuration of claim 1, wherein one or more cold stages of said cold head, which are not a coldest cold stage, are connected to one or more radiation shields in a heat-conducting fashion.

13. The cryostat configuration of claim 1, wherein said radiation shield comprises a container having liquid nitrogen which is connected in a heat-conducting fashion to said cold head of said cryocooler, wherein evaporated nitrogen is at least partially reliquefied by said cold head of said cryocooler.

14. The cryostat configuration of claim 13, further comprising a second heater disposed in or in contact said nitrogen container.

15. The cryostat configuration of claim 14, wherein said second heater is an electric heater.

16. The cryostat configuration of claim 1, further comprising a gas flow control valve disposed in said connection means between said first and said second suspension tubes and said neck tube.

17. The cryostat configuration of claim 1, further comprising a controllable circulating pump disposed in said connection means between said first and said second suspension tubes and said neck tube.

18. The cryostat configuration of claim 1, wherein the cryostat configuration contains a superconducting magnet arrangement.

19. The cryostat configuration of claim 18, wherein the superconducting magnet arrangement is part of an apparatus for magnetic resonance, for magnetic resonance imaging (MRI), or for nuclear magnetic resonance spectroscopy (NMR).