US20110271694A1

US20110271694A1 - Low-loss cryostat configuration

Info

Publication number: US20110271694A1
Application number: US13/067,041
Authority: US
Inventors: Marco Strobel; Gerhard Roth
Original assignee: Bruker Biospin GmbH
Current assignee: Bruker Biospin GmbH
Priority date: 2010-05-07
Filing date: 2011-05-04
Publication date: 2011-11-10
Also published as: GB201107479D0; DE102010028750B4; GB2480154B; GB2480154A; DE102010028750A1

Abstract

A cryostat configuration (10), with at least one cryostat (11), which has at least one first chamber (1) with supercooled liquid helium having a temperature of less than 4 K and at least one further chamber (2), which contains liquid helium having a temperature of approximately 4.2 K, a Joule-Thomson valve (3) being disposed in the first chamber, wherein the first chamber is separated from the further chamber by a thermally insulating barrier (4), wherein helium from the first or the further chamber expands through the Joule-Thomson valve into a pump-off pipe (13), which is in thermal contact with the helium of the first chamber and supercools the latter, and wherein the pump-off pipe is directly or indirectly in thermal contact with the further chamber during its further progression and is then connected to the inlet of a pump (14), is characterized in that the outlet of the pump and/or an outlet for evaporating helium of at least one of the cryostats is fluidically connected to the further chamber through a cryogen pipe (15), and that the cryogen pipe has a branch-off device (16), which returns a partial current of the helium located in the cryogen pipe into the further chamber. In this way, the helium consumption and therefore the operating costs are reduced while the pressure in the first chamber remains constant.

Description

This application claims Paris Convention priority of DE 10 2010 028 750.4 filed May 07, 2010 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a cryostat configuration, with at least one cryostat, which has at least one first chamber with supercooled liquid helium having a temperature of less than 4 K and at least one further chamber, which contains liquid helium at essentially atmospheric pressure having a temperature of approximately 4.2 K, wherein a Joule-Thomson valve is disposed in the first chamber, the first chamber being separated from the further chamber by a thermally insulating barrier, wherein helium from the first or the further chamber expands through the Joule-Thomson valve into a pump-off pipe, which is in thermal contact with the helium of the first chamber and supercools the latter, and wherein the pump-off pipe is directly or indirectly in thermal contact with the further chamber during its further progression and is then connected to the inlet of a pump.
Such a cryostat configuration is known from DE 40 39 365 C2 (U.S. Pat. No. 5,220,800).
Magnet systems for magnetic resonance equipment are subject to the highest achievable demands in terms of the magnetic field strengths and homogeneity.
At a resonance frequency of 600 MHz, a field strength of 14.1 T must be achieved. These high magnetic field strengths are technically best achieved using superconducting magnet coils having a superconducting short-circuiting switch.
The superconducting magnet coils only require energy during the charging phase and produce a high magnetic field in short-circuited operation for a long time after the power supply has been disconnected. Decay times until half the original field strength is reached are around 5000 years for modern superconducting magnets. This means that, in short-circuited operation over a period of hours and days, practically no change occurs in the magnetic field strength.
High stability over time is above all necessary in long-term measurements, especially in 2D and 3D measurements. This can only be achieved in superconducting short-circuited operation. The magnet coils are usually charged once and then produce a homogeneous magnetic field for many years once the power cables have been removed. In routine operation, typical intervals between helium refills of magnet systems are several months in the case of “low-loss” cryostats.
To obtain stronger homogeneous magnetic fields and a more stable superconductor, a publication by Williams et al. in “Rev. Sci. Instrum.” 52 (5), May 1981, American Institute of Physics, 649-656, proposes operating the superconducting magnet coil at a lower operating temperature than the normal temperature of liquid helium (T=4.2 K). This lower temperature is usually achieved by pumping off the liquid helium.
In the cited publication, a cryostat is proposed that has two concentric helium tanks, one nesting inside the other. The outer tank contains liquid helium at T=4.2 K under normal pressure (1 bar).
From this outer tank, a filling pipe for liquid helium leads to the inner tank so that the liquid helium can be moved from the outer to the inner tank. In the inner tank, in which the superconducting coil is located, the helium is pumped off down to a pressure of 40 mbar and thus cooled down to a temperature of 2.3 K.
One major disadvantage of this configuration is that the supercooled helium in the inner tank is under vacuum so that the electrical supply cables, especially those for charging the superconducting magnet coil, have to be routed through the cold vacuum system. This gives rise to sealing problems and also insulation problems due to the heat input into the cold vacuum reservoir through the supply cables brought in from an environment at room temperature and under normal pressure, which necessarily results in much reduced intervals between helium refills.
A further disadvantage is that no means are provided to lower the helium consumption required to operate this equipment, with the result that both enormous operating costs are incurred and only relatively short intervals between liquid helium refills are achieved, except in cases where it is in any event necessary to constantly fill the equipment with fresh helium during operation.
DE 40 39 365 C2 and U.S. Pat. No. 5,220,800 propose a system in which two temperature ranges are provided in a first and in a further chamber, wherein, in the first chamber, liquid helium, which enters from the further chamber under normal pressure and a temperature of T=4.2 K, is cooled by pumping off through a restrictor in a state of non-equilibrium.
This equalizes the pressure level in the first chamber with the pressure level in the further chamber. Because atmospheric pressure essentially prevails in the first chamber with the supercooled liquid helium, the problems associated with a vacuum bushing for the electrical supply cables to the superconducting magnetic coil do not occur.
Because of the vertical disposition of the first chamber underneath the further chamber, gravity counteracts flowback of the denser and therefore heavier supercooled helium from the lower cold reservoir into the upper warmer reservoir. This ensures defined flow conditions and there is no unwanted mixing of cold and warm helium in the upper reservoir.
A thermally insulating barrier not only prevents convection between the two chambers but, to a great extent, also heat transfer by thermal conduction from one chamber to the other. The barrier consists of two plates separated by a vacuum and consisting of a poorly thermally conducting material, such as stainless steel or plastic. The vacuum insulation prevents heat exchange between the upper and the lower reservoir.
To avoid unwanted cooling of the helium in the further chamber, an electric heating element is disposed in the further chamber usually in addition to the thermal insulation.
The vacuum is part of the single vacuum part of the cryostat, so that the barrier does not have to be separately evacuated.
In contrast to continuous tank systems, these measures cause a drastic reduction in the heat entering from the outside and are a precondition for a cryostat with low operating losses (“low loss”).
The electrical supply cables to the magnetic system and the supply cables for liquid helium are routed inside through the conduit passing through the tower or towers. This hollow conduit design results in a dual cryostat that can be used both at 4.2 K under normal pressure and in vacuum operation in the range, for example, from 1.8 K to 2.3 K.
In both operating modes, the cryostat has low-loss properties because, irrespective of the proportion of the helium flow that is pumped off and evaporates, the total enthalpy in both gas flows is essentially passed to the shield system of the cryostat.
Because the cryostat contains two chambers with helium at two different temperature levels, there are two exhaust gas flows at different pressure levels. One exhaust gas flow arises due to the helium evaporating from the further chamber at atmospheric pressure; the second exhaust gas flow is formed by the helium pumped off through the refrigerator under a pressure of approx. 40 mbar.
Depending on the operating state of the cryostat, the two exhaust gas flows have different strengths, and the exhaust gas flow from the further chamber may even cease altogether. For a low-loss cryostat, it is essential that the enthalpy contained in the exhaust gas be utilized as-completely as possible. For this purpose, it is necessary to distribute the two exhaust gas flows evenly among the various towers and the shields connected to them, whatever the strength of the two flows.
The object of this invention is to lower the helium consumption and therefore the operating costs still further as compared to prior art, while keeping the pressure in the first chamber as constant as possible.

SUMMARY OF THE INVENTION

The object is inventively achieved by fluidically connecting the outlet of the pump and/or an outlet for evaporating helium of the or of at least one of the cryostats through a cryogen pipe with the further chamber, and providing the cryogen pipe with a branch-off device, which returns a partial current of the helium located in the cryogen pipe into the further chamber.
Instead of releasing the total pumped-off helium into the atmosphere, as was previously the case, the inventive cryostat configuration leads part of the helium into the first chamber. The helium is re-condensed in the cryogen pipe, which becomes increasingly colder inside the cryostat. The thermal energy of the helium is brought into the further chamber, making a heating element superfluous.
The helium for re-condensation can originate from the same cryostat, into which the partial current is to be returned. This would be the case, for example, if only one cryostat were present. However, in a configuration having multiple cryostats, it is conceivable for the evaporated or pumped-off helium of one or more further cryostats to be input into one of the cryostats for re-condensation.
One especially preferred embodiment is characterized in that a pressure regulating device is provided, which keeps the pressure in the further chamber constant. This could be implemented, for example, using an actively or a passively regulated valve on the cryogen pipe. A constant pressure is indispensable for an even temperature distribution and especially important for highly sensitive NMR measurements.
In a further embodiment, a heating device is provided in the further chamber. Although the necessary heat input into the further chamber can only be achieved by the helium supplied to the cryostat, embodiments are conceivable in which pressure regulation is achieved by means of the evaporation rate of the helium from the further chamber.
It is advantageous if, in the embodiments stated above, the pressure regulating device sets the pressure in the further chamber to a settable target pressure that is greater than or equal to the ambient pressure of the cryostat configuration.
Alternatively, the pressure regulating device sets the pressure in the further chamber to a defined positive pressure above atmospheric pressure.
In a further embodiment, the cryogen pipe has at least one relief valve and/or at least one bursting disk. This ensures controlled pressure reduction in the event of an unexpected large increase in pressure.
An embodiment is also conceivable that is characterized in that the cryogen pipe contains a buffer vessel for the provision of an additional volume for the flowing helium. In this way, a reserve volume is constituted in case more helium has to be supplied to the cryostat. The buffer volume is also an additional means of keeping the pressure constant.
An embodiment is especially preferred in which the cryogen pipe has at least one filtering device for separating off impurities in the helium. Impurities that enter the first chamber can constitute significant heat input. Moreover, solids and frozen matter can be deposited, narrowing or even blocking pipes and valves. For that reason, the helium used must be of high purity.
It is preferred if the partial flow returned into the further chamber comprises between 20% and 80%, preferably between 25% and 60% of the total helium flow conveyed through the pump.
In a further conceivable embodiment, helium is input into the cryogen pipe from at least one further, physically separate cryostat. This embodiment is especially advantageous if multiple cryostats are installed in a place of work, such as a research institute. In this case, the evaporating helium from one cryostat can, for example, be input into another cryostat and cooled in the manner described.
A preferred embodiment is characterized in that a superconducting magnet coil is disposed in the first chamber.
In a variant of this embodiment, the cryostat configuration is part of NMR, MRI, or FTMS equipment.
In a further variant of the embodiment, the equipment comprises an ultrahigh-resolution high-field NMR spectrometer with a proton resonance frequency ≧800 MHz.
The first chamber and the further chamber can be disposed either one above the other or side by side.
Further advantages of the invention can be derived from the description and the drawing. The characteristics stated above and below can also be used individually or together in any combinations. The embodiments shown and described are not to be considered an exhaustive list but are intended as examples to explain the invention.
The invention is shown in the drawing and is explained in more detail using the example of the embodiments. The figures show:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 Embodiment of an inventive cryostat configuration with one cryostat with supercooled helium;

FIG. 2 Embodiment of an inventive cryostat configuration with one cryostat with supercooled helium and a further cryostat with helium, which are interconnected through a cryogen pipe;

FIG. 3 Embodiment of an inventive cryostat configuration with one cryostat with supercooled helium and a further cryostat with helium, which are interconnected through a cryogen pipe that leads to a condenser;

FIG. 4 Embodiment of an inventive cryostat configuration with multiple cryostats with supercooled helium and multiple further cryostats with helium, which are interconnected through a cryogen pipe;

FIG. 5 Embodiment of an inventive cryostat configuration with two cryostats with supercooled helium, which have a shared pump-off pipe, and a further cryostat with helium, wherein a cryostat with supercooled helium and the cryostat with helium are interconnected through a cryogen pipe.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an embodiment of an inventive cryostat configuration 10 with one cryostat 11 with supercooled helium. The cryostat 11 consists of a first chamber 1 with supercooled helium (temperature <4 K) and a further chamber 2 with liquid helium (temperature approx. 4.2 K), that are separated by a thermally insulating barrier 4. In the first chamber 1, a Joule-Thomson valve 3 is disposed through which the helium can expand from the further chamber 2 into the pump-off pipe 13, thus supercooling the first chamber 1. The helium is pumped off from the pump-off pipe 13 by a pump 14 and led to a cryogen pipe 15. In the embodiment depicted, the latter comprises a buffer vessel 18 to provide to the helium an additional volume that can serve as a pressure reserve and/or backflow reserve. A relief valve 6 with a bursting disk 7 prevents an excessive pressure in the cryogen pipe 15 if the pressure regulating device 17 of the branch-off device 16 fails or if the pressure cannot be kept constant for any other reason. To remove impurities through the pump 14, a filter 5 is also disposed in the cryogen pipe 15.
FIG. 2 shows a further embodiment of an inventive cryostat configuration 20. Here, the helium of a further cryostat 22, which works with liquid helium (4.2 K), evaporates into a cryogen pipe 25 constituted as a manifold, to which a buffer vessel 28 and a branch-off device 26 with a pressure regulating device 27 are also connected. The helium evaporated from the further cryostat 22 can now partially be input into the first cryostat 21 with supercooled helium, wherein the supercooling is performed by the expansion of helium in the Joule-Thomson valve 3 as shown in FIG. 1. Also for the case of the embodiment shown in FIG. 2, the helium expanded into the pump-off pipe 23 is pumped off by a pump 24. However, it is not thereby input into the cryogen pipe 25, rather released into the atmosphere.
In this way, the helium consumption of the entire cryostat configuration 20 is reduced from around 230 ml/h without helium return to around 170 ml/h.
FIG. 3 shows a further embodiment of an inventive cryostat configuration 30. In this case, the helium of a further cryostat 32, which works with liquid helium (4.2 K), evaporates into a cryogen pipe 35 constituted as a manifold, which leads to an external condenser 39 (not explicitly depicted). A buffer vessel 38 and a branch-off device 36 with a pressure regulating device 37 are also connected to the cryogen pipe 35. The helium evaporated from the further cryostat 32 can now partially be input into the first cryostat 31 with supercooled helium. The partial flow input into the first cryostat 31 now no longer has to be condensed by the condenser 39, whereby the latter is offloaded and can be rated for a smaller capacity. Also in this embodiment, the helium expended into the pump-off pipe 33 is pumped off by a pump 34 and released into the atmosphere.
FIG. 4 illustrates an embodiment of an inventive cryostat configuration 40, in which multiple further cryostats 42 are connected to a cryogen pipe 45 constituted as a manifold. A branch-off device 46 with a pressure regulating device 47 regulates the pressure in the cryogen pipe 45 and releases excess helium into the atmosphere. Part of the helium evaporated by the cryostat 42 is now supplied to the first cryostat 41 and condensed therein. The helium of the first cryostat 41 expanded into the pump-off pipe 43 is released into the atmosphere through a pump 44. The total consumption of such a configuration is thus reduced from approx. 460 ml/h without helium return to a minimum of approx. 340 ml/h.
Finally, FIG. 5 shows an embodiment of an inventive cryostat configuration 50, in which a further cryostat 52 is connected through a cryogen pipe 55 to a first cryostat 51. A branch-off device 56 with a pressure regulating device 57 controls the quantity of the helium input into the first cryostat 51. The first cryostat 51 shares the pump-off pipe 53 with a further cryostat 51′, which also works with supercooled helium. A pump 54 pumps the helium of the two cryostats 51, 51′ out of the pump-off pipe 53 into the atmosphere.

LIST OF REFERENCE SYMBOLS

1 First chamber (2 K He) 32 Cryostat (4.2 K helium)
2 Further chamber (4.2 K He) 33 Pump-off pipe
3 Joule-Thomson valve 34 Pump
4 Thermally insulating barrier 35 Cryogen pipe
5 Filter 36 Branch-off device
6 Relief valve 37 Pressure regulating device
7 Bursting disk 38 Buffer vessel
10 Cryostat configuration 39 Condenser
11 Cryostat (2K helium)
13 Pump-off pipe 40 Cryostat configuration
14 Pump 41 First cryostat (2 K helium)
15 Cryogen pipe 42 Further cryostat
16 Branch-off device 43 Pump-off pipe
17 Pressure regulating device 44 Pump
18 Buffer vessel 45 Cryogen pipe
46 Branch-off device
20 Cryostat configuration 47 Pressure regulating device
21 First cryostat (2 K helium)
22 Further cryostat 50 Cryostat configuration
23 Pump-off pipe 51 First cryostat (2 K helium)
24 Pump 51′ Further cryostat (2k helium)
25 Cryogen pipe 52 Further cryostat
26 Branch-off device 53 Pump-off pipe
27 Pressure regulating device 54 Pump
28 Buffer vessel 55 Cryogen pipe
56 Branch-off device
30 Cryostat configuration 57 Pressure regulating device
31 Cryostat (2 K helium)

Claims

1. A cryostat configuration comprising:

at least one cryostat, said cryostat having at least one first chamber disposed, structured and dimensioned to keep supercooled liquid helium at a temperature of less than 4 K, said cryostat also having at least one further chamber disposed, structured and dimensioned to keep helium under atmospheric pressure at a temperature of approximately 4.2 K, said cryostat having an outlet for evaporating helium;

a Joule-Thomson valve disposed in said first chamber;

a thermally insulating barrier disposed to separate said first chamber from said further chamber;

a pump-off pipe in thermal contact with helium of said first chamber, a further progression of said pump-off pipe being in direct or indirect thermal contact with said further chamber;

a pump, said pump having an inlet connected to said pump-off pipe to urge helium from said first or said further chamber to expand through said Joule-Thomson valve into said pump-off pipe, thereby supercooling helium in said first chamber, said pump also having an outlet;

a cryogen pipe in fluid connection between said further chamber and said outlet of said pump and/or said evaporating helium outlet of said cryostat, said cryogen pipe having a branch-off device, which returns a partial current of helium located in said cryogen pipe into said further chamber.

2. The cryostat configuration of claim 1, wherein said first chamber and said further chamber are hydrostatically connected to each other.

3. The cryostat configuration of claim 1, wherein said further chamber is disposed above said first chamber.

4. The cryostat configuration of claim 1, further comprising a pressure regulating device for keeping a constant pressure in said further chamber.

5. The cryostat configuration of claim 4, further comprising a heating device disposed in said further chamber.

6. The cryostat configuration of claim 4, wherein said pressure regulating device adjusts a pressure in said further chamber to a settable target pressure that is greater than or equal to an ambient pressure of the cryostat configuration.

7. The cryostat configuration of claim 5, wherein said pressure regulating device adjusts a pressure in said further chamber to a settable target pressure that is greater than or equal to an ambient pressure of the cryostat configuration.

8. The cryostat configuration of claim 4, wherein said pressure regulating device sets a pressure in said further chamber to a defined positive pressure above atmospheric pressure.

9. The cryostat configuration of claim 5, wherein said pressure regulating device sets a pressure in said further chamber to a defined positive pressure above atmospheric pressure.

10. The cryostat configuration of claim 1, wherein said cryogen pipe has at least one relief valve and/or at least one bursting disk.

11. The cryostat configuration of claim 1, wherein said cryogen pipe contains a buffer vessel for provision of an additional volume for helium located in said cryogen pipe.

12. The cryostat configuration of claim 1, wherein said cryogen pipe has at least one filtering device for separating off impurities in flowing helium.

13. The cryostat configuration of claim 1, wherein said partial flow returned into said further chamber comprises between 20% and 80% of a total helium flow conveyed through said pump.

14. The cryostat configuration of claim 13, wherein said partial flow returned into said further chamber comprises between 25% and 60% of a total helium flow conveyed through said pump.

15. The cryostat configuration of claim 1, wherein helium is input into said cryogen pipe from at least one further, physically separate cryostat.

16. The cryostat configuration of claim 1, further comprising a superconducting magnet coil disposed in said first chamber.

17. The cryostat configuration of claim 16, wherein the cryostat configuration is part of NMR, MRI, or FTMS equipment.

18. The cryostat configuration of claim 17, wherein the equipment comprises an ultrahigh-resolution high-field NMR spectrometer with a proton resonance frequency ≧800 MHz.