GB2492645A - Hermetically sealed cryocooler sleeve to avoid loss of cryogen during superconducting magnet quench - Google Patents

Abstract

A cryostat configuration with a vacuum vessel 1 and a cryogen vessel 2 built into it, and a sleeve 8, into which a cryocooler 7 is built, wherein the upper, warm end of the sleeve is connected to the outer jacket and the lower, cold end facing the cryogen vessel is hermetically sealed by a sleeve base 9, and wherein the cryogen vessel contains a superconducting magnet configuration 3, is characterized in that the cryogen vessel is hermetically sealed except for a gas capillary 13 and filled with gaseous fluid 12 at a pressure below the vapour pressure of the liquid phase of the fluid at the corresponding operating temperature and the coldest stage of the cryocooler is connected to the heat exchanger 11 disposed inside the cryogen vessel in a manner that ensures good thermal conduction. In this way, the superconducting magnet configuration can be cooled inside the cryogen vessel without cryogenic liquid and at the same time without direct mechanical coupling to the cryocooler, wherein it is thereby possible to dispense with handling of cryogenic liquids during the operating time and outflow of cold fluid is avoided should the superconducting magnet configuration quench.

Description

Cryostat The invention concerns a cryostat configuration with a vacuum vessel and a cryogen vessel built into it, and a sleeve, into which a cryocooler is built, wherein the upper, warm end of the sleeve is connected to the outer jacket and the lower, cold end facing the cryogen vessel is hermetically sealed by a sleeve base, the cryogen vessel containing a superconducting magnet configuration.

Such an assembly is known from US 2006/022779 Al.

The invention concerns a cryogenic system for cooling a superconducting magnet configuration, for example, for applications in nuclear magnetic resonance (=NMR) spectroscopy or magnetic resonance imaging (=MRI) Conventional superconducting magnet configurations are usually cooled in a cryogen vessel with liquid helium or liquid nitrogen to keep the temperature below the critical temperature. This involves at least partially immersing the superconducting magnet configuration into the liquid cooling fluid. This results in even cooling with a very stable temperature within the cryogen vessel. Such bath-cooled systems are usually deployed, for example, for NMR spectrometers. In these systems, liquid helium is used as the cooling fluid and has to be replenished at regular intervals because the heat input into the cryogen vessel produces continuous evaporation of the cooling fluid. Fig. 2 schematically depicts such a system of prior art according to US 2006/0021355 Al, US 2002/0002830 Al, and US 2006/022779 Al, cited above.

A cryogen vessel 2 is disposed in the interior of a vacuum vessel 1. The cryogen vessel 2 is at least partially filled with a liquid fluid 4, typically, liquid helium, and contains a superconducting magnet configuration 3, which produces a magnetic field. A room temperature tube 5 enables disposition of a measuring device, which is not depicted here, in the magnetic field. At least one discharging and filling opening 6 is provided to supply and discharge the cooling fluid 4 into and out of the cryogen vessel 2.

Rising helium costs and the availability of suitable cooling machines have resulted in methods being developed to minimize the consumption of liquid helium or to dispense with liquid helium altogether. Such systems are cooled using cryocoolers. To attain temperatures of 3 to 4 Kelvin, multi-stage coolers of the Gifford-MacMahon or pulse tube type are used.

Due to constant heat input into the cryogen vessel 2, the cooling fluid 4 evaporates and can be recondensed on the cold stage of the cryocooler 7.

System constraints mean that these configurations that recondense evaporating cooling fluid contain more or less liquid in the cryogen vessel, this liquid being in contact with the superconducting magnet configuration. The cooling fluid must be introduced at least during installation of the system. One important aspect of these configurations is the vibration decoupling between the cryocooler 7 and the magnet configuration 3 and the removability of the cryocooler 7 for service work, without the magnet configuration 3 having to be discharged. This is achieved by freely disposing the cold stage of the cryocooler 7 in the evaporation phase of the cooling fluid so that it is not directly connected to the magnet configuration 3. In such a device, the cryocooler 7 is built into a sleeve 8, which is connected to the vacuum vessel 1 at the upper end and to the cryogen vessel 2 at the lower end, so that the sleeve 8 is therefore open at the bottom, toward the cryogen vessel 2, thereby allowing the evaporated cooling fluid to condense directly on the cold stage of the cryocooler 7 and flow back into the cryogen vessel 2.

In a further configuration -described, for example, in US 2006/022779 Al -the sleeve 8 with the in-built cryocooler 7 is sealed tightly at the lower end by a sleeve base 9. The sleeve 8 then forms its own space that is sealed against the cryogen vessel 2 and the vacuum vessel 1. The evaporating fluid of the cryogen vessel 2 condenses on the underside of the tightly sealed sleeve 8 and heat transfer from the condensing fluid in the cryogen vessel 2 to the cold stage of the cryocooler 7 is effected via a separating wall with good thermal conduction properties.

One disadvantage of such configurations is dependency on liquid cryogens that are used for cooling and operation of the magnet configuration. This requires special equipment for filling and procurement of the necessary storage vessels.

In superconducting magnet configurations, for example, a section of the conductor can become normally conducting because of spontaneous conductor movements due to the magnetic forces acting on the superconductor. This can propagate to the entire coil. In such a magnet quench, the magnetic energy of the coil is transformed into heat within seconds and all the liquid cooling fluid 4 evaporates very quickly and results in a pressure increase in the cryogen vessel 2 and a heavy emission of cold gas. For that reason, design measures must be taken to ensure a sufficiently large tube cross-section between the cryogen vessel 2 and the environment for the outflowing cryocooled gas. In addition, the outflowing gas usually has to be guided out of the room in which the device is installed through a separate tube because the oxygen content in the surrounding air could otherwise sink to dangerously low values or people could be injured by the cold gas. A magnet quench must therefore be protected against by appropriately complex technical safety measures.

For this reason, it is desirable to be able to dispense with cryogen liquids in the cryogen vessel 2 altogether.

The object of this invention is to improve a cryostat configuration of the type described above by the simplest and least expensive possible technical means so that the superconducting magnet configuration can be cooled inside the cryogen vessel without cryogenic liquid and at the same time without direct mechanical coupling to the cryocooler. The user of the apparatus should be able to dispense with the handling of cryogenic liquids such as helium and nitrogen for the operating duration.

A further aim of the invention is to avoid cold outflowing fluid when the superconducting magnet configuration is quenched.

This object is achieved in a surprisingly simple and yet effective way in that the cryogen vessel is hermetically sealed except for a gas capillary and is filled with gaseous fluid at a pressure that is below the vapor pressure of the liquid phase of the fluid at the corresponding operating temperature, and that the coldest stage of the cryocooler is connected to a heat exchanger disposed inside the cryogen vessel in a manner that ensures good thermal conduction.

Numerous advantages of the inventive configuration over devices according to the state of the art result from the following aspects: The cryogen vessel with the superconducting magnet configuration remains sealed throughout the entire service life and therefore no additional cooling medium is required for operation. No cooling fluid therefore needs to be replenished during the life of the system.

The superconducting magnet configuration is cooled without contact via the heat exchanger disposed in the cryogen vessel because convection flows form within the gaseous fluid to ensure good heat transfer among the heat exchanger, cryogen vessel and the magnet configuration disposed therein.

Unlike a direct mechanical connection between the cryocooler and magnetic coil, with the inventive configuration, vibration decoupling is possible, which is a precondition for use as part of a high-resolution NMR or MIII spectrometer.

The cryogen vessel can be cooled fully automatically by means of the cryocooler because no cryogenic liquids have to be supplied.

In the case of unexpectedly large heat input into the cryogen vessel, for example, during a quench of the superconducting magnet configuration or a loss of the insulation vacuum in the vacuum vessel, outflow of large quantities of cold fluid can be avoided, which is unavoidable in bath-cooled systems. In this respect, the inventive configuration increases safety for users.

In a preferred embodiment, the cryogen vessel is only accessible from the outside via a thin gas capillary with poor thermal conduction properties, such as one made of austenitic steel. This gas capillary is guided through the outer wall of the vacuum vessel via a vacuum-tight bushing and has a shutoff valve that permits hermetic closure of the gas compartment of the cryogen vessel. It is advantageous if the cryogen vessel is already filled with a defined gas pressure via this gas capillary before cooling and if the capillary is then tightly closed outside the vacuum vessel, for example, using a shutoff valve. The cryocooler is built into a hermetically sealed sleeve and the coldest stage of the cooler is in good thermal contact with the base of the sleeve. Because fluid is no longer recondensed in the cryogen vessel, the otherwise necessary recondenser is replaced with a simple large-area heat exchanger within the cryogen vessel. The base of the sleeve is then in good thermal contact with this heat exchanger.

In a further advantageous embodiment of the inventive configuration, the heat exchanger in the cryogen vessel is constituted by a helical and hermetically sealed, closed tube. This tube is filled at the highest possible filling pressure with hydrogen, helium, neon, nitrogen, or a mixture of these gases and at room temperature, before the system is cooled and then hermetically sealed. The cooling reduces the pressure in this tube according to the isochoric equilibrium pressure. This closed tube then serves as an additional thermal buffer to keep the temperature in the cryogen vessel stable.

In a further embodiment of the inventive configuration, the sleeve with the built-in cryocooler is permanently connected to an external compressed-gas canister via a connecting tube with a pressure-reducing valve. A defined gas pressure is established in the sleeve at room temperature before cooling. This gas pressure is maintained throughout cooling whereby gas is constantly replenished from the gas canister.

Otherwise, the initial gas pressure in the hermetically closed sleeve would fall continuously due to the sinking average temperature inside the sleeve. When the pressure falls below the evaporation pressure of the supplied gas, a reservoir of the liquid phase of the gas used forms in the lower part of the sleeve. This liquid improves heat transfer from the base of the sleeve to the coldest stage of the cryocooler and enables the cryocooler to be disposed completely without contact with the base of the sleeve.

A further advantageous embodiment of the inventive configuration is used, in particular, to improve the heat transfer from the cold stage of the cryocooler to the heat exchanger inside the cryogen vessel. To achieve this, the heat exchanger is connected to a thermosiphon. The thermosiphon can be constituted as a tube, the beginning and end of which are disposed in the sleeve and which is guided in a vacuum-tight manner through the wall of the sleeve and the cover of the cryogen vessel. The thermosiphon works in such a way that the liquid fluid that has formed in the sleeve flows downward in the tube of the thermosiphon and evaporates on contact with the heat exchanger and then flows back to the sleeve as a gas. With this configuration, it is possible, in particular, to minimize the temperature gradient between the cryocooler and gas heat exchanger. As a further characteristic of this embodiment, it is therefore possible to dispense with good thermal contact between the sleeve base and the heat exchanger.

Further advantages of the invention derive from the description and the drawing. The characteristics stated above and below can also be used singly or in any combination. The embodiments shown and described are not to be understood as an exhaustive list but are examples for describing the invention.

The figures show: Fig. 1 a schematic cross-section of the inventive cryostat configuration with a hermetically sealed cryogen vessel filled with a gaseous fluid, a cryocooler built into a closed sleeve, and a heat exchanger in the cryogen vessel; Fig. 2 a schematic cross-section of a prior art cryostat configuration with a cryocooler built into a closed sleeve and a recondenser for recondensing the cooling fluid in the cryogen vessel; Fig.3 a schematic cross-section of the inventive cryostat configuration according to a further embodiment with a heat exchanger in the form of a gas-filled and hermetically sealed tube coil; Fig.4 a schematic cross-section of the inventive cryostat configuration according to a further embodiment with an external gas canister for filling the sleeve; and Fig.5 a schematic cross-section of the inventive cryostat configuration according to a further embodiment with a thermosiphon between the sleeve and heat exchanger.

Instead of a fluid in the liquid state, a fluid in a gaseous state is used to cool a superconducting magnet configuration inside a cryogen vessel. The fluid is cooled to the required operating temperature by thermal contact with a cryocooler.

The cryocooler is built into a hermetically sealed sleeve and the coldest stage of the cooler is in good thermal contact with the base of the sleeve.

Because fluid is rio longer recoridensed in the cryogen vessel, the recondenser required in configurations according to prior art, is replaced with a large-surface heat exchanger. The base of the sleeve is in good thermal contact with this heat exchanger. The adjacent spaces of the sleeve for receiving the cryocooler and the cryogen vessel for receiving the superconducting magnet configuration are therefore separated from each other in a hermetically sealed manner.

In this configuration, a superconducting magnet configuration can now be cooled via the gas atmosphere inside the cryogen vessel, which, among other things, permits improved vibration decoupling because no direct connection is necessary between the cryocooler and magnet configuration. Inside the sleeve, a different fluid or different pressure can be chosen, independently of the cryogen vessel.

By choosing a large-surface heat exchanger, the superconducting magnet configuration can be cooled much more effectively by a convection flow that forms than just by thermal conduction via the cooling fluid. To ensure the necessary heat transfer coefficient, the surface area of the heat exchanger should be at least 1000cm2.

For filling or discharging the cooling fluid, the cryogen vessel is only accessible via a thin gas capillary with poor thermal conduction properties, such as one made of austenitic steel. This gas capillary is guided through the outer wall of the vacuum vessel via a vacuum-tight bushing. It is advantageous if the cryogen vessel is already filled with the cooling fluid at defined gas pressure via this gas capillary before cooling, with the capillary then being tightly closed outside the vacuum vessel, for example, using a shutoff valve. The cryogen vessel is typically filled with a pressure of lbar at room temperature before cooling. During the cooling process, the pressure in the croyogen vessel will decrease according to the isochoric equilibrium pressure and the cooling fluid chosen such that the gaseous state is retained until the final temperature. If helium gas is used as the cooling fluid and a final temperature of 4K is chosen, the pressure of, for example, ibar will fall to l3mbar at room temperature without liquid helium forming. A higher filling pressure improves the convective thermal transfer from the heat exchanger to the superconducting magnet configuration within the cryogen vessel. The initial filling pressure therefore also depends on the mechanical design of the cryogen vessel and the maximum permitted pressure. The achievable final temperature is decisively determined by the cryocooler used and is chosen such that the superconducting magnet configuration can be operated as intended.

A considerable advantage of the inventive configuration over prior art is the safety aspect in the handling of cryogenic liquids. Because the cryogen vessel is filled at room temperature and then hermetically sealed, no measures need to be taken to drain the cooling fluid in case of a magnet quench because the magnetic energy of a superconducting magnetic coil is too low to heat it above room temperature. This ensures the safety of the system throughout the entire service life, and the operator of the system never comes into contact with cryogenic liquids or gases. On complete heating of the cryogen vessel during lengthy idle periods, the pressure in the cryogen vessel returns to its initial value.

Cooling or heating of the cryogen vessel can be performed fully automatically merely by switching the cryocooler on or off.

To improve the temperature stability within the cryogen vessel, the heat exchanger can also be constituted in the form of a closed and helical tube, which also provides a large surface area for heat transfer to the cooling fluid. This tube is filled, for example, with helium, neon, or nitrogen with the highest possible filling pressure of typically 200bar at room temperature before the system is cooled and hermetically sealed.

The cooling reduces the pressure in this tube according to the isochoric equilibrium pressure. If helium gas is used and an initial pressure of 200bar at 293k is chosen, a pressure of, for example, O.8lbar at 4K will result. In these circumstances, part of the gas also evaporates in the tube. This heat exchanger constituted as a closed tube then serves as an additional thermal buffer to keep the temperature in the cryogen vessel stable. This configuration has the advantage that the heat exchanger acts as a thermal reservoir if the cryocooler fails, for example, due to a power failure, and can maintain the superconducting magnet configuration for a short time of typically one hour to avoid quenching. This is because helium, in particular, has a very high specific heat at cryogenic temperatures as compared with solid bodies, which is improved still further because part of the helium is condensed and the evaporation enthalpy can additionally be used in the event of a temperature rise.

However, this principle can also be applied to other gases.

Because the sleeve with the built-in cryocooler forms a hermetically sealed space, a fluid can be filled into it independently of the cryogen vessel. In particular, if a fluid is used as opposed to a vacuum, the cold stage of the cryocooler can be mechanically decoupled from the sleeve base because the heat transfer is ensured by the filling fluid. In this way, the vibration input from the cryocooler onto the superconducting magnet configuration can be greatly reduced still further.

To fill the sleeve, it is convenient to connect a high-pressure gas canister with the desired fluid to the warm end of the sleeve through a pressure-reducing valve. With this configuration, the pressure inside the sleeve can also be kept constant during cooling because the pressure inside the sleeve falls with falling average temperature and the gas is automatically replenished. In particular, condensation of the latter in the lower coldest part of the sleeve can be achieved by the choice of a suitable fluid. The temperature of the liquid can be set via the set equilibrium pressure, provided the cryocooler reaches this temperature. Accordingly, helium, neon, or nitrogen, for example, can be chosen as the gas. Because the sleeve becomes warmer and warmer toward the upper end, a state of equilibrium and constant filling level can arise of their own accord.

A further advantage of this configuration is the thermal reservoir that the liquid constitutes. If the cryocooler fails, the liquid slowly evaporates in the neck tube and the temperature of the cryogen vessel and the superconducting magnet configuration can thus be kept constant for a certain time.

To improve the heat transfer between the base of the sleeve and the heat exchanger within the cryogen vessel, the liquid inside the sleeve can then be guided to the heat exchanger in a closed thermosiphon tube, where it is in good thermal contact with the former. The liquid in the thermosiphon evaporates in the contact zone and is guided back into the sleeve as vapor. With this configuration, the temperatures of the liquid inside the sleeve and the heat exchanger can differ only slightly, allowing the cooling fluid inside the cryogen vessel to cool to a lower temperature.

Special embodiments of the invention are described based on Figures 1 and 3 to 5: Fig. 1 shows an embodiment of the inventive cryostat configuration, in a vacuum vessel 1, to cool a cryogen vessel 2 with a superconducting magnet configuration 3 contained therein, a cryocooler 7 is built into a sleeve 8, which is hermetically sealed at the lower end by a sleeve base 9. The sleeve base 9 is in good thermal contact with a heat exchanger 11, which is disposed inside the cryogen vessel. The sleeve base 9 may be part of the cryogen vessel; sleeve 8 and cryogen vessel 2 however form two separate spaces. The surface area of the inventive heat exchanger 11 is much larger than that of the recondenser 10 in the configuration known from prior art according to Fig. 2.

The cryogen vessel is filled with a gaseous cooling fluid 12 and only accessible via a gas capillary 13. The gas capillary 13 is guided vacuum-tightly through the wall of the vacuum vessel 1 and is used to fill the cryogen vessel 2 with the cooling fluid 12 with a defined gas pressure before cooling. The superconducting magnet configuration 3 is cooled via a convection flow departing from the heat exchanger 11. The cryostat configuration also contains a room temperature tube 5 that permits access to the magnet center, for example, for NMR applications.

Fig. 2 shows a conventional configuration according to the prior art discussed above. The cryogen vessel 2 is filled with a liquid cooling fluid 4, and a superconducting magnet configuration 3 is immersed into the liquid cooling fluid 4. Evaporating cooling fluid condenses on a recondenser 10 and drips back into the liquid. For cooling the recondenser 10, a cryocooler 7 is built into a sleeve 9 closed at the lower end.

The cryogen vessel is connected to the vacuum vessel 1 via at least one discharging and filling opening 6.

Fig. 3 shows a further embodiment of the inventive configuration in which the heat exchanger 14 in the cryogen vessel 2 is constituted by a helical and hermetically sealed tube. This helical tube 14 is filled with a suitable fluid 12 at a high gas pressure before the cryogen vessel 2 is cooled, and then hermetically sealed. The heat exchanger 14 constituted as a helical tube is in good thermal contact with the base 9 of the sleeve 8 and with the cold stage of the cryocooler 7.

Fig. 4 shows a further advantageous embodiment of the inventive configuration, in which the sleeve 8 is permanently connected to an external compressed-gas canister 17 via a gas connection line 16 through a pressure-reducing valve. A defined gas pressure is established in the sleeve 8 at room temperature before cooling. This gas pressure is maintained throughout cooling whereby gas is constantly replenished from the gas canister 17. Otherwise, the initial gas pressure in the hermetically sealed neck tube would fall continuously due to the sinking average temperature inside the sleeve 8. When the pressure falls below the vapor pressure of the supplied gas, a reservoir of the liquid phase of the gas used forms in the lower part of the sleeve. This liquid 15 improves heat transfer from the base 9 of the sleeve to the coldest stage of the cryocooler 7 and enables the cryocooler 7 to be disposed completely without contact with the base 9 of the sleeve. The temperature of the liquid 15 can be set via the set equilibrium pressure, provided the cryocooler 7 reaches this temperature. Accordingly, helium, neon, or nitrogen, or a mixture thereof, for example, can be chosen as the gas. Because the sleeve 8 becomes warmer and warmer toward the upper end, a state of equilibrium and constant filling level can arise of their own accord.

Fig. 5 shows a further advantageous embodiment of the inventive configuration according to Fig. 4, which, in particular, provides improved heat transfer from the heat exchanger 11 to the cryocooler 7. For this purpose, the heat exchanger 11 is brought into good thermal contact with a thermosiphon 18. The thermosiphon 18 can be constituted as a tube and functions in such a way that the liquid 15 flows downward from the sleeve 8 in the thermosiphon 8, evaporates on contact with the heat exchanger 11 inside the thermosiphon 18, and the vapor rises in a second tube of the thermosiphon 18 and is guided back into the sleeve 8 above the liquid level. With this configuration, it is possible to minimize, in particular, the temperature gradient between the cryocooler 7 and the heat exchanger 11. As a further characteristic of this embodiment, it is therefore possible to dispense with good thermal contact between the sleeve base 9 and the heat exchanger 11.

List of reference symbols 1. Vacuum vessel 2. Cryogen vessel 3. Magnet configuration 4. Liquid cooling fluid 5. Room temperature tube 6. Discharging and filling opening 7. Cryocooler 8. Sleeve 9. Sleeve base 10. Recondenser 11. Heat exchanger 12. Gaseous fluid in the cryogen vessel 13. Gas capillary 14. Tube heat exchanger 15. Filling fluid in the sleeve 16. Connecting tube 17. Compressed-gas canister 18. Thermosiphon

Claims (1)

1. <claim-text>Claims 1. A cryostat having a vacuum vessel with a built in cryogen vessel, and a sleeve, into which a cryocooler is built, wherein the upper, warm end of the sleeve is connected to the outer jacket and the lower, cold end facing the cryogen vessel is hermetically sealed by a sleeve base, and wherein the cryogen vessel contains a superconducting magnet wherein the cryogen vessel is hermetically sealed except for a gas capillary and is filled with gaseous fluid at a pressure below the vapor pressure of the liquid phase of the fluid at the corresponding operating temperature, and the coldest stage of the cryocooler is connected to a heat exchanger disposed inside the cryogen vessel in a manner that ensures good thermal conduction.</claim-text> <claim-text>2. A cryostat according to claim 1, wherein the heat exchanger disposed inside the cryogen vessel has a surface area of at least 1000cm2.</claim-text> <claim-text>3. A cryostat according to any one of the preceding claims, wherein the heat exchanger disposed inside the cryogen vessel comprises a helical tube, which is filled with a gas comprising helium, hydrogen, neon or nitrogen at room temperature and is then hermetically sealed.</claim-text> <claim-text>4. A cryostat according to any one of the preceding claims, wherein the sleeve is filled with a gas or gas mixture comprising helium, hydrogen, neon or nitrogen and the gas pressure is set such that liquid forms at the lower end of the sleeve.</claim-text> <claim-text>5. A cryostat according to claim 4, wherein the coldest stage of the cryocooler is mounted without contact with the sleeve and with the sleeve base.</claim-text> <claim-text>6. A cryostat according to claim 4, including a thermosiphon, mounted between the sleeve and the heat exchanger disposed inside the cryogen vessel, wherein the thermosiphon is connected to the heat exchanger in a manner that ensures good thermal conduction so that the liquid evaporates in the thermosiphon and the vapor is guided back into the sleeve above the liquid level.</claim-text> <claim-text>7. A cryostat according to any one of the preceding claims, wherein the superconducting magnet is part of an apparatus for nuclear magnetic resonance.</claim-text> <claim-text>8. A cryostat according to Claim 7, wherein the superconducting magnet configuration is part of an apparatus for magnetic resonance imaging or for magnetic resonance spectroscopy, 9. A cryostat substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.10. A method of operating a cryostat according to any one of the preceding claims, comprising filling the cryogen vessel with helium gas at Ibar at room temperature via the gas capillary before cooling and then hermetically sealing the cryogen vessel.</claim-text>