GB2459104A

GB2459104A - Cryostat comprising a thermal sink located within an outer vacuum chamber

Info

Publication number: GB2459104A
Application number: GB0806430A
Authority: GB
Inventors: Neil Charles Tigwell; Stephen Paul Trowell; Andrew Farquhar Atkins
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-04-09
Filing date: 2008-04-09
Publication date: 2009-10-14
Anticipated expiration: 2028-04-09
Also published as: GB0806430D0; GB2459104B

Abstract

The invention relates to a method of cooling equipment, which can include superconducting magnet coils, within a cryostat (1, fig.1) comprising a cryogen vessel 2, a thermal shield 5 and an outer vacuum chamber 3. The method involves applying an initial vacuum to the outer vacuum chamber (OVC) and supplying cooling by a thermal sink (39, fig.11; 45), located within the outer vacuum chamber, to bring the vacuum to a predetermined vacuum pressure. Preferably, the thermal sink comprises a heat exchanger or heat absorbing structure, where cooling is supplied to the heat exchanger from a stream of helium gas. The thermal sink may comprise a heat exchanger with an integrated adsorption extraction unit 46, such as an activated getter or a high surface area charcoal device, for removing residual gases. Furthermore, the heat exchanger may be in thermal contact with the thermal shield. Preferably, the predetermined vacuum pressure is approximately one thousandth of a millibar (10 -3 mbar). In use, by condensing gas and vapours onto the cold surfaces of the thermal sink, the outer vacuum chamber is pumped down and further evacuated by a mechanism known as cryopumping.

Description

CRYOSTAT

This invention relates to a cryostat and a method of cooling equipment within a cryostat, in particular for cooling superconducting magnets for magnetic resonance imaging (MRI) applications.

In order to reduce convective and conducted heat loads, a vacuum is generated inside a cryostat, in an outer vacuum chamber (OVC) surrounding a cryogen vessel.

Currently, this vacuum is generated using a roughing pump followed by a turbo-molecular pump, with the remainder cryopumped onto a cryogen vessel surface by the addition of cryogens to the cryogen vessel. During this process, the OVC vacuum is deliberately softened with nitrogen gas to enable convection currents to cool the shields, and is then repumped.

The total heat load, and hence recondensing margin, varies significantly between magnet builds. Such inconsistencies can be attributed in part to the variable quality of final vacuum in the OVC. Furthermore, evacuating the OVC takes at least 48 hours by this method -up to 10% of the total assembly time.

In accordance with a first aspect of the present invention, a method of cooling equipment within a ciyostat comprising a cryogen vessel, a thermal shield and an outer vacuum chamber; comprises applying an initial vacuum to the outer vacuum chamber; and supplying cooling by a thermal sink located within the outer vacuum chamber to bring the vacuum to a predetermined vacuum pressure.

The present invention uses a thermal sink located in the outer vacuum chamber, outside the cryogen vessel to produce a reliable vacuum and repeatable cryogenic environment, particularly in magnet build, in a reduced timescale as compared with conventiona' methods of cooling. The pump down time and cost are reduced and the method lends itself to automation.

Preferably, the thermal sink comprises a heat exchanger, or heat absorbing structure.

In one embodiment, preferably the thermal sink comprises a heat exchanger and the cooling is supplied to the heat exchanger from a stream of helium gas.

Preferably, the predetermined vacuum pressure is of the order of i0 mbar.

Preferably, the method further comprises cooling the thermal shield, or equipment.

Preferably, the cooling comprises applying helium gas to the thermal shield or equipment.

Alternatively, direct shield cooling is applied using a retractable cold finger in direct thermal contact with the thermal shield.

Cooling the shield directly whilst applying the vacuum to the outer vacuum chamber enables the step of softening the outer vacuum chamber to be avoided, saving overall process time.

Preferably, the thermal sink comprises a heat exchanger and integral with the heat exchanger is provided an extraction unit for extracting residual gases present between the outer vacuum chamber and the thermal shield.

Preferably, the heat exchanger is in thermal contact with the thermal shield.

Preferably, the integral extraction unit comprises activated getters or a high surface area charcoal device.

The presence of the extraction unit integrated with the heat exchanger allows the shield to be cooled before cooling the cryostat, without causing contamination and impaired thermal performance due to the shield adsorbing residual gases.

Preferably, the method further comprises activating a refrigerator to cool the equipment to an operating temperature.

The refrigerator can be activated since the cryogen vessel is flushed with cryogen gas, e.g. helium, so there is no risk of contamination.

In one embodiment, the method further comprises removing the thermal sink from the outer vacuum chamber and allowing surfaces of the thermal sink to rewarm after a predetermined vacuum level is reached, such that adsorbed gas is released from the thermal sink.

Preferably, the method comprises applying further cooling to the equipment prior to filling the cryogen vessel with liquid cryogen.

Preferably, in transit, the cryostat is cooled by boiling cryogen.

In accordance with a second aspect of the present invention, a superconducting magnet system comprises a cryogen vessel, magnet coils, a thermal shield and an outer vacuum chamber; wherein the outer vacuum chamber is pumped down by supplying cooling to a thermal sink located within the outer vacuum chamber, outside the cryogen vessel.

Preferably, a cryogen gas is supplied to cool the thermal sink.

Preferably, the thermal shield further comprises an integrated heat exchanger and an extraction unit.

The extraction unit prevents the shield and insulation from adsorbing residual gases.

Preferably, the system further comprises a closed loop mechanical cooler for cooling the shield.

Cooling the shield with a Stirling cooler or equivalent reduces process time and helium costs.

Preferably, in use, additional shield cooling comprising a thermal sink, thermally linked to the thermal shield, is applied to oppose shield heating due to eddy currents.

An example of a magnet system and method of manufacture will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a typical magnet system to which the method of the present invention is applied; Figure 2a illustrates in more detail a section of the outer vacuum chamber of Fig. 1 at room temp before pump-out; Figure 2b illustrates the addition of a vacuum isolation valve to the section of Fig.1, still at room temp before pump-out; Figure 3 illustrates the section of Fig. 2 with tooling being added; Figure 4 illustrates the outer vacuum chamber being evacuated using a mechanical roughing pump; Figure 5 illustrates the steps of cooling the heat sink and shield and evacuating the OVC; Figure 6 illustrates the section once the outer vacuum chamber has been evacuated and the vacuum isolation valve closed; Figure 7 illustrates the step of removing the tooling; Figure 8 shows drop-off port fitment to the section; Figure 9 shows the drop-off port fitted to the section; Figure 10 shows the section when the process is complete; Figure 11 illustrates an alternative configuration for direct shield cooling using the method of the present invention; Figure 12 illustrates a detailed example of an embodiment of the present invention, showing gas flow for shield cooling; Figure 13 illustrates the embodiment of Fig. 12, showing gas flow for magnet pre-cooling; Figure 14 illustrates the embodiment of Fig.12 showing gas flow for ramping and filling; Figure 15 illustrates the embodiment of Fig. 12 showing gas flow in operation and in transit; and, Figures 16a and 16b illustrates temperature contours in a cryogen vessel under shipping conditions for turret cooling only (95K) and with additional cooling from a heat exchanger (63 K), respectively.

Figure 17 is a schematic timeline for the conventional manufacturing process and the process of the present invention.

Figure 1 shows a cross-section of a conventional cryostat 1 for use in an MRI system. A cooled superconductive magnet (not shown) is provided within a cryogen vessel 2, which is retained within an outer vacuum chamber 3. Coils of the magnet are immersed in a liquid cryogen 4 in the cryogen vessel 2. One or more thermal shields 5 are provided in the vacuum space between the cryogen vessel 2 and the outer vacuum chamber 3. A central bore 6 is provided, to accommodate a patient for examination.

An access neck 7 is provided to allow access to the cryogen vessel 2.

In this example, a two-stage cryogenic refrigerator 8 is removably connected to a cryogen vessel 2 by an interface sock 9 (also known as an interface sleeve), such that a first stage of the refrigerator cools the thermal radiation shield 5 and the second stage cools the cryogen vessel 2. A heat exchanger, cooled by the second stage of the refrigerator 8, is exposed to the interior of the cryogen vessel 2, for example by a tube 10. The refrigerator is located in a turret provided for the purpose, towards the side of the cryostat. Alternatively, the refrigerator may be located within an access turret 11 which retains the access neck 7, mounted at the top of the cryostat. The refrigerator provides active refrigeration to cool the cryogen within the cryogen vessel. The refrigerator is preferably arranged as a recondensing refrigerator, recondensing cryogen gas to a fluid. The refrigerator is, in operation, thereby enabled to reduce the consumption of clyogenic liquid by recondensation of boiled off cryogen back into its liquid state.

As described above, the conventional process for manufacturing of a magnet system of this type involves a number of pumping stages and a requirement to soften the vacuum in the outer vacuum chamber before re-pumping, before pre-cooling the magnet to operating temperature. As there is considerable variability in the total heat load and recondensing margin during the process, this leads to quality inconsistencies in the manufactured magnet system and can be quite time consuming.

The present invention improves the process of magnet manufacture via both vacuum quality and the time to achieve evacuation, as well as avoiding the requirement to soften the vacuum during pump down. With existing methods, the time each system spends on pump varies. The gas adsorbed onto the superinsulation surface varies as a function of the helium background and humidity at the time of assembly. The end result is that a variable volume of gas is finally adsorbed onto the cold surfaces inside the cryostat, leading to differences in emissivity performance. The invention also provides a more efficient method of cooling when a cryostat needs refilling with cryogen in service.

The figures illustrate the steps involved in the cooling process according to the present invention, showing a section of the outer vacuum chamber (OVC). Starting at room temperature. in Fig. 2a, a section 20 of the OVC 3 is configured with a relatively large access port 21, e.g. of diameter approximately 8 inches (approximately 20cm).

Optional shield cooling connections 22 are in place either side of the port 21. In Fig. 2b, a vacuum isolation valve 23 is added. In Fig. 3, tooling 24 is added which covers this port 21. The tooling includes a service vacuum chamber 25, copper plates 26, a pump out port 27 and leak check port 28. Cryogen gas lines 29, in this example helium, from outside the service vacuum chamber 25 connect to the shield cooling connections 22. As shown in Fig. 4, initial evacuation of the service vacuum chamber and the OVC 3 is carried out with a roughing pump, the flow 30 passing through the pump out port 27.

In Fig. 5, the mechanical roughing pump is isolated and cold helium gas at -20 K is introduced to cool the surface of a thermal sink (Fig. 13). By condensing gas and vapours onto the cold surfaces, the service vacuum chamber and OVC volume are evacuated 33 in a mechanism known as cryopumping. The warmed helium gas 32 returns from the OVC after cooling the thermal sink. Once a reasonable vacuum has been achieved, of order mbar, cold gas can also be introduced to the shield cooling circuit 31, 32 to cool the 50K shield, and magnet As shown in Figs. 6 and 7, once the required OVC vacuum has been achieved, the OVC port 21 is valved off by closing vacuum valve 23, the helium supply 32 is stopped and the service vacuum chamber let up to atmosphere 34. The tooling 24 is removed from the OVC (Fig, 7), including the cold pumping surfaces which are allowed to warm, releasing the adsorbed gas.

Finally, a drop-off port can be fitted using a port fitting chamber 35, as for conventional systems. The port fitting chamber 35 is evacuated 36 (Fig.8). After the port is fitted (Fig. 9), the port fitting chamber is removed and the assembly finished (Fig. 10).

In an alternative arrangement, shown in Fig. 11, negating the requirement for pipes cooling the shield, a retractable cold finger 39 is in direct contact with the shield 5. Positive engagement on the shield is ensured by the use of resilient bellows 40. This has the further advantage that with the addition of slots 41 cut in the side of an extending section 42, the vacuum space behind the shield can be pumped effectively.

Optionally, a thermal braid 54 is provided between the extending section 42 and the shield 5.

The arrangement and gas flow paths for shield and magnet cooling, ramping/filling, and in-service imagingishipping are illustrated in more detail in Figs. 12 to 15.

As shown in Fig. 12, a heat exchanger 45, mounted in good thermal contact with the shield 5, has an integrated adsorber unit (sorb) 46. Typically, the sorb unit comprises activated getters, or a high surface area charcoal device. As the shield S cools, the sorb unit 46 is the coldest part initially and so residual gases, within the vacuum space between the OVC and the helium vessel, cryopump in a controlled manner, so that the gases are condensed and immobilised on the sorb surface. If the sorb was mounted conventionally to the helium vessel 2 and the shield 5 was cooled before the helium vessel, then the shield and superinsulation 45 would adsorb residual gases, leading to possible contamination and impaired thermal performance. The shied is typically cooled to 50K.

The heat exchanger 45 mounted in good thermal contact with the shield 5 has an inlet pipe 48 with long thermal length (e.g. thin wall tube of poor thermal conduction) to the OVC 3. Ideally, to minimise heat load, this inlet pipe 48 should meander between layers of the superinsulation 47. The outlet pipe 49 is of similarly poor thermal conductivity and extends into the helium vessel 2. Suitably constrained convoluted bellows (not shown) may be used for all, or parts of, the inlet and outlet tubes 48, 49 to extend the effective thermal length of the tube, whilst minimising the overall length and allowing for differential movement between shield 5, helium vessel 2 and OVC 3. The heat exchanger 45 may be formed from an extruded aluminium omega section welded to the shield 5, connected to the inlet/outlet pipes 48, 49 by friction welds.

Cold helium gas, output 50 from a mechanical cooler flows through the pipes 48, 49 in the direction of the arrows. A two-way solenoid valve 51, as used in conventional venting, controls the gas flow in each case. With vacuum in the OVC 3, refrigerated cryogen gas 50 at 20 K cools the shield 5 then passes through the helium vessel 2 and out through turret 52, as shown by the arrows, partially cooling the magnet and flushing the helium vessel. Cooling the shields directly, whilst pumping the OVC, negates the need to soften the vacuum within the OVC, saving process time. The cryogenic refrigerator 8 can also be activated since the helium vessel is flushed with helium gas and so there is no risk of contamination.

A vacuum of i0 mbar is required before cooling the shields since when the cryogenic refrigerator fails on site and the shield warm up, then it is essential that the outgassing from the shield is not excessive, and can be recovered during the re-cooling process. The sorb may comprise activated getters, or high surface area charcoal to address this problem, if appropriate.

The magnet is then cooled further using a gas-cooled method (e.g. closed-loop mechanical cooling) and the cryogen vessel is filled with liquid helium. A reduction in process time and helium costs for magnet precool is achieved by cooling the shields with a closed-loop mechanical cooler, such as a Stirling cooler, or equivalent.

With shield temperatures < 75 K, the system is able to recondense immediately, eliminating the helium loss currently experienced (200 litres). This also represents a significant time saving (the current process calls for a 3-5 day wait after helium fill before the recondensing margin can be measured).

The method described is clearly suitable for automation, leading to further gains in process costs and repeatability. A repeatable cryogenic environment enables reduction in Cost of other cryostat Components (e.g. suspension, helium vessel wrapping, or multi-layer insulation 47).

Fig. 13 shows how with cold shields (at 50K) and OVC vacuum, the magnet is precooled in an established way for a Stirling process, coolant flowing in through the auxiliary vent 53 and out through the turret 52 and two way solenoid valve 51. As shown in Fig. 14, for ramping and filling, helium vessel gas release is as for standard systems, though the turret 52, with additional venting via the tubes 48, 49 cooling the shield is possible.

In operation and when shipped, helium gas from the vessel is diverted exclusively via the shield heat exchanger to cool the shields, flowing through the two way solenoid valve 51. A clear advantage for the shipping case where conventionally, boil off is of the order 50 litres/day (or 125000 J/day available from the helium gas enthalpy).

Heat load calculations show that the thermal loads from pipes 48, 49 when made from a 1 mm thick, 10 mm outer diameter stainless steel tube, of effective length cm are: From 300 Kto 50 K: 166mW From5OKto4K: 7.6mW The thermal conduction through static helium gas in such a tube is between 50K and4K<lmW.

Convection in the tube should be minimised to ensure the total heat load to 4K <10mW.

Analysis of the improvement in time to dry has been modelled. Fig. 16a iBustrates temperature distribution using conventional turret cooling only, where the maximum temperature of-i 77C (95K) is found in area a. This reduces through areas b: -i88C (85K), c: -i98C (75K); d: -208C (64K); e: -218 C (54K); f: -229C (44K); g: -239C (34K); h: -249C (23K) to the very small region of area i: reducing from -260C (13K) to a minimum temperature of -270 C (3K). The benefit of the additional cooling provided by the thermal sink of the present invention can be seen, in that the maximum temperature is now A: -210C (63K); B: -217C (56K); C: -223C (49K); D: -230C (43K); E: -237C (36K); F: -243C (29K); G: -250C (23K); H: -257C (16K) to the somewhat larger area of I: reducing from -263C (9K) to a minimum temperature of - 270C (3K). Under shipping conditions, the addition of a heat exchanger 1 m long of 10 mm diameter welded to the shield yields 68W of cooling power and reduces the maximum equilibrium temperature of the shield from 95K to 63K. For shields at 70K, boil-off is estimated to be 26 litres per day. This corresponds to a time to dry of 53 days, or an improvement by a factor of 2 over current designs.

An assembly as described above could be manufactured without the direct shield cooling pipes, and rely solely on the cold head activation to cool the shield.

However, the preferred embodiment includes the shield cooling pipes as these offer significant advantages for shipping and imaging conditions.

In summary, the process improvement, in terms of timescale, is highlighted in Fig. 17. Conventionally, the steps are to fit pumps 55, rough pump 56 the vacuum chamber, turbo pump 57 the vacuum chamber, soften 58 the vacuum within the outer vacuum chamber, re-pump 59 the outer vacuum chamber at 77K and precool 60 the magnet to 4.2K. In the present invention, tooling is fitted 61 and the chamber rough pumped 62. Then the chamber is further evacuated 63 by condensing gas and vapours onto the cold surface of a thermal sink. The cold head is started 64 and the magnet precooled 65 to 4.2K. By reducing the number of steps and increasing the effectiveness of those which are carried out, the time taken to bring the magnet down from room temperature to operating temperature can be seen to be significantly reduced.

An assembly according to the present invention offers significant improvements in pump down time, costs and potential for automation, as well as the process providing improved repeatability of the cryogenic environment between magnet builds. The system reduces heat loads on the helium vessel during shipping by making more effective use of the boil-off helium gas for cooling. Gains by the same method are achieved when ramping and also when gradient induced heating occurs during imaging.

Additional cooling of the shield is provided according to the present invention to oppose shield heating due to eddy currents in imaging, The equipment which can be cooled using a cooling method according to the present invention may be magnets for MRI systems, magnets for other types of system, such as particle accelerators, or nuclear magnetic spectroscopy, or equipment other than magnets which needs to be kept at low temperatures when operating. The specific examples described above refer to a helium cryogen, but other cryogens could be used, such as nitrogen, hydrogen, neon or similar types.

Claims

CLAIMS1. A method of cooling equipment within a clyostat comprising a cryogen vessel, a thermal shield and an outer vacuum chamber; the method comprising applying an initial vacuum to the outer vacuum chamber; and supplying cooling by a thermal sink located within the outer vacuum chamber to bring the vacuum to a predetermined vacuum pressure.
2. A method according to claim 1, wherein the thermal sink comprises a heat exchanger, or heat absorbing structure.
3. A method according to claim 2, wherein the thermal sink comprises a heat exchanger and the cooling is supplied to the heat exchanger from a stream of helium gas.
4. A method according to any preceding claim, wherein the predetermined vacuum pressure is at least of the order of 1 0 mbar.
5. A method according to any preceding claim, wherein the method further comprises cooling the thermal shield, or equipment.
6. A method according to claim 5, wherein the cooling comprises applying helium gas to the thermal shield or equipment.
7. A method according to claim 5, wherein direct shield cooling is applied using a retractable cold finger in direct thermal contact with the thermal shield.
8. A method according to at least claim 2, wherein the thermal sink comprises a heat exchanger and integral with the heat exchanger is provided an extraction unit for extracting residual gases present between the vacuum chamber and the thermal shield.
9. A method according to claim 8, wherein the heat exchanger is in thermal contact with the thermal shield.
10. A method according to claim 8 or claim 9, wherein the integral extraction unit comprises activated getters or a high surface area charcoal device.
11. A method according to any preceding claim, wherein the method further comprises activating a refrigerator to cool the equipment to an operating temperature.
12. A method according to any preceding claim, wherein the method further comprises removing the thermal sink from the outer vacuum chamber and allowing surfaces of the thermal sink to rewarm after a predetermined vacuum level is reached, such that adsorbed gas is released from the thermal sink.
13. A method according to any preceding claim, wherein the method comprises applying further cooling to the equipment prior to filling the cryogen vessel with liquid cryogen.
14. A method according to any preceding claim, wherein in transit, the cryostat is cooled by boiling ciyostat.
15. A superconducting magnet system comprising a cryogen vessel, magnet coils, a thermal shield and an outer vacuum chamber; wherein the outer vacuum chamber is pumped down by supplying cooling to a thermal sink located within the outer vacuum chamber, outside the cryogen vessel.
16. A system according to claim 15, wherein a cryogen gas is supplied to cool the thermal sink.
17. A system according to claim 15 or claim 16, wherein the thermal shield further compises an integrated heat exchanger and an extraction unit.
18. A system according to any of claims 15 to 17, wherein the system further comprises a closed loop mechanical cooler for cooling the shield.
19. A system according to any of claims 15 to 18, wherein, in use, additional shield cooling comprising a thermal sink, thermally linked to the thermal shield is applied to oppose shield heating due to eddy currents.AMENDED CLAIMS HAVE BEEN FILED AS FOLLOWS:-13 CLAIMS 1. A method of cooling equipment within a clyostat comprising a cryogen vessel, a thermal shield and an outer vacuum chamber; the method comprising applying an initial vacuum to the outer vacuum chamber; and supplying cooling by a thermal sink located within the outer vacuum chamber to bring the vacuum to a predetermined vacuum pressure.2. A method according to claim 1, wherein the thermal sink comprises a heat exchanger, or heat absorbing structure.3. A method according to claim 2, wherein the thermal sink comprises a heat exchanger and the cooling is supplied to the heat exchanger from a stream of helium o gas.4. A method according to any preceding claim, wherein the predetermined vacuum pressure is at least of the order of 1 0 mbar. c\'j5. A method according to any preceding claim, wherein the method further comprises cooling the thermal shield, or equipment.6. A method according to claim 5, wherein the cooling comprises applying helium gas to the thermal shield or equipment.7. A method according to claim 5, wherein direct shield cooling is applied using a retractable cold finger in direct thermal contact with the thermal shield.8. A method according to any preceding claim, wherein the thermal sink comprises a heat exchanger and integral with the heat exchanger is provided an extraction unit for extracting residual gases present between the vacuum chamber and the thermal shield.9. A method according to claim 8, wherein the heat exchanger is in thermal contact with the thermal shield.10. A method according to claim 8 or claim 9, wherein the integral extraction unit comprises activated getters or a high surface area charcoal device.11. A method according to any preceding claim, wherein the method further comprises activating a refrigerator to cool the equipment to an operating temperature.12. A method according to any preceding claim, wherein the method further comprises removing the thermal sink from the outer vacuum chamber and allowing surfaces of the thermal sink to rewarm after a predetermined vacuum level is reached, such that adsorbed gas is released from the thermal sink.C13. A method according to any preceding claim, wherein the method comprises o applying further cooling to the equipment prior to filling the cryogen vessel with liquid cryogen. c\'j14. A method according to any preceding claim, wherein in transit, the cryostat is cooled by boiling cryogen.15. A superconducting magnet system comprising a cryogen vessel, magnet coils, a thermal shield and an outer vacuum chamber; wherein the outer vacuum chamber is pumped down by supplying cooling to a thermal sink located within the outer vacuum chamber, outside the cryogen vessel.16. A system according to claim 15, wherein a cryogen gas is supplied to cool the thermal sink.17. A system according to claim 15 or claim 16, wherein the thermal shield further comprises an integrated heat exchanger and an extraction unit.18. A system according to any of claims 15 to 17, wherein the system further comprises a closed loop mechanical cooler for cooling the shield.19. A system according to any of claims 15 to 18, wherein, in use, additional shield cooling comprising a thermal sink, thermally linked to the thermal shield is applied to oppose shield heating due to eddy currents. CoCC C) cj