GB2414537A

GB2414537A - A cryostat with outer vacuum jacket insulation

Info

Publication number: GB2414537A
Application number: GB0423894A
Authority: GB
Inventors: Graham Gilgrass; Timothy John Hughes; Keith White
Original assignee: Siemens Magnet Technology Ltd
Current assignee: Siemens Magnet Technology Ltd
Priority date: 2004-05-25
Filing date: 2004-10-28
Publication date: 2005-11-30
Anticipated expiration: 2024-10-28
Also published as: GB0411599D0; GB2414537B; GB0423894D0

Abstract

In a cryostat comprising an outer vacuum jacket (4), a resealable (7) evacuation port (6), the outer vacuum jacket contains a cryogenic fluid vessel (2), surrounded by at least one thermal shield (3), wherein a solid phase thermal insulation material (5) is placed within a cavity between the vacuum jacket and the thermal shield. A hole (8a) is provided in the thermal shield in the vicinity of the evacuation port (6), and a corresponding hole (8b) is provided in the solid phase insulation material, thereby to facilitate evacuation of the volume between the thermal shield and the cryogenic fluid vessel. The hole in the solid phase insulation material may be sealed by a patch (9) of solid phase insulation material suitably dimensioned to overlap the edges of the hole. The patch (9) may be stuck or affixed to the insulation material (5) by using a slidable rod assembly (14, Fig 3B), or by using a spring (29, Fig 4).

Description

241 4537 A CRYOSTAT WITH IMPROVED OUTER VACUUM,IACKE,T INSULATION Mlkl

(Magnetic Resonance Imaging) systems are used for medical diagnosis.

retinirenent ol an MRI magnet is that it plot ace a stable, homogeneous, magnetic field. In s ortler to achieve stability it is common to use a supercorittucting magnet system which operates at very low temperature, the temperature being maintained by cooling the superconductor, typically by immersion in a low temperature cryogenic fluid, such as liquid helium. Cryogenic fluids. and particularly helium, are expensive. It is desirable that the magnet system should be designed and operated in a manner to reduce to a rninimur 0 the amount of cryogenic fluid used.

A superconducting magnet system typically comprises a set of superconductive windings for producing a magnetic field, a cryogenic fluid vessel which contains the superconductor windings, one or more thermal shields completely surrounding the cryogenic fluid vessel, and a vacuum jacket completely inclosing the vie or more thermal shicids. It is common practice to use a refrigerator to cool the thermal shields to a low temperature in order to reduce the heat load onto the cryogen vessel. It is also known to use a refrigerator to directly refrigerate the cryogen vessel, with the aim of reducing or eliminating the cryogen fluid consumption. In both eases it is well known that it is necessary to have a good go vacuum inside the vacuum jacket so as to reduce to a very low level the heat load by gas convection onto the cryogen vessel.

It is well known to use multilayer insulation ("superinsulaticn"), for example layers of' alumirlised polyester film such as that marketed by Austrian Aerospace under the CO()I,CAT brand, tall reduce; the rcdiatutl heat kind onto the thermal shield oi'a cryogenic system, and it is well known that the application of' such material must he light tight to achieve the optimum properties oi'tlle insulation; a hole of just I cm2 area will admit 46() mW ol room temperature radiated heat into a vessel at a temperature of 4K, which will seriously degrade the perforrllarlee of Mlkl magnet systems where the maximum tolerable heat load onto the cryogen vessel is in the region of'500 mW for a system with a single shiekl and a recondensing refrigerator. As is known in the art, the superirlsulation may be applied to any or all of' the thermal shields in a cryogenic system, hut is particularly berlet'ieial when applied to the outer. highest temperature shield. where it is most et'l'ective at reducing radiated heat load.

It is necessary to establish a flood vacuum within the vacuum jackets of superconducting MRI magnets. It takes considerable time to reach an adequate vacuum by pumping through the evacuation port as the superinsulation in the region of the port impedes gas flow. '1'lle pumping speed through the superinsulation surrounding the thermal shields is very slow. A method is proposed to increase the pumping speed whilst not degrading the superhsu ration et't'ecti veness.

It is known to punch holes in the sheets which form the supcrinsulation with the intention 0 ot increasing the porosity of the superinsulation and thereby reducing the time taken to evacuate the volume containing the insulation' but this is ol'little effect in increasing the through-conductance olives through the insulation for evacuation others from the regions ol'tile cryogenic vessel inside the superinsulation. It can take many hours of pumping to achieve a suitable vacuum in an MRI magnet system. 'I'he time taken to evacuate the system to an aceeptabie vacuum is a considerable part of tile overall build and test time ol' the system and reduction of this thile would be an economic advantage.

I'he present invention accordingly aims to provide methods and apparatus for enhanced pumping ol' the magnet system that does not degrade the performance ot' the superinsulation.

Summal y of the I nvention I'he present invention accordingly provides methods and apparatus as def ined in the 2s appended claims. I 'or exailipic a hole is prtvitictZ irvcrsing the superinsuiation and tile underlying thermal shield. 'I'he hole is suf't'iciently large k, enhance the evacuation process. A means t'or subsequently covering the hole in the insulation utter evacuation is provided whereby the heat insulating properties are restored.

The above and further objects characteristics and advantages of' the present invention will become more apparent with ret'erence to the t'ollowUlg description of certain embodiments giVcn by way ot'example only in con junction with the aecompanyhg drawings wherein: Fig. I shows a cross-section ot an Mlkl magnet system; Fig.2 shows an enlarged view of a pumping part of the system of T7ig.1, adapted according to an embodiment of the present invention; lig.3A - 313 show an enlarged view of a purllpirlg part ol the system of l ig.l, aclaptect accordhlg to an embodiment ot the present invention; and L'ig.4 shows an enlarged view of a pumping part ot the system of 1-'ig.1, adapted according to an embodiment ol the present invention.

Detailed description.

lo Figure I shows a schematic cross section of an MRI magnet system. Superconductor magnet I is housed in cryogen vessel 2 containing a cryogenic fluid 2a, such as licluid helium and is surrounded by one or more thermal shields 3, of which one is shown. [he shield(s) 3 are surrounded by vacuum jacket 4. A central bore 4a is provided, for receiving a patient to be examined. Superinsulation 5 is placed bctwoen outermost shields 3 and vacuum jacket 4. Attaclled to the outer shell ol' the vacuum jacket is evacuation port 6, which is sealed during magnet system operation by plug 7.

Idgure 2 shows the pumping port in more detail, and adapted according to an embodiment of the present invention.

JO

According to an embodiment ol the present invention a pumping hole 8a is cut in shield 3 to provide a gas pumping path Air evacuation ol the gas between shield 3 and cryogen vessel 2. A corresponding hole 8b is cut in the superinsulation 5. Superinsulation 5 covers shield 3. Alter evacuation of the magnet system, superirlsulation patch 9 is stuck to the surlacc of supcrii,sLilatic,i, 5 usir,g, an adhesive metliu 1 () which is preferably double sided adhesive tape, so as to cover hole Xb to provide a light tight seal, as is necessary to prevent thermal radiation passing Irons room temperature at plug 7 through hole 8 onto cryogen vessel 2. Evacuation port (j is typically 70 torn in diameter, and holes 8a, 8b are typically mrn in diameter. I'lug 7 is sealed to port 6 in a conventional manner with "()" ring 11.

During evacuation of the magnet system, gas pumping holes 8a, 8b are not obscured, arid permit free flow oi gas from between shield 3 and cryogen vessel 2 to be pumped away through evacuation port 6 by a suitable pump (not shown).

Referring now to lig.3, alter evacuation, a method whereby superinsulation patch 9 may be stuck to the surface of superinsulation 5 is discussed in more detail. Vacuum valve 12 is attached to port 6 and to a vacuum pump (not shown) lor evacuation purposes. When adequate vacuum is achieved within the volume between shield 3 and cryogen vessel 2, valve 12 is sealed by sealing Lange 13 shown schematically in the drawing. 'I'he pump is then detaciled. Such operation of valve 12 and sealing flange 13 is conventional h1 itsell' and well known to those skilled in the relevant art.

According to an aspect ol' the present hivcntiorl, assembly 14 is then attached in vacuum 0 tight manner to valve 12. Assembly 14 comprises shaped flange 15 and guide tube 16, which seals in a leak tight manner to valve 12 with "()" ring 17 and to stem I X with at least one "()" ring 19. Stem 18 is a sliding fit in guide tube 16. Attached to the end ol' stem 18 is an outer flange 20. Passing through stem 18 is rod 21 which is sealed in a leak tight manner to stem 18 with at least one ''O'' ring 22 and is a sliding fit in stem 18. An inner is flange 23 is attached to the end of rod 21. 'the other end of rod 21 is threaded and fitted with nut 24 bearing against the end ol' stem 18, so that inrier flange 23 can be drawn up against outer flange 2() so as to grasp tabs 25 which are attached to superinsulation patch 9 thereby holding patch 9 onto the end face of flange 23.

Stein 18 is initially withdrawn as shown h1 L'ig.3A so that flanges 20 and 23 and patch t) arc at position A, situated in the chamber Armed by valve 12, sealing flange 13 and shaped flange 15. This chamber is evacuated by a suitable pump attached to port 26 until the vacuun1 is at least close to that achieved in the magnet system.

As shown in Fig. Gil, the scaling flange i3 oldie vacuum valve 12 is now opened and stem 18 carrying flanges 20, 23 and patch 9 is pushed in to position so that patch 9 makes contact with the superinsulation 5 and the adhesive 10 sticks the patch ') to the superinsulation 5. Nut 24 is now loosened and outer flange 2() is separated from imier flange 23 so that tabs 25 are released. Stem 18 is now withdrawn to position A (I;ig.3A) and valve 12 closed again. The vacuum between flange 15 and closed valve 12 and sealing flange 13 is released and assembly 14 removed.

Plug 7 may now be fitted in a manner similar to the fitting cuff the patch 9, but this is not described, ShlCC this is conventional in itself: An alternative embodiment is shown in figure 4. According to this embodiment, the patch 9 may be removed if further vacuum pumping should be required at a later date. Plug 7 is fitted with a cage 27 which is used to house and restrain a guide stem 2X and a bias spring s 29. Stem 28 has, h1 contact with spring 29, small plate 30 onto which spring 29 acts, and on the other end large plate 31 which holds the patch 9. Superinsulation patch 9 is attached to plate 31 by adhesive tape 32 which prel'erably is of low heat conducting material with low emissivity, lor example aluminised Mylar. CNage 27 stem 28 and spring 29 are preferably made of low thermal conductivity material such as stainless steel so as to 0 conduct little heat to the patch 9. 'I:'his assembly is fitted in a manner similar to that described above lor the tatting ol'tile fixed patch 9, and so is not described in detail, other than to skate that the whole assembly of patch '), plate 31, cage 27, step 28, spring 29 and plug 7 may all be attached and removed together in a single operation.

While tile present invention has been described with reference to a limited number ot' specific embodiments, the invention is not limited to such embodiments. Various modifications and amendments may be made, as will be apparent to those skilled in the art.

I'or example, while the patch has been described as being attached by adhesive, other retaining means may be used - I'or example, mechanical fasteners such as VE:I,CRO(R) ho brand hoof; and loop fasteners

Claims

CLAIMS: 1. A cryostat comprising an outer vacuum jacket (4) having a

resealahie (7) evacuation port (6)' said outer vacuum jacket containing a cryogenic fluid vessel (2)' surrounded by at least c>ne thermal shield (3), wherein a solid phase thermal insulation material (5) is placed within a cavity between the vacuum jacket and the thermal shield eharacterised in that a hole (8a) is provided in the thermal shield in the vicinity of the evacuation port (6), and a corresponding hole (8b) is provided in the solid phase insulation material, thereby to facilitate evacuation of the volume between the thermal lo shield and the cryogenic tiuid vessel, said hole in the solid phase insulation material being sealed by a patch (9) ol solid phase insulation material suitably dimensioned to overlap the edges of the hole.

2. A cryostat according to claim 1 wherein the patch ol solid phase insulation material is retained in position by an adhesive positioned between the patch and the solid phase thermal hisulaliorl material in the regions of overlap.

3. A cryostat according to claim 2 wherein the adhesive is a double sided adhesive tape. go

4. A cryostat according to claim I wherein the patch cl solid phase insulation material is retaped in position by a bearer (31) resiliently biased into position by a spring (29).

5. A cryostat according to claw 4 wherein the resealable evacuation port is sealed by a removable plug (7), the spring acting between an inner surface of the plug and the bearer.

6. A cryostat according to any preceding clain1 wherein the solid phase insulation 3() material and the patch of solid phase insulation material comprise a multilaycr illStilIti(ll.

7. A cryostat according claim 6 wherein the multilayer insulation comprises layers of aluminised polyester film.

X. A metilod for evacuating a volume in a cryostat comprising an outer vacutn1 jacket (4), having a resealable (7) evacuation port (6), said outer vacuum jacket containing a cryogenic fluid vessel (2), surrounded by at least one thermal shield (3), wherein a solid phase thermal insulation material (5) is placed within a cavity between the vacuum jacket and the thermal shield, said volume IyLlg between the cryogenic tepid vessel and the thermal shield, lo the method comprising the steps ol' (a) connecting a pump to the evacuation port; (h) - unsealing the evacuation port; (c) - evacuating the volume through the evacuation port; and (d) resealing the evacuation port; l characterised in that the method f'urtiler comprises the following steps: - providing, a hole (Xa) h1 the thermal shield in the vicinity of'tile evacuation port (6), and a corresponding hole (Xb) h1 the solid phase insulation material; - after evacuation of'ttlC volume, and prior to resealing the evacuation port, sealing said hole in the solid phase hisulation material by applying, a patch (a)) of solid phase insulation material suitably dimensioned and positioned to overlap the edges of the hole.

9. A method according to claim X wherein the patch of'solid phase insulation material is retaped in position by an adhesive positioned between the patch and the solid phase thermal insulation material in the regions ol'overtap.

1(). A method according to claim 8 wherein the patch ol' solid phase insulation material is retained h1 position by a bearer (31) resiliently biased hito position by a spring (9')).

1 1. A method according to claim 10 wherein the resealable evacuation port is sealed by a removable plug (7), the spring acting between an inner surface of the plug and the bearer.

12. A method according to any of claims 8-1 1 wherein the method further comprises the steps of (a) - bck-'re evacuation of the volume, attaching a vacuum valve ( 12) to the evacuation port (6); (b) - connecthlg; a pump to the evacuation port through tl'c vaeuun1 valve; (c) - evacuating the volume through the evacuation port and the vacuum valve; (d) - sealing the vacuum valve; (c) - removing the pump from the vacuum valve; (I) connecting an assembly (14), comprising a patch carrier carrying a patch, to the vacuum valve in a vacuum-tight manner; (g) - connecting a pump to the evacuation port and evacuating the space between the assembly and the closed vacuum valve; (h) - opening the vacuum valve; is (I) - displacing the patch carrier towards the hole, such that the patch is located in position over the hole.

13. A method according to claim 12 when dependent on claim 11, wherein the patch carrier serves as the removable plug.

14. A method according to claim 12 wherein the patch carrier comprises two abuttable flanges (2(), 2), which initially are in abutment with patch retention means (25) entrapped therebetwoen, and wherein, once the patch is located in position over the hole, the abuttable flanges are rcicased Tom abutment, allowing the flanges to be withdrawn, while the patch rcmahis in position.

15. A method accortfing to claim 14 wherein, following the withdrawal of the abuttabie flanges, (k) - the vacuum valve ( 12) is sealed, (1) assembly 14 is removed; (m) - a removable plug is mounted onto the patch carrier or a similar dcvicc; (n) - the assembly (14) is again connected to the vacuum valve in a vacuum-tight manner; (p) - the space between the assembly ( 14) and the closed vacuum valve is again evacuated; s (q) - the vacuan1 valve is opened; and (r) - the removable plug is displaced towards the hole, such that the plug seals the resealable port (6).

16. A method according to any of claims X-15 wherein the solid phase insulation 0 material and the patch of solid phase insulation material comprise a multilayer insulation 17. A method according to claim 16 wherein the multilayer insulation comprises layers ol aluminised polyester film.

18. A method or a cryostat as described and/or as illustrated in Figs. 24 of the accompanying drawing. 1C)

Amendments to the clams have been fled as follows

CLATMS

I A cryostat COmpr!S!Tlg aTT outer- N/acunfll jacket, haviTTg a resealable evacuation port, S,TiCI oTTter V!CTT:Tm!cket CoTtaiTlTTl, i CTyr:'eTTiC flcTid vessel. SUrT-:uTded ty!t iest one tllc-rnT!l shTelci. wliereiTl a solTd plTTiSC thCrTTTal TnSTTit:ItioTT Tll-llerTT] TS placed witITTTT a cav!ty letweeT, l:lc VcrTT'rlT ITcket aTld tile tlTeri1lrl sliTeld, CITaT'TCteTiSed TTi Lliat a lolc TS pi-ovideci in the tTTerrTlal shield Tn tile vienity of the evacution port, aTld.T CtITUS}-,OTdiT]g} Iole TS pToVT6CCi ir tte solid prase TrTs!atiorT nTaterial, thereky to faeilTtate evacitTtion of the vohTme hetween the thermal shre]d and the cryogenic fluid vessel, saTd hole in the solid phase Tnsulation rmateT- Tal being sealed h y a patch ot solTd phase inslation mateTTal sTTrtahly dimensioned to overlap the edgres OT' the hole.

2. A cryostat aeeording to elaTrrT] wherein the patch of solicT phase insulation material is retained in position by an adhesTve posTtioned between the patch and the solid phase thcrnTa1 insulation matcria] ITT the regions of overlap.

3 A cryostat aeeording to elaTTn 2 whereiTl the adhe.sive rs a do,Thle sided adhesive tape 2() 4 A cryostat aeeordTng to claim] wherein the. patch of solid phase TnsulatiorT materTal TS rOtaTTlOd TT1 posit/on hy a bear-er resTliently based into post->n by a spring.

:' A ci-yostt accordirTg to CITitTT A 'wthCTCT'1 tile rese.lble evcuatior, port:s sealed s by a removable. ph.g, the SprirTg actirTg betweeTl aT1 inner surface of the pltTg and the bearer f' A eTyostat accorcdTno to aTTy preceding clairr whereirT the solid phase TnsulatoTl TtllOT-,tI. TT'CT tle tgatcl Tf sQltd ptTase TTsulatioTT Tll!lerTtI cotripr-se 1 iTultTlyeT ]O ins'laticn It 7 A cryostat according e]aim wherem the multTlayer insulation comprises layers of alumTmsec3 polyester film A method fciTevaeuatirT, a volume In a cryostat eomprisTngr an flouter vaeuTTm s 1aelel having a resealable evaenation port, said outeT- vacuum jaelet containing a eTyoger,e fluTcl vessel, suT1-oundec3 by at least oils tlerrTal slTTelci, where a solid lease thC'TTTTTi TTISUiatTon Tl:TOte.tia] is placed WTtiTTrT.T CiVTty betWeeT] the V.iCTTuTr JChCt aTIcl the thermal shield, said volurrie iyTng between the cTyogenie fluid, vessel a,,ct the thern.al shield, lo the method comprising the steps of (a) - eorTTTeetrng a pump to the evacuation poTt, (h) - unsealing the evaeualron port; (e) evacuating the volume through the evacuation port; and (d) - resealing the evacuation port; characterized in that the method l'uTt'heT- comprises the following steps - providing a hole in the thermal shield in the vicinity of the evacuation port, and a corresponding hole in the solid phase Insulation material; - after evacuation of the volume, and prior to resealing the evacuation port, sealing said hole En the solid phase insulation material by applying a patch of solid phase isolation material suitably dirnensionect and positioned to overlap the edges of the hole 9 A rTetlcd aeeotclTrTg to Elaine wlleT-ein the patch of solid please TTTsulatToT-T material TS retained In position by an adhesive pOSitiOTlCCt between the patch and the solid phase therrrTal Tnsulalon material in the regions of overlap 2> I () A illetlloct ceuTCtiTTg to elaTTT wTreTeiTi the patch of solid phase usu]alion maternal is retained In position by a heater resiliently biased Into poSItTC)n by a spring I I A method according to clang 1() wherein the resealable evacuat->n port Is sealed lo by a removable plug, the SpT-mg action Between Din mner Outrace of the thug anct the he arer em 12. A method according to any of claims S- 1 1 wherein the method further comprises the steps of: (a) - Ire evacuation of the volume, attaching a vacuum valve to the evacuation port; (b) connecting a pump to the evacuation port through the vacuum valve; s (c) evacuating the volume ttTrough the evacuation port anti the vacuum valve; (d) - sealing the vacua valve, (e) - removing the pump from the vacuum valve; (i) - connecting an assembly, comprising a patch carrier carrying a patcl- . to the vacuuin valve n1 a vac;;um-tTght manner; lo (g) connecting a pump lo the evacuation port and evacuating the space between the assembly and the closed vacuum valve; (h) - opening the vacuum valve; (j) - displacing the patch carrier towards the hole, such that the patch is located in position over the hole. is

13. A method according to claim 12 when dependent on claim] ], wherein the patch carrier serves as the removable plug.

14. A method according to claim 12 wherein the patch carrier comprises two abuttable flanges, which initially are in abutment with patch retention means entrapped therebetween, and wherein, once the patch is located in position over the hole. the abuttable flanges are released from abutment, allowing the flanges to be withdrawn, while the patch remains in position.

15. A method according to claim]4 wherein, following the withdrawal of the

abuttable flanges,

(ii) - the vactTuTn vTIve is sealed, (1) - the asser1Tb]y is removed; (TTi) - AT rCnTOVab]e p]iTT is TTToUTTted onto the patch caterer or a smi] aT device, So (n) - tlTe assernh]y TS a;iTTT CoTlT1eCtCd to tile vacrTcun valve TTT a vactTuTn-tirht nurrTeT, (p) - the space between the assembly and the closed vacuum valve is agahT evacuated; (cl) - the vacuum valve IS opened, and (r) - the removable plug Is dsplaccd towards the hole such that the p]Ug seals the rcsealabic port 16. A method accorciing to any of claims S-15 wl1erein the solid phase insulation s material anti the patch of.schd phase insulation mater-al comprise a mltilaycr stlatic'T 17 A method accor^dng to claim].6 whe.rcin the. n,ultlaycr insulation comprises lancers of altirrunisecl polyester l lrn.

18 A method or a cryostat as described and/or as ilhstrated In Fi,s 0-4 of the accompanying drawing