Abstract:
An over-pressure limiting arrangement for a cryogen vessel includes an access neck providing access into the cryogen vessel, a tubular structure extending through the access neck, a turret outer assembly joined leak-tight to the cryogen vessel and defining an interior volume that is separated from the atmosphere by a protective valve or burst disc, enclosing an upper extremity of the access neck and the tubular structure. An egress path defines a route for cryogen gas to escape from the turret outer assembly, and a pressure-responsive quench valve seals the egress path and opens when a differential pressure between the interior of the turret outer assembly and the interior of the egress path exceeds a predetermined value. An auxiliary burst disc, or a valve, is attached to the tubular structure within the turret outer assembly, with an inner surface thereof exposed to the interior of the tubular structure and an outer surface thereof exposed to the interior of the turret outer assembly.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present application relates to safety pressure limiting features on cryogen vessels, particularly in respect of cryogen vessels containing superconducting magnets of magnetic resonance imaging (MRI) systems. In particular, it relates to the advantageous arrangement of components of an auxiliary vent path, provided to limit pressure within the cryogen vessel in case of a quench of the superconducting magnet. 
     Description of the Prior Art 
       FIG. 1  shows a conventional arrangement of a cooled superconducting magnet  10  within cryogen vessel  12 , itself retained within an outer vacuum chamber (OVC)  14 . One or more thermal radiation shields  16  are provided in the vacuum space between the cryogen vessel  12  and the outer vacuum chamber  14 . In some known arrangements, a refrigerator  17  is mounted in a refrigerator sock  15  located in a turret  18  provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator may be located within an access turret  19 , which retains access neck (vent tube)  20  mounted at the top of the cryostat. The refrigerator provides active refrigeration to cool cryogen gas, typically helium, within the cryogen vessel  12 , in some arrangements by recondensing it into a liquid  22 . The refrigerator may also serve to cool the radiation shield  16 . As illustrated in  FIG. 1 , the refrigerator  17  may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield  16 , and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K. 
     A negative electrical connection  21   a  is usually provided to the magnet  10  through the body of the cryostat. A positive electrical connection  21  is usually provided by a conductor passing through the vent tube  20 . 
     For fixed current lead (FCL) designs, a separate vent path (auxiliary vent) (not shown in  FIG. 1 ) is provided as a fail-safe vent in case of blockage of the vent tube  20 . It is this auxiliary vent path which is the subject of the present invention. 
       FIG. 2  shows an example of a conventional over-pressure limiting protection arrangement  30 , designed to vent cryogen gas from the cryogen vessel in case of over-pressure, such as could occur following a quench of the magnet. 
     A superconducting magnet  10  is contained within a cryogen vessel  12  as discussed with reference to  FIG. 1 . A turret outer assembly  24  encloses upper extremities of access neck (vent tube)  20  and positive current lead  21 , and provides a normal exit path  26  for cryogen gas from cryogen vessel  12 . Turret outer assembly  24  is joined to the cryogen vessel in a leak-tight manner and defines an interior volume which is separated from atmosphere by a protective valve and/or burst disc, and forms part of normal exit path  26 . The protective valve and/or burst disc in the illustrated example is quench valve  32 . 
     In the event of a quench, the cryogen vessel  12  is vented to atmosphere via the vent tube  20  in the access turret  19  through the interior volume of the turret outer assembly  24  and quench valve  32 . Quench valve  32  includes a valve plate  34  which is held against valve seat  36  by a spring arrangement  38 . Cryogen egress tube  40  leads exit path  26  to atmosphere, or to a cryogen recuperation facility, essentially at atmospheric temperature. In case of over-pressure within cryogen vessel  12 , a corresponding pressure of cryogen gas within the turret outer assembly  24  acting on the inner side  34   a  of the valve plate  34  will exceed the pressure acting on the outer side  34   b  of the valve plate sufficiently to overcome the force of the spring arrangement  38  and open the valve  32 . Cryogen gases will escape, maintaining the pressure within the cryogen vessel at an acceptable level. Once the pressure in the cryogen vessel and the interior volume of the turret outer assembly  24  drops below the pressure needed to keep the quench valve  32  open, spring  38  will press the valve plate  34  back into contact with valve seat  36 . 
     Part of the valve plate  34  may be formed by a burst disc, not visible in the drawing as it lies in the plane of the valve plate  34 . In case the differential pressure across the valve plate becomes much higher than the pressure at which the quench valve  32  should open, for example if the quench valve  32  sticks, or the pressure increase within the cryogen vessel is extremely rapid or severe, the burst disc will rupture and cryogen gas will then escape through a hole left by the burst disc and out of the cryogen vessel  12  through the interior volume of the turret outer assembly  24  and egress tube  40 . This burst disc is typically a declared regulatory pressure relief safety device, provided to rupture in the event of quench valve failure. 
     In addition to the declared safety device, an auxiliary vent path  42  is provided, through a tubular positive current lead  21  to atmosphere via an external room-temperature tube  44  fitted with its own auxiliary burst disc  46 . Auxiliary vent path  42  does not pass through the interior volume of the turret outer assembly  24 . The auxiliary burst disc  46  is designed to rupture when a differential pressure across it meets a certain value, in excess of the differential pressure at which quench valve  32  is designed to open, and in excess of the differential pressure at which the bust disc within valve plate  34  is designed rupture. 
     It is known that air ingress into the access neck  20  may cause ice to form in region  48 , between the inner wall of the access neck  20  and the positive current lead  21 . If sufficient ice forms in this region, it may form a constriction, and cryogen gas may not be able to freely escape in case of a quench. A differential pressure may exist across the blockage, reducing the differential pressure across the quench valve  32 . 
     On the other hand, the positive current lead  21  passes into the cryogen vessel more deeply than the ice-forming region  48 , to the level of temperatures usually so cold that any air ingress into the access neck  20  freezes onto the access neck in region  48  and before it can reach the lower end of the positive current lead  21 . The interior of the tubular positive current lead  21  may therefore be assumed to be free of ice. As there is no blockage in the positive current lead, the full differential pressure between the interior of the cryogen vessel  12  and atmospheric pressure in the egress tube  40  will apply across the auxiliary burst disc  46 . Burst disc  46  is designed to rupture at a pressure high enough that it can only be reached if the quench valve  32  and its burst disc have failed to protect the cryogen vessel as designed. 
     Typically, quench valve  32  is designed to open in response to a 0.5 BAR (50 kPa) differential pressure between the high pressure side  34   a  exposed to the interior volume of the turret outer assembly  24  and the low pressure side  34   b  exposed to the interior of the egress tube  40 . The burst disc within the quench valve is typically designed to rupture in response to a differential pressure of 1.4 BAR (140 kPa), and the auxiliary burst disc  46  is typically designed to rupture in response to a differential pressure of 1.8 BAR (180 kPa). These values are chosen to protect the cryogen vessel in all circumstances, but are sufficiently separated that the quench valve  32  will open without damage to the burst disc within the quench valve unless the quench valve is stuck, and that the auxiliary burst disc  46  will only rupture in response to a cryogen vessel pressure so high that it is clear that neither the quench valve  32  nor the burst disc within the quench valve are going to open. 
     This arrangement has certain drawbacks, which the present invention seeks to alleviate. 
     In present arrangements such as shown in  FIG. 2 , the auxiliary burst disc  46  is permanently subjected to the full differential pressure between the interior volume of the turret outer assembly  24  and the cryogen vessel on one side and the egress path  40 , which is at approximately atmospheric pressure, on the other side. This differential pressure may approach the pressure at which the auxiliary burst disc  46  is designed to rupture. 
     During a quench event which is vented through the auxiliary burst disc  46 , the pressure within the cryogen vessel may approach the maximum allowable working pressure of the cryogen vessel, due to the constriction of escaping gas in the “room-temperature” tube  44  and the rapid expansion of this cryogen gas due to heating as it passes through the “room temperature” tube  44 . It would be preferable from this point of view to provide a room temperature tube  44  of increased cross-section, but this would have the undesired effect of increasing the height of the overall system. 
     In the event of rupture of the auxiliary burst disc  46 , air can be drawn back into the auxiliary vent path  42  once the over-pressure within the cryogen vessel has ceased. This can cause a buildup of ice within the tubular positive current lead  21  which is difficult to detect or remove. 
     A further disadvantage is the cost of the external room-temperature pipe work  44  and seals required to interface the auxiliary vent path  42  to the remainder of the equipment. The external pipe work  44  adds to overall system height, which causes integration problems in siting the cryostat. Any external joints, seals, welds etc. all have the potential to cause leaks into the vent path during normal service, and so their number should preferably be reduced. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these, and further, problems by relocating the auxiliary burst disc to the tubular positive current lead  21 , preferably to the top of the tubular positive current lead  21  within the turret outer assembly  24 . In the event of the normal exit path  26  becoming blocked or restricted, cryogen gas escapes via the tubular positive current lead to atmosphere via the turret outer assembly and through the quench valve or burst disc. 
     UK patent GB2472589 proposes a single vent path in a similar application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional cryostat arrangement housing a superconducting magnet. 
         FIG. 2  shows a conventional over-pressure limiting arrangement. 
         FIG. 3  shows an over-pressure limiting arrangement according to an embodiment of the present invention; 
         FIG. 4  shows a step in a servicing method for the over-pressure protection arrangement of  FIG. 3 . 
         FIGS. 5A and 5B  illustrate a valve which may be used in certain embodiments of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIG. 3 , an over-pressure limiting arrangement according to an embodiment of the present invention comprises an auxiliary burst disc  50  placed at or near the upper extremity of the tubular positive current lead  21 , within the turret outer assembly  24 . The inner surface  50   a  of the auxiliary burst disc  50  is exposed to the interior of the tubular positive current lead  21 , and thence to the interior of the cryogen vessel  12 . The outer surface  50   b  of the auxiliary burst disc  50  is exposed to the interior of the turret outer assembly  24 . The auxiliary burst disc may be mounted onto a carrier  51  which is mounted by pillars  52  onto a removable cover plate  54 , which may be fastened by bolts  56  and sealed by o-ring  58  to the turret outer assembly  24 . 
     As for the conventional arrangement of  FIG. 2 , the normal exit path  26  in the arrangement of  FIG. 3  is through access neck  20 , turret outer assembly  24  and through quench valve  32 . However, according to the present invention, the auxiliary vent path  42  is also through the turret outer assembly  24  and through quench valve  32 . Escaping cryogen gas may flow through auxiliary burst disc  50  and between pillars  52  which surround the auxiliary burst disc  50  and mount the carrier  51  onto a removable cover plate  54 , as shown. 
     Provided that normal exit path  26  is not blocked, over-pressure within the cryogen vessel will cause increased differential pressure between inner  34   a  and outer  34   b  sides of the valve element  34  of quench valve  32 . Once that differential pressure becomes sufficient to overcome the force applied by the spring  38 , the quench valve  32  will open and release cryogen gas from the interior volume of the turret outer assembly  24  into the egress tube  40 , to reduce the pressure in the cryogen vessel. Once a sufficient amount of cryogen gas has escaped to reduce the pressure within the cryogen vessel to a normal level, the force applied by spring  38  is sufficient to close the quench valve  32 . 
     In the event of the normal exit path  26  becoming blocked or restricted, typically by a build-up of ice in region  48  between the inner wall of access neck  20  and tubular positive current lead  21 , pressure within the cryogen vessel will build up until the differential pressure between inner  50   a  and outer  50   b  sides of auxiliary burst disc  50  is sufficient to cause the auxiliary burst disc to rupture. Cryogen gas then escapes through the tubular positive current lead  21  to atmosphere via the turret outer assembly  24  and through the quench valve  32 . Unlike with the conventional auxiliary burst disc arrangement of  FIG. 2 , the auxiliary vent path  42  closes once a sufficient amount of cryogen gas has escaped to reduce the pressure within the cryogen vessel to a normal level, and the force applied by spring  38  is sufficient to close the quench valve  32 . 
     In such an arrangement of the present invention, the auxiliary burst disc  50  may be designed to rupture at a relatively low differential pressure. This will occur if sufficient pressure differential exists between the cryogen vessel  12  and the turret outer assembly  24 , which will only occur if some blockage is present in the normal egress path  26 . The auxiliary burst disc may be designed to open at a differential pressure of 0.5 bar (50 kPa), regardless of whether the quench valve  32  is open or closed. Quench valve  32  may be designed to open at a typical differential pressure of 0.5 BAR (50 kPa), so the auxiliary burst disc  50  should rupture in response to a 1 BAR (100 kPa) pressure difference between the cryogen vessel  12  and the egress path  40 , assuming a total blockage of normal exit path  26  in the region  48 . Due to the constrictions in the auxiliary vent path  42 , the pressure in the cryogen vessel may rise during venting, but in this example is unlikely to exceed 1.4 BAR (1400 kPa) above the pressure in the egress tube  40 , which is typically at atmospheric pressure. 
     In an alternative arrangement, the auxiliary burst disc  50  may be replaced by a valve. There are certain advantages that may be achieved in this way. The valve may have a lower opening pressure than the burst disc, and may be arranged to open during any quench, whether the gap between the access neck and the positive current lead  21  is clear or not, so as to share cryogen flow between normal exit path  26  and auxiliary vent path  42 . 
     The valve should be arranged to re-seal after a quench, and would not require replacing each time it opened, which is the case for a burst disc. Some leakage of the valve may be acceptable, as there would be no leakage of cryogen to egress tube  40  under normal conditions, as quench valve  32  would remain closed. In its simplest form, a spring-loaded flap valve may be used. It may be preferred to include a burst disc within the valve, similar to the arrangement used with the quench valve  32 , to ensure opening of the auxiliary vent path  42  even in case of the valve sticking closed. Removable cover plate  54  should still be provided, to allow for inspection and replacement of the valve. 
     The present invention provides auxiliary burst disc  50  or valve closing auxiliary vent path  42  in normal operation, and which opens into the turret outer assembly  24 , upstream of the quench valve  32 , when required. 
     The auxiliary quench path is accordingly protected by the declared regulatory pressure relief safety device, the burst disc in quench valve  32 , in the same way as the normal egress path  26 . 
     The differential pressure across the auxiliary burst disc  50  is greatly reduced, as compared to the differential pressure experienced by auxiliary burst disc  46  of conventional arrangements such as illustrated in  FIG. 2 , as the differential pressure across the auxiliary burst disc  50  is now the pressure differential between the cryogen vessel  12  and the interior of the turret outer assembly  24 , rather than the pressure differential between the cryogen vessel  12  and atmosphere. This pressure differential is approximately halved, which means the rupture pressure of the burst disc can be correspondingly reduced. The pressure within the turret outer assembly  24  at the point of bursting of the auxiliary burst disc  50  may be predicted by conventional methods of Computational Fluid Dynamics, or may be measured by experimentation. 
     The present invention enables reliable operation of the auxiliary burst disc  50  and the auxiliary vent path  42  at a lower cryogen vessel pressure in the event of a quench through the auxiliary vent. In a normal steady-state situation, the differential pressure across the auxiliary burst disc  50  is zero, as pressure within the tubular positive current lead  21  will equalize with pressure within the turret outer assembly  24  by flow of cryogen gas through the normal exit path  26 . This makes unwanted rupture of the auxiliary burst disc very unlikely. Auxiliary burst disc  50  will rupture only if a pressure differential exists between the cryogen vessel  12  and the volume enclosed by the turret outer assembly  24 . This in turn will only occur if the normal exit path  26  is substantially blocked in the access turret  20 , and either the pressure within the cryogen vessel has increased more rapidly than cryogen has been able to flow through normal exit path  26  to equalize with the pressure in the turret; or the quench valve  32  has at least partially opened, reducing the pressure within the turret outer assembly  24 . Ice formation in region  48  may form a constriction, but is unlikely to completely block the normal exit path  26 . In case of over-pressure within the cryogen vessel, some gas will flow through the constriction at  48  to partially open the quench valve  32 . This partial opening of the quench valve will increase the differential pressure across the auxiliary burst disc  50  and cause it to rupture. Even with cryogen gas flowing through the normal exit path  26  and quench valve  32 , the differential pressure across the auxiliary burst disc  50  will increase as the normal exit flow path becomes restricted due to ice build-up, typically in the region  48 . 
     The auxiliary burst disc  50  is concealed within the turret outer assembly  24 , and so is very unlikely to be mechanically damaged. In the conventional arrangement of  FIG. 2 , the outer casing of the auxiliary burst disc  46  is located at the very top of the cryostat, in a position which is exposed to possible mechanical damage during siting, or service operations. 
     The auxiliary burst disc  50  is in an air-free atmosphere, within the turret outer assembly  24 , during normal service. The chances of any air ingress into the vent path  42  within the tubular positive current lead  19  are extremely low. 
     After venting of cryogen gas through a ruptured auxiliary burst disc  50 , the chances of air ingress into the vent path  42  within the tubular positive current lead  19  are very low as the quench valve  32  re-seals the turret outer assembly  24  from atmosphere. It would only be possible for air ingress to reach beyond the ruptured auxiliary burst disc  50  if burst disc of the quench valve  32  is ruptured. Under these circumstances the air ingress would be shared between the normal  26  and the auxiliary  42  vent paths. 
     The cost of providing, fitting and maintaining the conventional external room-temperature pipework  44 , with its seals etc. would be saved. 
     The height and installation complexity of the cryostat is reduced with the arrangement of the present invention. 
     In the case of a quench causing venting of cryogen gas through the auxiliary burst disc  46  of the conventional arrangement of  FIG. 2 , a significant proportion of the pressure drop between the cryogen vessel  12  and the egress tube  40  is in the constrictive room-temperature pipework  44 , whose cross-section tends to be minimized to reduce the overall height of the system. In the arrangement of the present invention, the constrictive room-temperature pipework  44  is functionally replaced by the much larger cross-section of the turret outer assembly  24 , quench pipe  60  and quench valve  32  or its burst disc. 
     The turret outer assembly  24  and quench valve  32  operate at room temperature so consequently remain ice free. There is no risk of the normal exit path  26  and the auxiliary vent path  42  from becoming obstructed due to a build-up of ice in the turret outer assembly  24  and quench valve  32 . Even when cold during ramping and filling operations, no ice builds up in these regions. 
     Advantageously, the auxiliary burst disc  50  may be attached by pillars  52  to a plate  54  sealing a port in the service entry plate  62 , part of the structure of the turret outer assembly  24 . As illustrated in  FIG. 4 , such arrangement enables the auxiliary burst disc  50  to be removed easily for replacement, or to allow other service operations to be carried out. 
     As is conventional in itself, the auxiliary burst disc  50  could be fitted with electrical contacts to enable an alarm signal to be sent to the magnet supervisory system in the event of rupture of the auxiliary burst disc. This has the benefit of allowing remote diagnosis of disc rupture, enabling appropriate service action to be planned. In an example embodiment, the disc rupture sensing contacts could be wired in series with a refrigerator pressure sensor input, which may be arranged to switch off the refrigerator  17  in the event of auxiliary burst disc rupture. This would provide for a remote indication of auxiliary burst disc rupture without having to make any change to the magnet supervisory system. In addition, such an arrangement would reduce the chance of air ingress by turning off the refrigerator immediately and so allowing the cryogen vessel pressure to build up until a safety valve opens, which may be quench valve  32 . Cryogen gas will flow out of the cryogen vessel at a rate determined by the thermal influx into the cryogen vessel, significantly reducing air ingress. 
     In an alternative embodiment, a sight glass could be fitted such that a visual inspection may be performed following a quench to determine whether the auxiliary burst disc  50  needs to be replaced. This is particularly simple in the case of arrangements such as shown in  FIGS. 3-4 , as a sight glass may be positioned in the cover plate  54 , directly above the auxiliary burst disc  50 . 
     During ramping of current into the magnet  10 , liquid cryogen  22  is boiled off, and cold escaping cryogen gas cools the auxiliary burst disc  50 . Similarly, the auxiliary burst disc  50  will be cooled when the cryogen vessel  12  is filled, or topped-up, with liquid cryogen  22 . Due to the material properties of a typical burst disc, this cooling will raise the burst pressure of the burst disc by approximately 10-20%. In preferred embodiments of the present invention, the burst pressure of the auxiliary burst disc  50  may be substantially less than the burst pressure of the auxiliary burst disc  46  of the conventional arrangement, as explained above, and so the increase in burst pressure on cooling is proportionately lower. The position of the auxiliary burst disc  50  in the arrangement of the present invention also reduces the significance of the increase in burst pressure. Such temperature-dependent variation in burst pressure of burst discs is well understood among those skilled in the art, such variation may be compensated for during manufacture. The use of INCONEL® austenitic nickel-chromium-based superalloys for the disc material also reduces the effect by up to 50% as compared to other materials commonly used for burst discs, for example stainless steel. 
     As illustrated in  FIG. 3 , but more clearly visible in  FIG. 4 , a tube  64  could be bonded to the auxiliary burst disc  50  to prevent air ingress into the vent path through any gap between the auxiliary burst disc and the tubular positive current lead  21 . When installed, the tube  64  passes inside the tubular positive current lead  21  to a depth beyond the expected depth of freezing of air components, illustrated by region  48  in  FIG. 3 . As illustrated, this may conveniently be achieved by bonding a tube  64  of fiberglass-reinforced plastic (GRP) onto carrier  51  which also carries the auxiliary burst disc  50 . Alternatively, the burst disc carrier  51  could be sealed to the top of the tubular positive current lead. However, in any normal operational situation, the concentration of air in the turret outer assembly  24  would be already at a very low level, so the risk of significant air influx through a small leak may be considered minimal. 
       FIG. 4  shows the auxiliary burst disc being removed for replacement. As shown, it is a relatively simple matter to remove bolts  56  and withdraw cover plate  54 , bringing auxiliary burst disc  50  on its carrier  51  with it. If necessary, the burst disc  50  may be replaced, but more conveniently, the burst disc carrier  51  may be replaced, carrying a new burst disc. Any tube  64  attached to the burst disc carrier  51  may be removed and attached to the replacement burst disc carrier  51 . It may be preferred to simply replace the whole assembly of burst disc  50 , burst disc carrier  51 , pillars  52 , cover plate  54  and any tube  64  when a service or replacement of burst disc is required. 
     In an alternative series of embodiments, as illustrated in  FIGS. 5A and 5B , the auxiliary burst disc  50  is replaced by a valve  68  attached to the upper end of the tubular positive current lead  21 . Such valve may be of similar construction to the quench valve described, or may be any known type of safety valve suitable for carrying the desired flow rate of escaping cryogen. In  FIG. 5A , a suitable spring-loaded flap valve  68  is shown closed, in cross-section, mounted at an upper extremity of the tubular positive current lead  21 . An inner surface  72   a  of the valve is exposed to the interior of the tubular structure, and an outer surface  72   b  of the valve is exposed to the interior volume of the turret outer assembly ( 24 ). 
       FIG. 5B  shows the same valve  68  open. The spring-loaded flap valve  68  includes a valve seat  70  mounted atop the tubular positive current lead  21 . A valve flap  72  closes a through hole  73  in the valve seat. The valve flap is typically pivoted at a pivot  74 , and is urged into its closed position by a coil spring  76 . Such flap valves are common mechanical arrangements, but the materials chosen for the flap valve in the present invention must be suitable for operation at cryogenic temperatures. It may be preferred to mount such as valve atop a tube  64  such as illustrated in  FIG. 4  to enable easy removal and replacement of the valve. However, by its nature, the valve  68  should not need to be replaced at every activation, and mounting direct to the tubular positive current lead  21  as shown in  FIGS. 5A, 5B  may be found sufficient. 
     Similar to the arrangement shown in  FIGS. 3 and 4 , the valve  68  may be mounted onto a carrier  51  which is itself mounted by pillars  52  onto a removable cover plate  54 . 
     Types of valves other than the described flap valve may be used, as appropriate. In some arrangements, the valve may be located within the tubular positive current lead structure  21 . 
     As such a valve would be situated in a cryogen-rich atmosphere within the turret outer assembly, the amount of air that could leak into the vent path may be regarded as insignificant. Also, this valve would preferably be self-closing, removing the need for inspection and replacement which is necessary where auxiliary burst discs  50  are used. 
     While the present invention has been described with reference to a limited number of specific embodiments, numerous variations will be apparent to those skilled in the art, within the scope of the appended claims. For example, the auxiliary burst disc  50 , or a valve performing its function, need not be positioned at the top of the positive current lead  21 , but may be positioned at any convenient position along the auxiliary vent path  42  between the lower end of the positive current lead  21  and the quench valve  32 , such as inside the positive current lead  21 . Although the present invention has described feature  21  as a “positive” current lead, this term is used in a descriptive, not limiting, manner reflecting present conventional electrical arrangements. The present invention may equally be applied to situations in which the magnet  10  is connected to a positive supply terminal through the material of the cryogen vessel, while a negative current lead, similar to feature  21  shown in the drawings, may provide connection between the magnet and a negative supply terminal. Furthermore, the present invention may employ a tubular structure similar to that illustrated at  21  in the drawings but which is not used as a current lead at all. 
     Quench valve  32  may be replaced by any suitable pressure limiting device, for example a simple burst disc. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.