The present invention relates to cryostat vessels for retaining cooled equipment such as superconductive magnet coils. In particular, the present invention relates to access arrangements for cryostat vessels, which enable electrical current leads to enter the cryostat vessel to supply current to the cooled equipment; venting arrangements allowing cryogen gas to escape from the cryostat, and providing access for refilling with cryogen when required; and turret arrangements for retaining refrigerators in thermal contact with the cryogen.
A negative electrical connection 21a 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
The present invention aims to overcome or at least alleviate numerous identified disadvantages of the conventional design. The present invention aims to allow the access turret to be moved from the top of the system to the side, combined with the refrigerator turret. This provides reduced overall system height and offers benefits in ease of manufacture and reduction of scrap as will be described below. The conventional separation of the access turret and the refrigerator turret means that two separate access ports (holes) must be provided in the cryogen vessel. The present invention aims to reduce this to a single access port. This will simplify assembly of the cryogen vessel and reduce thermal influx to the cryogen vessel by reducing the number of thermal paths into the cryogen vessel. Each port needs to be sealed during final assembly of the cryostat by welding of the appropriate turret, and welding into position of vent tube 20 and refrigerator sock 15. Such welding, to thin-walled components, is difficult to achieve, and is the source of some manufacturing difficulties, reworking and scrap. The present invention also aims to eliminate the need for welding to thin-walled turrets during final assembly of the cryostat. Electrical connections have conventionally been provided to superconducting magnets within cryostats as follows. Referring briefly to
A disadvantage of the conventional termination configuration is that the contact resistances of the joints between the flexible current leads 21, 21a and the vent tube 20 and auxiliary vent 40 dissipate heat at the base of the vent tube 20 within the cryogen vessel 12. This raises the temperature of adjacent cryogen gas during ramping, through conduction and convection of cryogen gas in the cryogen vessel. Typically, existing systems are intended to operate with cryogen vessel gas temperatures of order 5 K for typical liquid helium cryogen. Variance in contact resistance at the point where flexible leads 21, 21a from the magnet are connected to vent tube 20 and auxiliary vent 40 causes power dissipation during ramping, and far higher cryogen gas temperatures than intended, on some systems. This is known to result in excessive quenching frequency and a number of cryostat reworks. Higher stability outer coils are conventionally provided to compensate for this.
Returning to
The present invention accordingly provides methods and apparatus as defined in the appended claims.
The above, and further, objects, characteristics and advantages of the present invention will become more apparent from consideration of the embodiments described below, given by way of examples only, together with the accompanying drawings, wherein:
Conventionally, the access turret 19 and refrigerator turret 18 are two separate entities which require two ports (holes) in the cryogen vessel 12 and some awkward welding and assembly operations, to assemble the respective turrets to the cryogen vessel. As discussed, this also leads to significant amounts of current flowing through the material of the cryostat and possibly also the refrigerator.
The present invention provides a turret sub-assembly replacing the conventional access turret and a refrigerator turret, which contains a vent tube and a refrigerator sock as well as provision for electrical connections to the magnet. The turret sub-assembly can be built and tested before being assembled as a single unit to the cryogen vessel. This provides a simpler more robust build sequence, being a feature of the invention. By testing the turret sub-assembly before assembly to the cryogen vessel, observed defects can be rectified, avoiding damage or scrap of the cryogen vessel in the case of a fault. The turret sub-assembly can be leak tested offline, before assembly to the cryogen vessel, reducing the risk of failure on the cryogen vessel when rectification is more difficult and expensive. Many of the formerly difficult assembly operations such as welding thin walled components are performed during manufacture of the turret sub-assembly, with a relatively simple process remaining for mounting the turret sub-assembly onto the cryogen vessel.
The termination box 30 accordingly serves as a common interface between the vent tube 32, refrigerator sock 34 the cryogen vessel and the OVC.
The turret sub-assembly 24 of
Particular advantages of the present invention flow from arrangement of electrical connections within the terminal box 30. As with conventional fixed current lead (FCL) designs, flexible current leads from the magnet must be terminated onto the fixed current leads of the vent tube 32 and auxiliary vent 40. As illustrated in
According to a preferred feature of the present invention, flexible current leads are joined to the auxiliary vent 40 and the vent tube 32. More preferably, these joints are located inside the termination box 30. This may be by any usual means such as bolting, soldering, welding, braising. Any heating caused by the resistive nature of the electrical connections between the flexible current leads and the auxiliary vent 40 and the vent tube 32 then takes place within the termination box 30. This heat is conducted to the refrigerator or taken by cryogen gas escaping through the vent tube 32 or auxiliary vent 40, or is absorbed in latent heat of evaporation of liquid cryogen partially flooding the termination box 30. Little of such heat will reach the cryogen vessel to heat the cryogen therein.
Since the negative current path is typically through the material of the cryostat, most of the negative return current passes through the material of the refrigerator sock 34 and vent tube 32. The close proximity of the refrigerator sock 34 to the negative current lead termination in the termination box 30 minimises the current flow through the cryogen vessel, reducing the heating effect on the cryogen vessel as compared with conventional arrangements such as shown in
In operation, the termination box 30 is preferably partially flooded with liquefied cryogen so as to cover the negative lead termination, thereby eliminating the negative lead connection as a source of heating to the cryogen gas in the cryogen vessel.
Conventional arrangements such as shown in
The arrangement of the present invention minimises the generation of warm gas in the cryogen vessel, enabling significant potential reductions in magnet wire costs with improvements in recondensing margin, that is, the required power of the recondensing refrigerator, and ease of assembly of the cryostat as a whole. The improved thermal environment during ramping could avoid the need for the known higher stability outer coils, conventionally provided to compensate for instabilities caused by heated gas in the cryostat. In turn, this has been determined to enable a cost saving of the order of GB£1000 (US $2000) per magnet assembly in superconducting wire costs for the outer coils.
Typically, the components illustrated in
By combining the access turret 32 and refrigerator turret 34 into a single turret sub-assembly 24, the present invention enables a more robust manufacturing route, at least in that no welding of thin walled components is required during assembly to the cryogen vessel. The combination of the conventional access turret 19 and refrigerator turret 18 into a turret sub-assembly 24 provides better access to the thin walled components for welding and assembly operations. This means that the likelihood of a failed weld is reduced, and the consequences of such a failed weld are not as severe as in the conventional manufacturing route, as only the turret sub-assembly 24 need be re-worked, with no damage to the cryogen vessel.
Close coupling of the vent tube and refrigerator sock has a number of other advantages. As illustrated in
As is well known to those skilled in the art, turret components such as vent tube 32 and refrigerator sock 34 represent paths for heat influx to the cryogen vessel. Such turret components are accordingly relatively high temperature components. The use of the turret sub-assembly 24 of the present invention, comprising termination box 30, serves to separate relatively high-temperature turret components from the cryogen vessel. This avoids a significant portion of the known problem of heating of cryogen gas in the cryogen vessel by thermal influx through the material of the turret components. This usefully enables cheaper magnet designs, since an equivalent cooling may be achieved with a less-powerful refrigerator. The reduced heating of the cryogen gas inside the cryogen vessel also reduces the likelihood of magnet quench.
A significant advantage provided by the present invention lies in the improved assembly method, particularly when joining the turret sub-assembly 24 comprising vent tube 32 and the refrigerator sock 34 to the cryogen vessel 12. As shown in
As illustrated in
Final assembly is accordingly rendered far simpler than in the conventional arrangement wherein thin walled vent tube 32 and refrigerator sock 34 are welded into ports on the cryogen vessel, separately and in difficult welding operations. By contrast, the present invention requires only a single welding operation of relatively thick-walled components which are easily accessible through and/or around the termination box.
In the final assembly, both the vent tube with auxiliary vent and the refrigerator sock are located towards the side of the cryostat, rather than being located at the top. This enables the overall height of the system to be reduced and access to the refrigerator and vent tube is simplified, making servicing operations simpler. As will be described below, the present invention also provides advantages in location of, and access to, electrical connections to the magnet.
Advantages provided by the present invention include the following:
Relatively high temperature components such as turret and electrical connections are placed remote from the cryogen vessel, in the path of escaping cryogen gas, thereby reducing heat input to the cryogen vessel.
Close thermal coupling of the vent tube and the refrigerator sock improves cooling of the vent tube, requiring less cooling power from the refrigerator and hence improving the recondenser margin.
The electrical termination points of flexible leads can be welded or bolted, increasing reliability of the joints, and reducing the resistance of the joints which in turn reduces heat generation within the system.
By situating the flexible current lead terminations nearer to the bottom of the cryogen vessel, reduced lengths of uncontrolled flexible current leads are present in the cryogen vessel.
By providing for partial flooding the termination box, electrical connections of flexible current leads to the access turret and access tube may be contact cooled by liquid cryogen.
Coupling the access turret and refrigerator turret together in proximity to both positive and negative electrical terminations reduces current flow through the cryogen vessel. Conventionally, the negative earth point is located on the refrigerator turret 18 and the refrigerator itself is plugged in to the refrigerator turret and hence earthed, so current flows through all parts of the OVC, refrigerator and refrigerator turret.
By providing both positive and negative electrical connections in close proximity to grounded components such as the refrigerator and refrigerator sock, the current path through resistive elements is shortened and heat influx to the cryostat is reduced.
The final assembly process is lower risk, more repeatable and requires less time than existing design, since the turret sub-assembly is pre-tested, and the final assembly of the turret sub-assembly onto the cryogen vessel is a simple welding task. Only one port in the cryogen vessel needs to be sealed, as opposed to the two ports required in the conventional arrangement of separate refrigerator turret and access turret.
The relocation of both vent tube and refrigerator sock to the side of the cryostat improves access to these components for easier servicing. Such arrangement also enables simpler and smaller looks covers, improving the aesthetic appearance of the final system, and reducing patients' fear of the system by making it appear smaller.
For fixed current lead (FCL) designs, there is a requirement to extend magnet current leads from the magnet to the base of the vent tube. The body of the cryostat itself typically serves as the negative terminal. Conventionally, flexible current leads 21, 21a extend from the base of the magnet and is bolted to the base of the vent tube 20 and auxiliary vent 40, as shown for example in
A disadvantage of the conventional flexible lead termination arrangement as illustrated in
An aspect of the present invention provides an arrangement which combines the functionality of the auxiliary vent 40 and current leads to minimise the heat input to the cryogen vessel during ramping, reducing the likelihood of quench during operation and reducing risk of errors during assembly.
An embodiment of the present invention illustrating this aspect is schematically shown in
In further contrast with conventional arrangements, the negative lead connection point 66 is displaced away from the interior of the cryogen vessel 12. Rather, the negative lead connection point 66 is exposed to a flow of cryogen gas up the vent tube 32 and auxiliary vent 40. The negative lead 64 may be connected to the vent tube 32, as shown in
The turret sub-assembly 24 with termination box 30 configuration of the present invention enables welding or other connection of a joint 40b joining the auxiliary vent extension piece 40a to the auxiliary vent 40 and bolting of the negative current lead at the relevant connection point 66 once the turret sub-assembly 24 has been mounted to the cryostat. Contact resistances for both positive and negative current leads are less variable than for conventional soldered designs.
Cryogen gas escaping from the cryogen vessel 12 passes through and around auxiliary vent 40 and its extension piece 40a, offering efficient cooling and removal of any heat generated by current flowing through the auxiliary vent and its extension piece.
In an alternative arrangement, the negative lead connection point is provided at an interface between the magnet former and the interior surface of the cryogen vessel, or with a short flexible lead to the interior surface of the cryogen vessel. In solenoidal-type arrangements, where the cryogen vessel is hollow cylindrical, the negative lead connection point may be provided on the interior surface of the cryogen vessel bore. Such embodiments are advantageous in that current flows through the material of the cryogen vessel and through the cryostat without direct warming of the cryogen gas. The negative lead connection point may even be arranged to be cooled by direct contact with liquid cryogen. Such improvements to the thermal environment of the coils during ramping become increasingly important when minimum cryogen inventory systems are considered. A secondary effect of such arrangements is that assembly of the access turret is simplified, where space is critical at the turret-cryogen vessel interface, as no negative lead connection need be established at that position. Such connection arrangements may be used independently of the positive connection arrangements employing the auxiliary vent described above, and independently of the turret subassembly of the present invention.
This aspect of the present invention accordingly provides a novel arrangement for the auxiliary vent and current lead assembly in fixed current lead access turret arrangements. The novel arrangement minimises the generation of warm gas in the cryogen vessel and combines the functionality of components, reducing cost and complexity. A simpler manufacturing process is enabled.
The present invention enables a low-cost fixed current lead (FCL) turret design, in turn enabling cheaper magnet designs which are more predictable in performance and less likely to require reworking during manufacture.
While the present invention has been described with particular reference to certain embodiments, it will be apparent to those skilled in the art that many variations of the described embodiments are possible, and remain within the scope of the invention as defined by the appended claims.
While specific reference has been made to helium cryogen, it will be apparent that any suitable cryogen may be used. References to “positive” and “negative” current leads, terminations and so on are used as convenient labels only, reflecting common practice in the art. Of course, the positive and negative electrical connections may be reversed, without departing from the scope of the present invention. If required, alternating voltages and currents may be applied to the described current leads, terminations and so on, without departing from the scope of the present invention.
Number | Date | Country | Kind |
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0618141.6 | Sep 2006 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB07/50538 | 9/13/2007 | WO | 00 | 3/12/2009 |