GB2348693A - Suspension system for a cryostat - Google Patents

Abstract

A suspension system for a cryostat comprises a strut arrangement (100) for mounting a 'cold' vessel inside a 'warm' vessel. The strut arrangement (100) comprises first and second tensile bands (130, 140) connected together via an intermediate link (106). Each band (130, 140) is carried by respective roller units (124, 126, 142, 144, 170, 172, 174, 176), roller units (170, 172, 174, 176) being supported in the intermediate link (106). Roller unit (124, 126) carrying one end of band (130) is connected to the 'warm' vessel via a clevis/bracket arrangement (120, 122, 122a, 122b), and roller unit (142, 144) carrying one end of band (140) is connected to the 'cold' vessel. Tensile band (130) comprises a glass/epoxy (GRP) composite material which has a relatively low thermal conductivity at high temperatures and tensile band (140) comprises a carbon/epoxy (CFRP) composite which has a relatively low thermal conductivity at low temperatures.

Description

IMPROVEMENTS IN OR RELATING TO SUSPENSION SYSTEMS The present invention relates to improvements in or relating to suspension systems, and is more particularly, although not exclusively, concerned with suspension systems for mounting vessels in cryogenic dewars such as superconducting magnet cryostats for magnetic resonance imaging (MRI) apparatus.

Cryostats for MRI apparatus generally comprise at least two cylindrical vessels, an inner vessel and an outer vessel. The inner vessel is mounted within the outer vessel and is spaced apart from it. The inner vessel, also known as a'cold'vessel contains or supports the superconducting magnet which needs to be kept at an extremely low temperature in order to operate. The outer vessel is also known as a'warm'vessel, which in this case means close to ambient temperature. In order for the apparatus to operate efficiently, the two vessels must be thermally isolated from one another as far as possible. Accordingly, the annular space between the two vessels comprises a vacuum chamber to prevent convective heat transfer between the'warm'vessel and the'cold'vessel. The inner vessel is often suspended within the outer vessel by means of tensile members which have a high aspect ratio to minimise conductive thermal losses.

A'cold'vessel has a temperature typically below 20K and is suspended in a vacuum chamber. One or more radiation shields are placed between the surface of the'cold'vessel and the vacuum chamber to intercept thermal radiation from the'warm'vessel at some intermediate temperature to prevent it reaching the'cold'vessel. Both the'cold'vessel and the radiation shields are suspended by members having a low thermal conductivity.

However, radiation shields and'cold'vessels tend to utilise separate suspension systems which means that there are two separate mechanisms for thermal conduction from the'warm'vessel.

Typical materials used for suspension members are fibre reinforced composites and stainless steel, and the thermal conductivity of these materials changes with temperature to a greater or lesser extent. For example, at room temperature carbon/epoxy (CFRP) composites are an order of magnitude more thermally conductive than glass/epoxy (GRP) composites, yet at 1 OK they are an order of magnitude less thermally conductive.

It is therefore an object of the present invention to provide an improved suspension system in which materials from which suspension members are made are utilised within their optimum temperature range.

It is a further object of the present invention to provide a combined suspension system for both radiation shields and'cold'vessels which optimises thermal efficiency.

In accordance with one aspect of the present invention, there is provided a suspension system for connecting a first element at a first temperature to a second element at a second temperature, the first and second temperatures having substantially different values, the system comprising: a tensile member for connecting the first element to the second element ; first connecting means for connecting the tensile element to the first element; and second connecting means for connecting the tensile element to the second element; characterised in that the tensile member comprises a first component having a first thermal characteristic attached to the first connecting means, and a second component having a second thermal characteristic attached to the second connecting means, the first and second thermal characteristics being substantially different, and in that the first and second components are connected together via an intermediate component which is at a temperature intermediate the first and second temperature.

Advantageously, by combining suspension member materials so that each material is being used within its optimum temperature range, maximum thermal efficiency of a cryostat can be achieved. For example, a suspension member which is made from GRP at its'warm'end and CFRP at its'cold' end has a higher thermal efficiency than one made entirely of one or other material.

Thermal efficiency is further optimised by combining the suspension systems so that the'cold'vessel and the radiation shields share the same members.

Moreover, the higher specific strength of CFRP compared to GRP means that the actual conductivity of components can be even lower at low temperature due to higher possible aspect ratio for the suspension member.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which: Figure 1 illustrates a first embodiment of a suspension member in accordance with the present invention; Figure 2 illustrates a second embodiment of a suspension member in accordance with the present invention ; and Figure 3 illustrates connection of a suspension member to a radiation shield.

Whilst the present invention is to be described with reference to the suspension of cryostats, it will readily be appreciated that suspension members of the present invention can be utilised in any application where the effect of thermal conductivity between elements to be suspended is a concern.

In accordance with the present invention, a system for optimising suspension conductivity by combining materials is described. The system also enables both'cold'vessel and radiation shields to be suspended from the same system.

As discussed above, a cryostat comprises at least two cylindrical vessels, an inner vessel and an outer vessel. The inner vessel is mounted within the outer vessel and is spaced apart from it. The inner vessel is generally known as a'cold'vessel and the outer vessel as a'warm'vessel.

The annular space between the two vessels comprises a vacuum chamber to prevent convective heat transfer between the'warm'vessel and the'cold' vessel. The inner vessel is mounted within the outer vessel by means of a suspension system comprising at least three or six strut arrangements.

However, it will be appreciated that the strut arrangements are substantially identical therefore only one strut arrangement will be described in detail for each of the embodiments below.

It will readily be understood that in order to support the'warm'vessel within the'cold'vessel, the ends of the strut arrangement need to be connected to respective brackets mounted within the'warm'vessel and on the'cold'vessel.

A first embodiment of a strut arrangement 100 in accordance with the present invention is shown in Figure 1. The strut arrangement 100 comprises an outer tensile component 102 and an inner tensile component 104. The terms'outer'and'inner'refer to the location of the component, that is, whether the tensile component is nearer to the'warm'vessel or the'cold vessel'. In this case, the'outer'tensile component is nearer to the'warm' vessel and the'inner'tensile component is nearer to the'cold'vessel. The outer tensile component 102 is connected to the inner tensile component 104 by means of an intermediate link 106.

In this embodiment, component 102 comprises a clevis portion 120 which is anchored to the'warm'vessel (not shown), and a bracket portion 122 connected to the clevis portion 120. The bracket portion 122 is generally U-shaped and has arm portions 122a, 122b. A ball or spherical member 124 mounted on a shaft 126 is located between arm portions 122a, 122b of bracket 122 with the ends of shaft 126 passing therethrough as shown. Ball or spherical member 124 provides a self-aligning support for one end of a continuous band 130. The other end of the band 130 is connected to the intermediate link 106, and will be described more fully below. The band 130 comprises a tensile band and is made of a glass/epoxy (GRP) composite which has a relatively low thermal conductivity at'high'temperature. By the term'high'temperature is meant a temperature which is close to that of the 'warm'vessel-typically ambient temperature, the difference in temperature between the'cold'vessel and the'warm'vessel being typically 289K.

Typically, the bracket (not shown) in the'warm'vessel receives the clevis portion 120 which is then held in place by means of a nut (not shown).

Component 104 comprises a continuous band 140 which is carried by a ball or spherical member 142 which is mounted on a shaft 144, and connected to the intermediate link 106 at the other end. Band 140 comprises a second tensile band and is made of a carbon/epoxy (CFRP) composite which has a relatively low thermal conductivity at'low'temperature. By the term'low'temperature is meant a temperature which is close to that of the 'cold'vessel.

Component 104 is attached to the bracket (not shown) on the'cold' vessel (not shown) by inserting shaft 144 carrying ball or spherical member 142 into a hole (not shown) formed in the bracket.

The intermediate link 106 connects the inner and outer tensile components 102,104 together and provides a rigid connection for radiation shield mounting points. The link 106 comprises a bracket assembly 160 comprising two parallel plates 162,164 which are spaced apart by spacer 166. Each plate 162,164, has respective holes-only holes 162a, 162b, 162c in plate 162 being visible-formed therein for supporting shafts 170,172 of respective roller members 174,176 and spacer 166 as shown. Roller member 174 carries the other end of the band 140, and roller member 176 carries the other end of band 130, the respective shafts 170,172 of the roller members 174, 176 passing through holes 162a, 162b and the corresponding holes (not shown) in plate 164. Spacer 166 passes through hole 162c and the corresponding hole (not shown) in plate 164.

Spacer 166 comprises a block 180 housing a stud member 182 by which at least one radiation shield (not shown) is attached to the strut arrangement 100.

Turning now to the embodiment of a strut arrangement 200 shown in Figure 2. As before the strut arrangement 200 comprises an outer tensile component 202 and an inner tensile component 204 which are connected together by an intermediate link 206. However, in this embodiment, the outer tensile component 202 comprises a continuous band 220 which is carried by a ball or spherical member 222 which is mounted on a shaft 224, and connected to the intermediate link 206 at the other end. Band 220 comprises a tensile band and is made of a GRP composite. Ball or spherical member 222 is attached to a bracket (not shown) formed in the'warm'vessel with shaft 224 engaging a hole (not shown) formed in the bracket.

Component 204 comprises a clevis portion 240 which is anchored to the'cold'vessel (not shown), and a bracket portion 242 connected to the clevis portion 240. The bracket portion 242 is generally U-shaped and has arm portions 242a, 242b. A ball or spherical member 244 mounted on a shaft 246 is located between arm portions 242a, 242b of bracket 242 with the ends of shaft 246 passing therethrough as shown. Ball or spherical member 244 provides a support for one end of a continuous band 250. The other end of the band 250 is connected to the intermediate link 206. In this case, the band 250 comprises a second tensile band and is made of a CFRP composite.

The intermediate link 206 connects the inner and outer tensile components 202,204 together and provides a rigid connection for radiation shield mounting points. The link 206 comprises a bracket assembly 260 comprising two parallel plates 262,264 which are spaced apart by spacer 266. Each plate 262,264, has respective holes-only holes 262a, 262b, 262c in plate 262 being visible-formed therein for supporting shafts 270,272 of respective roller members 274,276 and spacer 266 as shown. Roller member 274 carries the other end of the band 250, and roller member 276 carries the other end of band 220, the respective shafts 270,272 of the roller members 274, 276 passing through holes 262a, 262b and the corresponding holes (not shown) in plate 264. Spacer 266 passes through hole 262c and the corresponding hole (not shown) in plate 264.

Spacer 266 comprises a block 280 housing a stud member 282 by which at least one radiation shield (not shown) is attached to the strut arrangement 200.

Figure 3 illustrates how a radiation shield is attached to an intermediate link. In Figure 3, intermediate link 300 is shown extending through an aperture 302 in a radiation shield 304. It will be understood that intermediate link 300 is identical to intermediate links 106,206 described with reference to Figures 1 and 2, and no further explanation will be given.

Radiation shield 304 has a suspension bracket 306 attached on one surface thereof, the bracket 306 retaining a stud member 308 which extends therefrom. Stud member 308 is attached to block 310 of spacer 312 as shown. Stud member 308, block 310 and spacer 312 are identical to stud members 182,282, blocks 180,280 and spacers 166,266 described with reference to Figures 1 and 2 above.

It will readily be appreciated that the lengths of the inner and outer components 102,104,202,204 are chosen so that the operating temperature of the intermediate link 106,206 will be the same as that of the radiation shield (s) (not shown) attached to the link 106,206 via stud member 182,282.

This eliminates a thermal conduction path between the two components 102, 104, 202, 204 and the radiation shield. Therefore, the cross sectional area of the stud members 182,282 is not critical thermally, and stresses can be optimised.

Alternatively, the lengths of the components 102,104,202,204 can be arranged such that the intermediate link 106,206 is'forced'to the same temperature as the shields via a rigid connection of high thermal conductivity. This further optimises the thermal performance of the strut arrangement.

Moreover, the stiffnesses of the inner and outer components 102,104, 202, 204 are chosen so as to optimise the dynamic characteristics of the system. For a cryostat of a superconducting magnet used for magnetic resonance imaging (MRI), for example, it can be detrimental to the image quality if the radiation shields and'cold'vessel containing the superconducting magnet vibrate independently. This produces magnetic field instabilities due to the generation of eddy currents. By combining the suspension for the'cold'vessel and the radiation shield (s), the generation of eddy currents are reduced and an increased magnetic field stability is obtained.

Although the tensile members have been described as tensile bands, it will readily be appreciated that they could also be tensile rods made from the GRP and/or CFRP as mentioned above.

Claims (10)

1. CLAIMS: 1. A suspension system for connecting a first element at a first temperature to a second element at a second temperature, the first and second temperatures having substantially different values, the system comprising: a tensile member for connecting the first element to the second element; first connecting means for connecting the tensile element to the first element; and second connecting means for connecting the tensile element to the second element ; characterised in that the tensile member comprises a first component having a first thermal characteristic attached to the first connecting means, and a second component having a second thermal characteristic attached to the second connecting means, the first and second thermal characteristics being substantially different, and in that the first and second components are connected together via an intermediate component which is at a temperature intermediate the first and second temperature.
2. 2. A system according to claim 1, wherein the first and second components comprise tensile elements.
3. 3. A system according to claim 2, wherein at least one tensile element comprises a tensile rod.
4. 4. A system according to claim 2, wherein at least one tensile element comprises a tensile band.
5. 5. A system according to claim 4, wherein said at least one tensile band comprises a glass/epoxy (GRP) composite material.
6. 6. A system according to claim 4, wherein said at least one tensile band comprises a carbon/epoxy (CFRP) composite material.
7. 7. A system according to claim 5 or 6, including two tensile bands, one of the tensile bands comprising a GRP composite material and the other tensile band comprising a CFRP composite material.
8. 8. A cryostat comprising an outer'warm'vessel and an inner'cold' vessel, the inner vessel being mounted within the outer vessel by at least one system according to any one of the preceding claims.
9. 9. A cryostat according to claim 8, further comprising at least one radiation shield mounted on the intermediate link.
10. 10. A suspension system substantially as hereinbefore described with reference to the accompanying drawings.