Patent Application
20090188261
This invention relates to a superconducting magnet, such as used in a Magnetic Resonance Imaging system and in particular to a cryostat for such a magnet which minimises heating of cryogen held within the cryostat.
Magnetic Resonance Imaging (MRI) imaging systems utilise large superconducting magnets which require cooling to liquid helium temperatures for successful operation. A cryostat is provided to enclose the magnet and to hold a large volume of the liquid helium to provide the cooling. Liquid helium is very expensive and thus the cryostat structure is designed to minimise its loss through heating from the environment where the imaging system is located. A multilayer structure is provided which is designed to prevent heat passing into the helium by conduction, convection and radiation.
The structure comprises a helium vessel which is innermost, a radiation shield spaced apart form the helium vessel, a number of layers of aluminised polyester sheet (Mylar(RTM) foil) and insulation mesh, and then the outer vessel. This structure is evacuated during manufacture to minimise heat transfer from the outer vessel by convection and conduction.
To support the helium vessel in a spaced apart relationship to the radiation shield and the outer vessel it is known to provide a support structure, for example comprising carbon fibre bands. These extend from brackets welded to the outer surface of the helium vessel to brackets formed on the inner surface of the outer vessel. The bands extend through the radiation shield and the various layers of reflective Mylar(RTM) aluminised polyester sheet and insulation mesh at an angle to provide sufficient bracing against movement during transport of the magnet to its site of operation. To cater for the possibility of poor handling during shipping, the bands have to be provided in sufficient numbers and strengths to prevent, or at least restrain, relative movement of the helium vessel with respect to the outer vessel. Five G impacts are factored for in the design although once installed the bands will just have a maximum loading of just one G. Thus, the bands are in effect over-engineered to cater for handling during transport to an extent that far exceeds the loading they will experience once the imaging system is installed.
It will now be appreciated that in order to cater for the handling loads by providing such bands or similar structures, a large number of holes will be created through the insulation and the radiation shield and these will provide pathways for radiation and conduction of heat to the helium vessel which will lead to heating of the vessel. A loss of helium will therefore result which adds significantly to the running costs of the imaging system.
The present invention arose in an attempt to alleviate this problem.
According to the invention there is provided a cryostat comprising a set of superconducting magnet coils, a cryogen vessel for containing cryogen for cooling the superconducting magnet coils, an outer vessel containing the cryogen vessel and an insulation structure disposed between the outer vessel and the cryogen vessel, a support structure within the outer vessel for supporting the cryogen vessel in spaced apart relationship to the outer vessel and a limiter for limiting relative movement of the cryogen vessel with respect to the outer vessel. The limiter has a deployed condition and a stowed condition. When in the deployed condition, the relative movement of the cryogen vessel is limited by the limiter and when in a stowed condition, the relative movement is limited by the support structure. The limiter moves between at least one of the deployed and stowed conditions to the other of the deployed and stowed conditions in response to the generation of a magnetic field by the superconducting magnet coils.
By providing a limiter for limiting the relative movement, it is possible to provide movement limitation during transit. The limiter may be stowed once the magnet has been located at its site of use. This means that the support structure may be optimised for use when the imaging system is installed rather than for catering for excessive loads during transit. Accordingly, the effect of the support structure on the insulation of the cryogen vessel at its site of use is reduced.
In the described embodiment of the invention, the support structure is a set of carbon fibre bands as known in the prior art but these are fewer in number and/or gauge than in known arrangements. Alternative support arrangements, known in themselves, such as carbon fibre rods, steel rods or bands, fibreglass rods or bands, may be used and may each be used in smaller number than in conventional systems, as a result of the present invention. The cross section of the elements of the support structure may also, or alternatively, be reduced. Accordingly, the insulation structure is more efficient since the holes created in it are fewer and/or smaller. Furthermore, the cost of the support structure is reduced. The insulating structure in the described embodiment comprises a radiation shield and layers of aluminised sheet, and is evacuated.
The cryogen vessel in the described embodiment is designed to hold helium but other cryogens may be used depending upon the imaging system magnet properties.
Preferably, the limiter is provided for limiting relative movement of the helium vessel, and is deployed by a spring bias.
Preferably, the limiter will be moved to a stowed position using attractive force provided by operation of the imaging system magnets. This is advantageous since it avoids the need to provide other motive power to return the limiter to a stowed position.
A specific embodiment of the invention will now be described by way of example only, with reference to the drawings of which:
As is shown in
References to “inner” and “outer” refer to the radial direction of the cryostat 1 as a whole.
The helium vessel 2 is supported in a spaced apart relationship to the other components by a series of carbon fibre bands 7. These pass through the radiation shield 4 and the insulation layers 6 between respective brackets 8 and 9 on the helium vessel 2 and outer vessel 5 respectively. According to an aspect of the present invention, the bands 7 are designed to take a loading of only 1.5 G.
Spaced, preferably equiangularly, about the circumference of the helium vessel 2 are three motion limiters 10. These are shown in the figure in their deployed state where they are separated at their inner ends from the helium vessel by a small clearance gap 120 and are fixed into cups 5a in the profile of the outer vessel 5. If the helium vessel 2 moves during transit beyond the dimension of the clearance gap 120 then it will be stopped by the inner end of at least one limiter 10, with the mechanical load transferred outwards into the outer vessel 5 by the limiter.
The piston 101 is preferably hollow. This reduces thermal conduction through the material of the piston. Of course, the piston may be solid, particularly if required to support the necessary mechanical load. Located within a void in the cylinder 106 and preferably immediately below the bearing surface 104 is a deployment mechanism 109. This comprises a disc 110 which includes a step 111 and is rotatable about an axis pin 112. Attached to the disc 110 is a pivot arm 113 carrying at its outer end a ball 114 of ferrous material. An eccentrically located bias riding pin 115 is fixed off axis on the disc 110 and rides as the disc rotates against a leaf spring 116. The leaf spring 116 is fixed between two pins 117 in the cylinder body.
A number of features are provided to reduce heat migration via this mechanism. Firstly, as already described, the materials are chosen to reduce this. In this case, the use of predominantly glass re-enforced plastics material for the cylinder 106 and piston 101. Secondly, the piston inner end area is reduced relative to the rest of the piston, to reduce the transfer of heat to the piston. Thirdly, a layer of reflective foil 118 may be applied to the innermost portion of the cylinder 106. Fourthly, the piston contact area to the cylinder is reduced by the use of the bore riding ring 105 and bearing surface 104. Preferably, the piston wall does not touch the cylinder other than by bore riding ring 105 and bearing surface 104.
To reduce heat transfer even further, the end face 103 is preferably thermally connected by a metallic strip or braid 119 to the radiation shield 4. This cools the end of the piston down to the temperature of the radiation shield itself. Further, the reflective layers 6 abut the end 102 of the piston 101. A reflective layer 118a is preferably provided adjacent the piston on the helium vessel 2.
It will be seen that there is a gap 120 in this deployed state between the helium vessel 2 and the end of the piston 103 to cater for expansion and contraction of the components and to avoid heat being continuously conducted directly to the helium vessel from the piston. However, if during transit the helium vessel moves, it will traverse the gap 120 to abut the piston end 103 and mechanical load will be transferred to the outer vessel 5.
When the cooled magnet is safely located at its operating site, the magnets 3 are ramped up, that is to say, current is introduced and a magnetic field is generated. This results in the ferrous ball 114 being attracted inwards towards the helium vessel 2 by the magnetic field. This in turn causes the disc 110 to rotate in the direction of labelled arrow 121. The disc 110 moves against the spring bias provided by the leaf spring 116 against pins 117 until the step 111 is parallel to the end face 104 and the end face falls back into the step under the action of the piston spring 108. This gives the stowed condition of the limiter as shown in
While the present invention has been described with particular reference to cooled superconducting magnets for MRI imaging systems, it will be clear to those skilled in the art that the present invention may apply to cryogenically cooled superconducting magnets for any purpose, such as nuclear magnetic resonance spectroscopy, particle acceleration and so forth. Furthermore, while the present invention has been described with reference to superconducting magnets cooled by immersion in liquid helium in a cryogen vessel, it will be apparent to those skilled in the art that the invention may be applied to magnets cooled by other cryogens, such as nitrogen, hydrogen, neon, and so on, as determined by the material of the superconducting magnet. Some cooled superconductive magnets are not cooled by immersion in liquid cryogen in a cryogen vessel. Rather, cooling loops or direct refrigeration may be used. In such arrangements, the present invention may be employed to restrain displacement of the magnet, by arranging the limiters 10 to bear against a mechanically robust part of the magnet structure, such as a mechanical former.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0801255.1 | Jan 2008 | GB | national |
