Refrigeration – Storage of solidified or liquified gas – Including cryostat
Patent
1994-11-23
1996-07-23
Capossela, Ronald C.
Refrigeration
Storage of solidified or liquified gas
Including cryostat
335300, F25B 1900
Patent
active
055378293
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a cryostat assembly.
2. Description of the Related Art
Cryostat assemblies are used in a variety of fields to provide a cooled region within which is positioned an instrument such as a detector or a magnet particularly where it is desired to provide a superconducting magnet.
One of the problems with conventional cryostats is that they are susceptible to low frequency vibrations which, in the case of, for example, a spectrometer, can lead to deterioration in the accuracy of the results which are obtained. The reason for the deterioration is that the time varying perturbation of the magnetic field on the nuclear magnetic resonance (NMR) or ion cyclotron resonance (ICR) sample which results from the vibrations will lead to undesirable artifacts in the measurements made.
SUMMARY OF THE INVENTION
In accordance with the present invention, a cryostat assembly comprises an outer vessel within which is provided an inner vessel for containing an item to be cooled, the inner vessel being suspended from the outer vessel by a number of low thermal conductivity support members secured at one end to points on the outer vessel substantially at the height of the centre of gravity of the system constituted by the assembly and item in use and at the other end to the inner vessel at points not higher than the said points on the outer vessel.
We have realised that conventional assemblies arrange for the various vessels to be supported towards the top from the outer, usually vacuum, vessel leading to a "pendulum" effect in response to low vibration input. In contrast, the invention arranges for the inner vessel to be suspended from the outer vessel from points between the ends of the outer vessel, typically substantially mid-way along the outer vessel substantially in line with the centre of gravity of the system. A modal analysis of an assembly constructed according to the invention has shown that the lowest modal frequencies are significantly higher with the invention than those obtained with conventional assemblies.
In practice, the one ends of the support members do not need to exactly align with the centre of gravity of the system, and for example the one ends of the support members may be secured to the outer vessel at points situated within 10% of the overall height of the assembly above or below a horizontal plane containing the centre of gravity.
Typically, the other ends of the support members will be connected to the base of the inner vessel but in principle other secure points are possible. In general, the fixing point to the vessel should, as far as vibration reduction is concerned, be strong, therefore the corner of the vessel, or a rigidly supported part of it, would be best. Since the base of a typical Helium vessel is thick, this would be a good point of attachment.
In some cases, the cryostat assembly may comprise just the outer vessel and the inner vessel. Normally, however, additional vessels and/or thermal shields are provided. In the preferred arrangement, the inner vessel comprises a first coolant containing vessel, the assembly further comprising a second coolant containing vessel surrounding the first and suspended from the outer vessel, and optionally a thermal shield surrounding the first coolant vessel and suspended from the second coolant vessel.
In the case where the assembly is to be used to cool a superconducting magnet, the magnet will be placed in the inner vessel with the first coolant typically being liquid Helium, while the second coolant is liquid nitrogen.
Typically, there will be three substantially equally angularly spaced support members suspending the inner vessel from the outer vessel. The ends of the support members attached to the outer vacuum vessel maybe attached via a continuous ring or by individual mounting arrangements.
The term "low conductivity" is well understood in the art, a typical example being 170 watts/meter.
The support members could comprise glass fibre reinforced epoxy resin rods but
REFERENCES:
patent: 2863297 (1958-12-01), Johnston
patent: 3782128 (1974-01-01), Hampton et al.
patent: 4038832 (1977-08-01), Lutgen et al.
patent: 4721934 (1988-01-01), Stacy
patent: 4976110 (1990-12-01), Reid
Daniels Peter D.
Jones Francis J.
Capossela Ronald C.
Oxford Instruments Ltd.
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