Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
1998-09-21
2001-02-27
McDermott, Corrine (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
Reexamination Certificate
active
06192690
ABSTRACT:
The present invention relates to load bearing means for use in NMR cryostat systems as a support or suspension element.
FIG. 1
shows a typical cryostat for a superconducting magnet with a warm clear bore for NMR applications. The cryostat comprises a nitrogen tank
1
or an 80K aluminium radiation shield, and a helium tank
2
housing the magnet
4
. An intermediate shield
3
is provided usually at a temperature of 42K.
An NMR system provides access to a room temperature bore of the cryostat for introducing an NMR probe to the magnet centre. The probe usually comprises a sample and radio-frequency coils for inducing transverse magnetisation
The various components comprising a cryostat or storage tank for cryogenic liquids are generally either suspended or supported by each other. Thus, a suspension system has various tasks, namely carrying the magnet or shield loads, and e.g. keeping the position of the cold mass during shipping and maintaining the internal clearances between the radiation shields during installation as well as at repeated cooldown or thermal cycling. Meeting all these requirements means that the suspension imposes a heat load onto the internal parts of the cryostat, and this causes a higher boil-off of liquids, although the percentage of total heat load is dependant on the overall size of the system.
Various parts of the cryogenic components such as radiation shields have to be cooled, either by using liquids by means of heat exchange between the evaporating gas and the surface to be cooled, or by other means such as conventional GM-cryocoolers.
The first attempts at installing GM-type coolers were made about fifteen years ago and, up to now, only very few applications use externally installed piston-driven coolers for shield cooling in NMR systems, because the piston movement induces vibrations and eddy currents which in turn affect the homogeneity or the signal resolution, in that the signal is distorted by the superimposed frequency spectrum.
However, due to the absence of moving parts in a pulse tube refrigerator, apart from the pressure wave travelling down the tube, the induced level of vibrations transmitted to the internal cryostat structure is several orders of magnitude lower than with piston-driven coolers. Such pulse tube refrigerators can be directly mounted and very little vibration experienced. Thus it is feasible to apply the whole spectrum of means for directly contacting or coupling a radiation shield to a cooler, such as soldering, bolting, riveting, screwing, clamping, gluing, welding, sliding, pressing, or by means of shrink-fitting or spring-loading, or mechanically by using a lever-actuated contacting system. Most probably, the radiation shield is made of highly-conducting materials such as aluminium or aluminium alloys etc., whereas the helium can incorporating the superconducting magnet is made of stainless steel or epoxy or aluminium or other non-magnetic material or a combination of several different materials.
An aim of the present invention is to combine both aforementioned functions, namely suspending and cooling, and to use the pulse tube refrigerator to effect these tasks.
The pulse tube refrigerator can be arranged in series or in parallel along the axis of a cryostat, or could be single- or multi-staged, or if the second/third stage of the pulse tube reaches a temperature where liquids could be liquefied, could provide an additional liquefying stage with attached heat exchanger. The pulse tube refrigerator could also suspend or support a superconducting magnet where the latter is usually made of NbTi, Nb
3
Sn or HTS material, and is housed within a cryostat structure. Since the pulse tube refrigerator is used as either a suspension or support member and an integral part, or so-called “invisible part” of a cryostat it can also simultaneously cool the shields affixed to it and at the same time carry shield and or magnet loads.
A further aim of the invention is to use a neck pulse tube or a room temperature bore pulse tube for NMR systems as a suspension system.
It is understood and self-explanatory that due to its design a pulse tube refrigerator is defined as being a combination of two tubes, namely the pulse tube itself and the tube housing the regenerator.
According to the present invention, there is provided load bearing means including at least one pulse tube refrigerator arranged to support or suspend members in an NMR system, while simultaneously cooling the members.
The pulse tube refrigerator may be a multi-stage pulse tube refrigerator.
The load bearing means may be a combination of single and multi-stage pulse tube refrigerators.
The pulse tube refrigerator may have different geometric structures.
The regenerator tube of the pulse tube refrigerator is interconnected with the pulse tube may have different geometric structures.
REFERENCES:
patent: 5237825 (1993-08-01), Menzi et al.
patent: 5295355 (1994-03-01), Zhou et al.
patent: 5339650 (1994-08-01), Hakamada et al.
patent: 2 301 426 (1996-12-01), None
Drake Malik N.
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
McDermott Corrine
Oxford Magnet Technology Limited
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