The present invention relates to cryostat construction and in particular it is related to the construction of cryostats which are employable in nuclear magnetic resonance (NMR) imaging systems and/or which contain superconducting coils which are cooled by a coolant fluid such as liquid helium. The present invention more particularly relates to a configuration of supporting ties which exhibit reduced cross-sectional area since thermal contraction stresses normally present have been effectively eliminated by the present configuration.
A cryostat is a containment vessel designed to thermally isolate its interior from exterior ambient temperature conditions. To achieve the desired degree of thermal isolation, multiple nested vessels may be employed with each vessel being designed to function at one of a sequence of temperatures, with the interior temperature being the coldest. In order to provide the desired thermal isolation and yet at the same time provide a cryostat which may be readily transportable, even when filled with coolant, minimal mechanical contact between the various inner and outer cryostat vessels is required. Accordingly, a system of ties may be employed. These ties preferably comprise a low thermal conductivity material such as titanium or a glass fiber and epoxy composite. For example, a system of ties may include a set of at least three ties disposed on each end of an annular vessel. Each tie extends transversely from the outer vessel to an interior annular vessel thus providing a mechanical connection between the circumference of the outer vessel and the circumference of the inner vessel. A system of such ties is more particularly described below. However, the introduction of cryogens into the cryostat produces changes in dimension as a result of thermal contraction of the tie material and of the vessels themselves. Accordingly, ties have been required to have larger cross-sections to compensate for the thermal stresses that are present, in addition to stresses due to weight alone and stresses that arise from cryostat transport. However, it is generally undesirable to have a tie exhibiting a cross-sectional area larger than is necessary because of increased thermal conduction through the tie between the interior and exterior cryostat vessels. Accordingly, if the thermal contraction stresses could be eliminated, supporting ties could be employed which exhibit reduced cross-sectional areas and therefore provide greater thermal isolation for the inner cryostat vessel.