Research and development of a large capacity, ultra-high-vacuum-compatible liquid helium cryostat using vapor-cooled shields

Abstract

A novel scheme of a large capacity, Ultra-High-Vacuum (UHV) compatible helium cryostat was designed for a Scanning Probe Microscope (SPM) system, using vapor-cooled shields. Three heat exchangers attached to the shields maximize cooling power by vapor from cryogen without the aid of superinsulation or a nitrogen jacket. To reduce conductive and radiative heat transfer into cryogen, the inner part of the cryostat are only supported by three narrow pipes with multi-functions, and double radiation shields are installed around a dewar. Besides, thermal and mechanical elements, such as thermal contraction, heat capacity, and thermo-acoustic oscillations, are considered for the cryostat design. Temperature of each shield at thermal equilibrium and holding time of cryogen was computed by a C/C++ code. The simulation showed that the cryostat is compatible with both liquid helium and nitrogen, and the holding time for 32 L of liquid helium was expected to be 10 days. However, the actual performance was not as good as the expectation due to poor conductance of heat exchangers. Hence, a revised design of heat exchangers was suggested. The validity of the revised design was demonstrated by calcu-lating its conductance and cooling efficiency, and it will be tested in the future.