Sensor for Distinguishing Liquid-Vapor Phases of Superfluid Helium

Abstract

Liquid helium is an indispensable cryogen for devices operating at temperatures below 10 K. Examples are semiconductor and superconductor far-infrared detectors and superconductive sensors, magnets, and logic elements. For planned scientific experiments associated with space missions, such as the Stanford/NASA gyroscope test of general relativity [¹], the proposed Large Space Telescope, and the Shuttle Infrared Telescope Facility, the use of liquid helium as cryogen, probably as a superfluid, is considered essential or highly desirable. The application of liquid helium in the zero-gravity environment poses problems concerning the spatial distribution of liquid within the dewar, both for thermal reasons and because dynamic behavior may affect the spacecraft attitude control system. A small, high-accuracy, rapid-response low-power-dissipation sensor is needed to determine the distribution of liquid in the flight dewar with minimum disturbance to the fluid. Standard techniques using pure superconductor wire or carbon resistors [^2,3] have proved inapplicable in the enclosed isothermal environment of a superfluid-filled dewar. A new sensor, based on the resistance change of the normal-superconductive transition, has been developed and is described in this paper. The sensor can be made suitable for use in either normal or superfluid helium by proper choice of material for the superconducting coating.

Paper presents results of one phase of research carried out at Jet Propulsion Laboratory, California Institute of Technology, under contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration.