Heat Transfer by the Circulation of Supercritical Helium

Abstract

Since the earliest days of low-temperature research, it has been universal practice to maintain cryogenic environments by immersion into a boiling liquid. Although highly satisfactory for many laboratory purposes, the bath immersion technique has been extrapolated to a number of modern applications for which a forced circulation technique would be considerably more effective. Cases of particular interest include systems requiring continuously variable temperature, systems requiring optical access at large aperture, systems located inaccessibly (masers at the focus of radio or radar telescopes, air-borne detectors, vacuum evaporators, etc.), and systems too massive to be cooled effectively by immersion (for reasons either of cooldown efficiency, local temperature stability, or explosive evaporation hazard). The latter case applies especially to large superconducting magnets where it has been recognized quite recently [¹] that performance depends very critically on the maintenance of a stable thermal environment in the immediate vicinity of the superconducting material. A forced circulation system may prove more effective in providing adequate thermal inertia than reliance on conduction to a local heat reservoir in the form of normal metal.