Survey of investigations on sudden loss of vacuum to air around equipment cooled with liquid helium

Abstract

Several large scale superconducting devices such as high field magnets and SRF particle accelerators use large quantities of liquid helium for cryogenic operation. Large helium containers or dewars are routinely used close by these devices to provide liquid helium buffer. These systems as well as the storage containers have an insulation vacuum space to separate the warm surrounding and the cold helium space. In addition to the insulating vacuum, SRF accelerators have their beamlines immersed in baths of liquid helium. Accidental loss of the insulating or the beamline vacuum is considered to be the worst case failure scenario of these systems. Following an accidental rupture, warm atmospheric air will rapidly flow into the vacuum space and flash solidify on surfaces cooled by liquid helium. The thermal energy released by solidifying air will ultimately make its way to the liquid helium, cause it to violently boil, and the helium containment to pressurize to potentially dangerous levels. Although imperative to design of adequate pressure relief devices, analyzing the effects of such loss of vacuum in real systems (dewars, magnet cryostats, SRF beamline) is inherently difficult due to the complex interplay of heat and mass transfer from solidification of air and boiling liquid helium. Fortunately, the last two decades have witnessed several pilot projects that performed controlled loss of vacuum into well-instrumental liquid helium experiments and incremented the understanding of the subsequent heat and mass transfer, both in air and liquid helium. This paper provides a comprehensive review of these experiments and their findings in order to guide future investigations of this critical safety issue.