Hydrogen contamination in liquid helium

Abstract

Abstract For some years now, massive problems are often occurring when operating standard flow cryostats with pumped liquid helium. There are a large number of reports from various parts of the world, describing blockage problems arising typically within a few hours. Standard equipment for measuring material properties at temperatures below 4.2 K is massively affected. Hydrogen contamination within the used liquid helium could be identified as direct cause. With helium evaporating in the narrow throttling passages, hydrogen in solid state accumulates and is forming blockages. Usually internal capillaries or inlet valves are concerned. In consequence, the helium flow is reduced or ceasing completely, the operating temperature of the cryostat can’t be upheld, and the measurement run must be interrupted. Extremely low concentrations in the sub-ppm range are sufficient to block the helium flow within a few hours of operation. This contribution contains first quantifications regarding these contamination. A semi-quantitative analysis method using a narrow flow resistor, as well as gas chromatographic investigations led to new findings. Effects within the liquid helium supply that give rise to the problem were scrutinized. The collected results should lead to an understanding and to feasible solutions of the problem.