Reducing the hydrogen content in liquid helium

TL;DR

This paper investigates the presence of molecular hydrogen in liquid helium, measures its levels, and proposes practical methods to significantly reduce hydrogen content, thereby preventing cryostat blockages and improving low-temperature physics experiments.

Contribution

It introduces the first measurement of hydrogen in helium gas and presents effective strategies to eliminate hydrogen-related cryogenic issues.

Findings

Hydrogen impurities cause cryostat blockages.

Implementing purification and catalytic conversion reduces hydrogen levels.

Blockages have been eliminated for over two years.

Abstract

Helium has the lowest boiling point of any element in nature at normal atmospheric pressure. Therefore, any unwanted substance like impurities present in liquid helium will be frozen and will be in solid form. Even if these solid impurities can be easily eliminated by filtering, liquid helium may contain a non-negligible quantity of molecular hydrogen. These traces of molecular hydrogen are the causes of a known problem worldwide: the blocking of fine-capillary tubes used as flow impedances in helium evaporation cryostats to achieve temperatures below 4,2K. This problem seriously affects a wide range of cryogenic equipment used in low-temperature physics research and leads to a dramatic loss of time and costs due to the high price of helium. Here, we present first the measurement of molecular hydrogen content in helium gas. Three measures to decrease this molecular hydrogen are afterward proposed; (i) improving the helium quality, (ii) release of helium gas in the atmosphere during purge time for the regeneration cycle of the helium liquefier's internal purifier, and (iii) installation of two catalytic converters in a closed helium circuit. These actions have eliminated our low-temperature impedance blockage occurrences now for more than two years.