A fast and easy to use device for testing the quality of liquid helium

Abstract

Measurement systems for low temperature research in material science commonly use 4He evaporation cooling techniques based on impedances made with capillary tubes. These techniques use isenthalpic gas expansion to extend the temperature range achievable from 4,2 K, the liquid helium normal boiling point temperature, down to 1 K. However, they have a worldwide reported issue: the occasional blockage of the impedance that generates nuisances and a waste of helium and financial resources. We have recently shown that the mechanism of capillary blockage is the freeze of molecular hydrogen traces present in the liquid helium bath when the liquid helium is not sufficiently clean. Here we present a simple device based on a pumping system with a micro impedance to test the cleanliness of the liquid helium. We used a Focused Ion Beam (FIB) to drill the impedance hole in a copper foil with a thickness of 35 μm. This device makes possible to test the liquid helium quality in a few minutes before transferring the liquid into a blockage sensitive apparatus.