An experimental set-up to study the blockage problem in the flow of liquid helium (LHe) in small impedances

Abstract

The presence of Hydrogen (H2) traces in LHe has disastrous consequences for some laboratory equipment such as evaporation cryostats, PPMS and MPMS. The working principle of those equipment to achieve temperatures below 4.2 K consists in pumping LHe through a capillary tube. If LHe contains H2 traces, that capillary will get blocked after some time. In order to unblock it, it is needed to heat the equipment at room temperature (RT) with the loss of liquid He and time. With the aim to solve this problem, the University of Zaragoza in collaboration with Quantum Design company developed a method to produce H2 free LHe. In this work, we present a study to determine what is the minimum H2 concentration in He gas that can generate blockages when the He gas is liquified. To carry out this proposal, we have developed an experimental setup to produce LHe contaminated with H2 in a controller manner. This setup can be divided in two stages (see figure): the first one consists in mixing the He gas (at RT and pressure larger than atmospheric) with a known H2 concentration to then be liquified (at 4.2 K and atmospheric pressure), whereas the second one consists in the detection of H2 traces present in the liquified He-H2 gas mixture by using a H2 detector developed by us, whose effectiveness has been proved in other labs around the world. This device consists of a flow microimpedance assembled to a stainless steel thin tube. The microimpedance is immersed in LHe while the other end of the tube is connected to a vacuum pump and a pressure sensor. If the tested LHe contains H2 traces (with an enough concentration to produce blockages), the pressure reading will decrease (in a few minutes) to the baseline of the vacuum pump.