Cryosorption Pumping of Hydrogen and Helium at 20 °K

Abstract

The feasibility of pumping hydrogen and helium gas in large vacuum chambers by adsorption at low temperatures has been investigated. The adsorptive capacity of seven selected adsorbents for hydrogen and helium was determined under equilibrium conditions at temperatures between 20 ° and 40 °K and at pressures from 3×10−8 to 1×10−3 Tort. The adsorptive capacity for hydrogen on coconut charcoal and on several types of molecular sieves at 20 °K was found to be very large. Above 30 °K, adsorptive capacities were found to be too low to be of practical interest. Cryosorption pumping under constant hydrogen gas leak was demonstrated for three of the adsorbents studied: molecular sieves 5A and 13X, and coconut charcoal. However, only the adsorbent surface directly exposed to the gas flow was found to be effective. The capacity and pumping efficiency of this surface were strongly dependent on its temperature. As a result, the adsorbent must be well shielded from radiation, and good thermal contact between the adsorbent bed and the refrigerant must be assured. It is concluded that cryosorption is a promising technique for pumping hydrogen in large space simulation chambers, but cryosorption pumping of helium at 20 °K does not appear practical.