Cryopumping of hydrogen on stainless steel in the temperature range between 7 and 18 K

In cryogenic vacuum accelerator systems operated at temperatures below 20 K, hydrogen is typically the predominant residual gas component. The understanding of the cryopump mechanism of hydrogen on cryogenically refrigerated surfaces is thus a crucial prerequisite for a reliable design of such vacuum systems. Adsorption isotherms represent an experimental approach to understand the cryopump effect as a physisorption process of gases on cryogenic surfaces. Using a specially designed pumping system, the adsorption of hydrogen on an electropolished stainless steel surface was experimentally investigated in the temperature range between 7 and 18 K. The analysis of the measured H₂ adsorption isotherms showed that the Dubinin-Radushkevich-Kaganer (DRK) isotherm, which is commonly used to theoretically describe the cryosorption process in the submonolayer coverage range, can be applied in the entire temperature range studied. The adsorption isotherms agree sufficiently well with the prediction of the DRK theory. Additionally, results on measurements of the sticking probability of H₂ on stainless steel are presented for the above mentioned temperature range. Using the measured isotherms and sticking probabilities, the harmonic mean sojourn times of H₂ molecules cryosorbed on a stainless steel surface were calculated as a function of temperature and surface coverage.