Characteristics, advantages, and possible applications of condensation cryopumping

A few years ago, serious doubts were raised about the possibility of obtaining very low pressures by means of condensation cryopumping. In fact, the equilibrium pressure of condensed H2 was found, in various laboratories, to deviate from the Clausius–Clapeyron law below about 3 K and to reach a temperature independent value ranging from 10−11 to 10−9 Torr. This phenomenon has since been investigated and clarified such that H2 saturation pressures as low as 10−14 Torr have been obtained in our laboratories. In parallel, a drastic improvement of the cryogenic aspect of cryopumping has made it possible to achieve the very high pumping speeds characterizing this pumping technique while still keeping the operating cost very low. The performance of a few cryopumps made at CERN will be presented and critically compared to those of pumps of different types. Although developed for extremely low pressures (below 10−11 Torr), we will show that these cryopumps could be advantageously used for higher pressure applications.