A vacuum cryopump typically consists of a vacuum-tight vessel with a valved inlet, containing a highly absorbent material such as a synthetic zeolite microporous on the scale of 1 to 10 Å (Angstrom units), and an enclosing cryogenic vessel. In industrial cryopumps, the expansion of compressed helium is used to cool a cold head, so the process is contiuous. Although the gas is trapped in the cryopump throughout normal operation, the cryopump can remain cold for months or even years in normal high and ultrahigh vacuum operation. At some point, the pump is shut down, and allowed to heat up. The trapped gasses evaporate and are flushed out, a process know as regeneration. Since cryopumps don't use any oil in the vacuum side, they are used when very clean pumping is needed.
Sorption pumps are a type of cryopump that is often used as roughing pumps to reduce pressures from the range of atmospheric to on the order of 10-3 torr, while lower pressures are achieved using a finishing pump (q.v. vacuum). As the sorbent saturates, the effectiveness of a sorption pump decreases, but can be recharged by heating the zeolite material (preferrably under conditions of low pressure) to a temperature near but below the breakdown point of the zeolite material's porous structure.
