Chapter 2.5 - Cryogenic Pumps

Abstract

Cryogenic high-vacuum pumps provide clean, high-speed pumping of all gases. They function through a combination of condensation and adsorption of gases and vapors on surfaces held at very low temperatures. Most cryopumps employ sets of surfaces held at two specific temperature ranges by a reciprocating mechanical refrigerator using helium gas as a refrigerant. The helium is supplied at high pressure and room temperature by a remote compressor, and returns to the compressor at low pressure but only slightly warmer than room temperature. All cold parts of the refrigerator lie within the vacuum environment of the pump, although there are no moving parts within the vacuum. This approach produces a rugged, cost-efficient vacuum pump that is suitable for many applications. The housing of a cryopump is typically a stainless steel or aluminum vacuum vessel with a high-vacuum flange for mounting to the high-vacuum valve on the system. A two-stage cryogenic refrigerator is bolted or welded into the vessel. The refrigerator surfaces exposed to the vacuum volume are stainless steel. Massive copper heat stations, sometimes plated with nickel for corrosion protection and appearance, are brazed to the cylinder of the refrigerator for efficient heat transfer to the condensing arrays.