ROOM-TEMPERATURE PUMPING CHARACTERISTICS FOR GAS-MIXTURES OF A ZR-AL NONEVAPORABLE GETTER

The main pumping for the vacuum system of the large electron positron (LEP) collider, which has been commissioned at CERN in the Geneva area, is achieved by a linear getter pump. In distinction with the usual operating conditions of nonevaporable getter (NEG) pumps, which consist in maintaining the getter hot to continuously diffuse into the getter bulk the gases adsorbed on its surface, the LEP pump will be kept at ambient temperature and heated only intermittently. This difference has required a study on the behavior of a NEG pump under these unusual operating conditions, which result in a progressive reduction of pumping speed with increasing quantity of the gas adsorbed. It was found that the pumping speed reduction may be described in terms of a progressive saturation of the NEG surface inside the cracks and porosities which are present in the getter film thickness. The formulation developed permits one to reconstruct the pumping curves of a NEG, provided that a few basic geometrical and physical quantities are known. This description, which had been applied to individual gases in a previous paper, is extended here to the case in which mixtures of different gases are pumped. © 1990, American Vacuum Society. All rights reserved.