Enhancement of Noble Gas Pumping for a Sputter-Ion Pump

Abstract

Enhancement of noble gas pumping has been observed for cathodes of different configuration and material. On the basis of the fact that erosion proceeds at the central portion of a cathode exposed to a Penning discharge, while sputtered material piles up at the circumferential area of the cathode, the ratio of the number of sputtered atoms per unit time to the number of impinging gas ions per unit time has been considered to be the controlling factor. A multiholed tantalum plate combined with conventional flat titanium cathode gives a pumping speed for argon about 40% as large as the pumping speed for nitrogen. With a flat tantalum cathode in one side and a flat titanium cathode in the opposite, the pumping speed for argon was about 27% of that for nitrogen. Both pumps, however, showed some 20% to 25% less speed for nitrogen compared with a conventional pump having a pair of flat titanium cathodes. A pump equipped with multiholed copper plates instead of multiholed tantalum plates was also tested, which did show an argon pumping speed twice as large as that of a conventional pump. During a long test run for the so-called argon instability, the multiholed tantalum pump did not show an appreciable pressure pulse at least for 700 h at 2×10−5 Torr with a constant throughput of argon.