Behavior of titanium sublimation and sputter ion pumps in the 10-11 torr range

In the proposed Intersecting Storage Accelerator, ISABELLE, 1200 pumping stations will be used to keep two 3 km long rings at a pressure of 1 x 10-11 torr. It is clear that in such a large system, price and simplicity are very important. A study was made on a well instrumented room temperature vacuum system with a surface area of 6000 cm2 and a volume of 17 l, pumped by a Ti-ball and an ion pump. Total pressure and residual gas composition were measured by a Helmer gauge and a quadrupole mass spectrometer. The final pressures of approximately 1 x 10-11 torr were reached with Ti films and a 20 l s-1 sputter-ion pump. The main residual gas components were hydrogen and methane. The ultimate pressure was determined primarily by the ability of Ti films to pump H2 which depends on the method of laying down the film and the surface temperature. Previous surface preparation seemed relatively unimportant. Two methods of pumping CH4 were investigated: (1) cracking the molecules on hot tungsten filament; and (2) conventional sputter-ion pumping. Sputter-ion pumping was found to be more efficient and preferable