Impurity and recycling control with gettering in ATF

The vacuum vessel of the Advanced Toroidal Facility (ATF) is Ti-gettered with a surface coverage of 70%. The major effects of gettering are: (1) reduction of the oxygen, carbon, and nitrogen content in the plasma and (2) improved density control due to wall pumping of the working gas hydrogen. The overall leak rate in ATF is 2{times}10{sup {minus}4} Torr-l/s which is too high for successful plasma operation. Ti-gettering is routinely employed every morning prior to operation and compensates for this shortcoming by reducing the partial pressure of nitrogen and other residual gas components to the low 10{sup {minus}9} Torr range which is close to the RGA background pressure. Rate-of-rise measurements at this stage show only argon and some methane. The argon is used to monitor the leak rate. In addition to impurity reduction, gettering leads to low recycling of the working gas which appears to be crucial for density control in ATF. The capacity of the gettered surface is large enough to show a strong effect even after 24 hours. An extensive data base on the short-term and long-term effects of gettering on the residual gas composition and its effects on plasma performance has been established over the past threemore » years and will be discussed in this paper. 9 refs., 7 figs.« less