Chromium getter studies in the tokamak fusion test reactor

We have studied the effects of the deposition of thin films (—0.1 μm) of chromium onto — 70% of the torus area of the Tokamak Fusion Test Reactor (TFTR). The purpose of these experiments was to test the difference between high surface coverage and high pumping speed gettering schemes with respect to minimizing oxygen impurity generation in high power tokamak discharges. The initial Cr deposition had significant effects on vessel outgassing and subsequent plasma performance: the outgassing of H₂0, CO, and CO₂ decreased by a factor of 10, oxygen impurity radiation decreased by a factor of 2, the plasma Z_eff decreased from 1.3 to 1.1, and the plasma density limit increased by 20%. This improvement correlates with a significant reduction of the edge radiation as the density limit is approached. The effects of the initial and subsequent Cr depositions were relatively long lasting, exhibiting time constants of the order of weeks. We attribute the observed impurity reduction to a modification of the oxide surface on the vessel wall which is apparently a significant impurity source for oxygen.