Discharge cleaning experiment in the JFT-2 tokamak with surface observation by AES

The effects of wall conditioning by tokamak discharges with low toroidal magnetic field have been studied by observing the concomitant changes in discharge characteristics, wall conditions, and residual gases. After 2500 shots of discharge cleaning, stable reproducible plasmas with Zeff=4.5 were obtained with the equilibrium surface composition of vacuum wall samples which consists of the limiter and wall materials (molybdenum and 304 stainless steel, respectively), carbon, and oxygen. Depth profiling by Auger/sputter techniques shows that the mixed layers composed of the same elements as the surfaces were formed on the samples. The layers are thicker by 5–10 times on the sample located at the minimum inner radius of vacuum chamber than on the one located 3.4 cm further away from the plasma. The results indicate that the wall condition is determined by deposition of the metallic particles during the discharge and adsorption of the residual gases during the interval between discharges.