LIMITER SURFACE OBSERVATION BY INFRARED TECHNIQUES, AES, AND SEM IN THE JFT-2 TOKAMAK

The surface characteristics of molybdenum rail-type limiters in the JFT-2 tokamak have been studied by means of infrared techniques, AES, and SEM. The maximum temperature of the limiter surface during the discharge was 550°C which is low enough to neglect the evaporation. The limiter surface was found to be eroded uniformly by exposure to the plasma, probably by ion sputtering. Microarcing with arc spots of 0.5 - 20 μm diameters was observed on the limiter exposed only to stable discharges. Since the arc current of microarcing is estimated to be 1 - 40 A, the return current can be supplied with electron flow to the nearby area of arc spots. Microarcing as well as ion sputtering may play important roles in limiter material injection into the plasma. © 1979.