TIME-RESOLVED MEASUREMENTS OF IMPURITY DEPOSITION IN ISX

Time-resolved measurements of impurity deposition were made in the plasma edge region of ISX-A. A cylindrical sample located 20 mm behind the limiter was exposed to the plasma in synchronization with the discharges to give a time resolution of ~8 ms. The cylinder was withdrawn to an analysis chamber where the impurities deposited as a function of time were determined by Auger electron spectroscopy. Later analysis by Rutherford ion backscattering was used to quantify the results. The primary metallic impurities were identified as titanium, iron and chromium. The time evolution of these impurities is correlated with conditions in the plasma during the discharge. All the impurities observed were found to accumulate on the sample at a minimum rate during the stable central portion of the discharge, and at a maximum rate at times when the plasma was unstable. © 1979.