Improvement of wall conditioning of the tandem mirror GAMMA 10 by ECR discharge cleaning

Abstract Wall conditioning and results of ECR discharge cleaning (ECR-DC) in GAMMA 10 are described. Improvement of the wall condition is evaluated quantitatively in terms of residual gas analyses, dynamic pressure, visible spectroscopy and plasma parameters. These results show that light impurities and gas desorption from the wall by plasma discharges are reduced by more than one order of magnitude during a great number of plasma shots. The ECR-DC for 30 h with simultaneous baking considerably improves the wall condition. Light impurities and gas desorption are observed to be strongly reduced to lower levels which have not been attained without ECR-DC. Spectroscopic and diamagnetic loop measurements indicate that the electron temperature as well as ion temperature increases. Consequently, ECR-DC shortens dramatically the wall conditioning period by 2 to 3 weeks which corresponds to more than 1000 pulsed plasma shots.