ECR plasmas for wall conditioning of the HT-7 tokamak

Abstract

Electron cyclotron resonance (ECR) plasmas produced by radio frequency (RF) waves were investigated for wall conditioning, with a toroidal field of 0.066-0.088 T in the HT-7 superconducting tokamak. Effective wall conditioning was obtained. The influences of toroidal magnetic field, radio frequency (RF) powers, pressures and working gas species on removal rates have been investigated by the partial pressures of the relevant gases. It was shown that ECR plasmas were toroidally homogeneous on the HT-7 tokamak, and the radial position of ECR plasmas was controlled by adjusting the toroidal magnetic field. Moreover, ECR plasmas were more localized by imposing a vertical field. In the experiment, RF powers made little difference, whereas higher pressure brought better cleaning. It was also observed that ECR discharge cleaning in deuterium resulted in a higher removal rate than that in helium under similar parameters. It was shown that the removal rate of hydrogen in ECR discharge cleaning is 5-10 times lower than that in ion cyclotron resonance (ICR) discharge cleaning.