Removal of re-deposited layers and release of trapped hydrogen by He/O-ICR plasma in a HT-7 superconducting tokamak

Oxidation experiments by ion cyclotron resonance discharge in a gas mix of oxygen and helium (He/O-ICR) have been performed in a HT-7 in the presence of a permanent magnetic field of 1.5-2.0 T at wall temperatures of 400 to 470 K. Two kinds of gas mixture ratios of 4 : 1 and 1 : 1 (helium to oxygen) were used. With the same filling rate of oxygen, a higher pressure of He in the He/O-ICR plasma is beneficial for removal of co-deposition and reduces oxygen retention. For the same filling pressure, both the oxygen retention rate and the removal rate of H and C atoms during the He/O-ICR experiment were lower than that in the pure O-ICR experiment. The influence of ICR power and filling pressure on hydrogen and carbon removal rates was analysed. The highest removal rates of H and C atoms up to 5.4 × 10²¹ atoms/h and 7.2 × 10 ²¹ atoms/h, respectively, were obtained in 40 kW He/O-ICR cleaning with a ratio of He/O of 4 : 1 at 9.8 × 10^-2 Pa. By He-ICR cleanings and baking in helium gas, most oxygen retained on the wall was sufficiently removed before the subsequent plasma discharge. High power and high pressure He-ICRF cleanings are effective in removing the oxygen retained in the walls. Plasma discharges could be recovered after a few tens of disruptive plasma discharges.