Oxidation experiments associated with ion cyclotron resonance discharges (O-ICR) have been performed in HT-7 in the presence of a permanent magnetic field of 1.5-2.0 T. The influence of ICRH power and filling pressure on hydrogen and carbon removal rates was analysed. Total numbers of 5.70 × 10 22 H-atoms, 1.6 × 10 22 D-atoms and 2.35 × 10 22 C-atoms had been removed within eight O-ICR cleanings. An ∼20.5 nm co-deposited film on average was removed from the limiters and liners with an area of 12 m 2 . About 1.73 × 10 22 O-atom retention in an O-ICR experiment corresponds to a coverage of 1.44 × 10 16 O cm −2 . The highest removal rates of H, D and C-atoms of up to 2.64 × 10 22 atoms h −1 , 7.76 × 10 21 atoms h −1 and 1.49 × 10 22 atoms h −1 , respectively, were obtained in a 40 kW, 9 × 10 −2 Pa O-ICR cleaning, corresponding to a removal rate of co-deposits of about 317 nm/day (7.2 g/day for carbon). In a 50 min He-ICR cleaning after the O-ICR experiment about 5.39 × 10 21 oxygen retention was removed. Also the influence of the oxidation experiment on the subsequent plasma operation was studied. Normal plasma discharges could be recovered after a few hours of disruptive plasma discharges.

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 × 10 21 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.