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.

The evolution of recycling behaviour has been investigated during long pulse discharges in the HT-7 tokamak after ICRF boronization (C2B10H12) using the H/(H+D) ratio and the edge recycling coefficient R. After boronization, impurity reduction is observed, attributed to the fresh boron film, but the recycling coefficient can exceed unity due to a large amount of hydrogen absorbed in the film, leading to an uncontrollable density rise and discharge termination. When the H/(H+D) ratio was reduced to less than 25%, the electron density was easily controlled. The longest discharge, up to 240 s with central electron temperature Te(0) of about 1.0 keV and central electron density ne(0) of 0.8 × 1019 m−3, was achieved following boronization. After many discharges the effectiveness of boron film was weakened, and the density rise was correlated with an increase in both carbon and oxygen radiation which limited the duration of long pulse 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.

MARFE phenomena on HT-7 tokamak is studied in this paper. A best correlation has been found between the total input ohmic power and the product of the -edge line average density and Z , j j . M.4RFEs which is strongly correlated with impurity density, always occurs at Z,,, = 3 -8 ohmic discharges. In HT-7 tokamak high Z , j j discharges, it is found that the MARFEs usually occur a t values of Z t i ; p in the range of 0.5-0.7, where p = r r ~~n , / ~, ( 1 0 ~~ MA-' m-'). In the case of good wall condition ( Z , j f = 1 -2)) the onset of MARFEs have not been observed before reaching the Greenwald density limit-on HT-7. An improved confinement mode plasma which is induced by the MARFE is observed, the global particle confinement time increases 1.9 times. T h e relaxation time between the MARFE event trigger and the L-H transition is about 1 . 4 ms, the followed L-H transition time is 1.9 ms, the improved confinement mode phase is maintained for about 40 ms. MARFE cools the plasma edge, and the electron density profileis observed to become more narrow and peaking