PSI issues toward steady state plasmas in the HT-7 tokamak

The main efforts of HT-7 superconducting tokamak are directed to quasi-steady state discharges and relevant physics. New doped graphite with an SiC gradient coating as a limiter material and ferritic steel used to reduce the ripples has been developed. Significant progress in obtaining high-performance discharges under quasi-steady state in HT-7 has been realized. The long pulse discharges with Te∼1 keV and central density ∼1×1019 m−3 have been obtained with a duration up to 20 s. Recycling has been studied in LHCD plasmas. The characteristic times of the recycling coefficients seem to be correlated with the physical absorption and adsorption. The uncontrolled density increase is accompanied by the impurity influx originated mainly from the parts of the inner vessel, located far from the plasma edge, which are caused mainly by slow heating by the radiated power and/or fast ion loss.