Intrinsic plasma impurity behaviours are surveyed by passive spectroscopy in different operation conditions, but with emphasis on long pulse discharges in the HT-7 tokamak. Some complicated behaviours and time evolution are shown for carbon and oxygen impurities resulting from plasma–wall interactions. Uncontrollable increase in density is observed, correlated with impurity concentrations, injected energy and local heat flux on limiter. Under certain conditions, high impurity radiation and uncontrollable density can reduce LHCD efficiency and thus lead to discharge terminations. A major aim of RF wall conditioning is to reduce impurity levels in the plasma. Furthermore, a new operation mode has been successfully developed, by means of LHCD power feedback control, to obtain full non-inductive current driven and long duration plasma. As a result, the longest pulse discharge with a Te(0) ∼ 1.0 keV, ne(0) ∼ 0.75 × 1019 m3 has been achieved with a duration of over 306 s.
Core impurity transport in Alcator C-Mod L-, I- and H-mode plasmas