Behaviors of impurity and hydrogen recycling on the HT-7 tokamak

Behaviors of impurity and hydrogen recycling are investigated based on passive spectroscopy measurement on HT-7. During LHCD long pulse discharges, impurity and hydrogen recycling shows complicated aspects in different phases depending on wall properties. An uncontrollable density increase is observed, correlated with impurity concentrations, injected energy and local heat flux on the limiter. Under certain conditions, high impurity radiation and uncontrollable density can reduce LHCD efficiency and thus lead to discharge termination. To reduce impurity levels, RF wall conditioning was applied. After boronization, high hydrogen emission led to uncontrollable density rise. When the H/(H + D) ratio was reduced to less than 25%, the electron density was easily controlled. The longest discharge up to 306 s with central electron temperature Te(0) ∼ 1.0 keV and central electron density ne(0) ∼ 0.8 × 1019 m−3 was achieved. The evolution of recycling behavior has been investigated during long pulse discharges in terms of H/(H + D) ratio and the edge-recycling coefficient R.