Determination of best operation parameters of electron cyclotron wall conditioning in KSTAR

Dependence of EC injection power (0.6 and 1.4 MW), EC injection width (0.1–1.0 s) and prefilled He pressure (2.8×10−5∼2.2×10−3 mbar) on D2 outgas efficiency is investigated at a toroidal field of 2.4 T in a poloidal trapped particle configuration with an EC wave frequency of 140 GHz (X-mode) and a second harmonic resonance layer near the machine center (1.73 m). With increasing EC injection power, D2 outgas increases clearly. The dependence of the EC power injection is the most pronounced between the scanned parameters. It is found that an optimum prefilled He pressure for D2 outgas exists under the condition of a constant EC injection power. It is observed that both high EC injection power and the optimum prefilled He pressure enhance the ion flux to the center stack, suggesting that the D2 outgas originates from the center stack.