Role of pre-ionization in NBI plasma start-up of Heliotron J using non-resonant microwave heating

Abstract Here, we report on role of pre-ionization using non-resonant 2.45 GHz microwave heating ( P < 20 kW) in plasma start-up neutral beam injection (NBI) for heliotron configurations low power NB ) under condition. A rapid electron towards burn-through the low- Z impurities was observed experimentally early phase when seed density produced by enough start-up. Beam time to increased with decreasing and a critical condition successful experimentally. Proper timing gas fuelling is expansion because not significant. 0-dimensional (0D) model analysis NBI developed this study well reproduces experimental results. The 0D clarifies physical mechanism described as follows: (1) produces sufficient ions immediately after injection, (2) heat up electrons that promote ionization/dissociation background neutrals, (3) process acts positive feedback loop resulting further burn-through. shows corresponds state at which equal loss due conduction ionization/dissociation.