Electron Heating by ICRF Mode-Conversion Heating in LHD

Electron heating regime using with radio-frequency waves in the ion cyclotron range of frequencies (ICRF) has been investigated in helical magnetic configuration. Mode-conversion scenario was applied to the LHD plasmas. When the wave frequency is change to be lower than that in the standard ICRF heating (minority ion heating) condition, the resonance layers are moved away to the peripheral region of plasma. Two-ion hybrid resonance (mode-conversion) layers still locate inside of plasma and fast waves launched by the ICRF antenna are converted to the ion Bernstein waves at these layers. The plasma is successfully sustained by mode-conversion heating only. The measured high-energy ion component shows no ion tail, which is observed in the minority ion heating case. Position of mode-conversion layer is closely related with the ratio of hydrogen ions to helium ions. Hydrogen ice pellet injection was tested to supply the hydrogen ions in the plasma core region. The plasma, which collapsed without the repetitive ice pellet injection was sustained during mode-conversion heating assisted by the repetitive pellet injection. Feasibility of steady state operation using mode-conversion heating is assessed. The plasma duration more than 80 sec. is achieved so far. The experimental result was compared by using 1D full wave simulation code. The electron absorption profile was slightly different when the hydrogen ion ratio was changed.