Preferential electron-cyclotron heating of hot electrons and formation of overdense plasmas

Three electron-cyclotron-heating techniques, which preferentially couple microwave power to different energy segments of the electron distribution function, have been experimentally investigated in the AMPHED facility [C. Bodeldijk, Special Supplement, Nucl. Fusion 26, 184 (1986)]. Whistler waves launched from the high-field mirror throat are strongly absorbed in a single pass across the resonant interaction layer, producing highly overdense cold background plasma but no relativistic hot particles. On the other hand, ordinary waves launched from the system side wall are only weakly damped, giving rise to local cylindrical cavity modes and preferential coupling to hot electrons in the 100 keV region. Low levels (≤5%) of upper-off-resonance heating power were shown to be most effective for preferential hot-electron plasma formation, with ∼100% of the injected power being absorbed by the energetic electrons.