Plasma electron acceleration in a non-resonant microwave heating scheme below the electron cyclotron frequency

Microwave heating of over-dense plasmas is routinely achieved in the stellarator TJ-K. In contrast to usually applied heating scenarios in tokamaks or stellarators, an operational regime is described where the heating occurs well below the electron cyclotron frequency but still well above the lower hybrid frequency. Microwave energy is deposited at the so-called O-resonance, accompanied by a small population of high-energy electrons with an energy up to 100 keV. These electrons are responsible for a significant toroidal net current and the occurrence of hard x-rays. Requirements for achieving this heating regime are described together with a novel scheme for plasma-electron acceleration.