Resonant and Nonresonant Electron Cyclotron Heating at Densities above the Plasma Cutoff by O-X-B Mode Conversion at the W7-As Stellarator

The extension of the experimentally accessible plasma densities with electron cyclotron heating beyond the plasma cutoff density and the removal of the restriction to a resonant magnetic field, both via mode conversion heating from an O-wave to an X-wave and, finally, to an electron Bernstein (O-X-B) wave, was investigated and successfully demonstrated at the W7-AS stellarator. In addition to the heating effect, clear evidence for both mode conversion steps was detected for the first time.