Electron cyclotron current drive in a lower hybrid current drive plasma

Abstract

Electron cyclotron (EC) current drive experiments were carried out in lower hybrid current drive (LHCD) plasmas on the WT-3 tokamak. Using oblique injection of EC waves, plasma current is ramped up in a frequency range ω/Ωe ≃ 1.2-1.4, where Ωe is the EC frequency at the plasma centre. This ramp-up is ascribed to selective EC heating of fast electrons in LHCD plasmas via the fundamental EC resonance at the upshifted frequency due to the Doppler effect. Fast electron momentum distributions are estimated from bremsstrahlung hard X ray spectra. These distributions indicate that the parallel momentum input to the fast electrons in the relativistic energy range from oblique EC waves is effective for efficient EC current drive. Current ramp-up is also observed in the second harmonic frequency range ω/ Ωe ≃ 1.9-2.2. In this case, bulk electrons are heated as well as fast electrons and no definite cause of current ramp-up is clarified.