Observation of lower-hybrid-current drive in the JIPP T-II torus

Abstract

Current driven by injecting lower hybrid waves has been observed in low-density plasmas in the JIPP T-II. It is confirmed that RF-driven current is generated by momentum transfer from lower hybrid waves to suprathermal electrons with an energy of 8–25 keV. The driving efficiencies in tokamak and stellarator configurations are 1 kAkW⁻¹ and 0.3 kAkW⁻¹, respectively, at a power level of 40 kW. Enhanced electron cyclotron emission due to pitch-angle scattering of RF-driven suprathermal electrons is observed, and spikes in loop voltage and X-ray bursts appear coincidentally. In a long RF-pulse, these rapid changes advance to relaxation oscillations. It is concluded that the pulsating changes originate in the instantaneous scattering of RF-driven suprathermal electrons by the unstable waves excited at an anomalous Doppler resonance.