FAST-WAVE AND ELECTRON-CYCLOTRON CURRENT DRIVE IN THE DIII-D TOKAMAK

The non-inductive current drive from directional fast Alfven and elect ron cyclotron waves was measured in the DIII-D tokamak in order to dem onstrate these forms of radiofrequency (RF) current drive and to compa re the measured efficiencies with theoretical expectations. The fast w ave frequency was 8 times the deuterium cyclotron frequency at the pla sma centre, while the electron cyclotron wave was at twice the electro n cyclotron frequency. Complete noninductive current drive was achieve d using a combination of fast wave current drive (FWCD) and electron c yclotron current drive (ECCD) in discharges for which the total plasma current was inductively ramped down from 400 to 170 kA. For steady cu rrent discharges, an analysis of the loop voltage revealed up to 195 k A of non-inductive current (out of 310 kA) during combined electron cy clotron and fast wave injection, with a maximum of 110 kA of FWCD and 80 kA of ECCD achieved (not simultaneously). The peakedness of the cur rent profile increased with RF current drive, indicating that the driv en current was centrally localized. The FWCD efficiency increased line arly with the central electron temperature as expected; however, the F WCD was severely degraded in low current discharges owing to incomplet e fast wave absorption. The measured FWCD agreed with the predictions of a ray tracing code only when a parasitic loss of 4% per pass was in cluded in the modelling along with multiple pass absorption. Enhanceme nt of the second harmonic ECCD efficiency by the toroidal electric fie ld was observed experimentally. The measured ECCD was in good agreemen t with Fokker-Planck code predictions.