Decrease in trapping effects for off-axis electron cyclotron current drive in high performance plasmas

Increased efficiency of off-axis electron cyclotron current drive (ECCD) has been obtained in discharges in DIII-D with higher electron beta, and these results have been understood by studying computationally the particle fluxes in velocity space driven by the EC waves. The relativistic cyclotron resonance tends to shift away in velocity space from the trapped-passing boundary for low-field-side launch as the electron temperature and/or density rise, increasing the net efficiency toward that of the trapping-free Fisch-Boozer level. Calculations with the CQL3D Fokker-Planck code illustrate clearly that the flux in velocity space due to rf-induced diffusion moves away from the trapping boundary for values of βe which have been realized in the experiments.