High power density electron cyclotron heating and current drive in DIII-D

Numerical calculations of the electron gyro-orbits passing through a mildly diverging, coherent Gaussian beam of X-mode second harmonic electron cyclotron waves in realistic tokamak geometry have been performed, simulating off-axis ECCD experiments on DIII-D. The object of this work is to ascertain whether CD efficiency is increased nonlinearly beyond quasilinear theory. An enhanced absorption would increase the Fisch-Boozer current while reducing the back-ward Ohkawa current, leading to increased off-axis current drive efficiency. At lower, quasilinear power levels, a direct comparison between QL diffusion coefficients obtained using (1) the standard multi-ray-tracing-based CQL3D QL calculation, and (2) the present numerical orbit calculation shows agreement. This result provides an independent validation of the CQL3D model. At higher power, the calculations show that nonlinear modification of velocity-space diffusion from that predicted by rf quasilinear theory sets in at rf injected power above 2 MW. The DIII-D experimental powers of 0.5 MW per gyrotron are well within the quasilinear regime, and modification of tile QL calculated efficiency is not indicated.