Effect of Particle Transport on the Measured Electron Cyclotron Current Drive Profile at High Relative Power Density

Recent experiments on the DIII-D tokamak have examined the effect of particle transport on the electron cyclotron current drive (ECCD) profile using measurements of the magnetic field pitch angles by motional Stark effect polarimetry. While previous ECCD studies on DIII-D did not observe any clear effects of transport, these new experiments at high ECCD power, low density, and radiation temperatures above 20 keV clearly demonstrate that the ECCD profile can be reduced and broadened compared to the Fokker-Planck code CQL3D predictions assuming no radial transport. A diffusion coefficient of [approximate]0.4 m² /s is required in CQL3D to reproduce the experimental ECCD profile at high relative power densities, while smaller diffusion coefficients are needed at low relative power densities. This level of transport is comparable to the effective particle transport rate needed to maintain the density profile but an order of magnitude less than the electron thermal diffusivity. While radial transport of the current-carrying electrons is potentially detrimental for applications that rely on strong localization of the noninductive current, this effect should be negligible on ITER owing to its large size and low relative power density.