MOMENTUM DIFFUSION IN CURRENT DRIVE BY ELECTRON-PLASMA WAVES

Fast electron acceleration and velocity diffusion are investigated in wavepackets formed by large-amplitude waves with relativistic phase ve locities. In tokamaks, such waves are generated, e.g. in current drive by free-electron lasers where large-amplitude electron plasma waves o r lower hybrid waves are generated. At small field intensities, the ve locity diffusion in wavepackets with Gaussian-like envelopes is found to be well described by the quasilinear approximation. At large field intensities, trapping effects become important, and deviations from qu asilinear diffusion are found. It is demonstrated that these deviation s may have a significant effect on the velocity distribution of the el ectrons. A wave-induced runaway-like phenomenon of relativistic electr ons is discussed.