STATISTICAL-THEORY OF WAVE-PROPAGATION AND MULTIPASS ABSORPTION FOR CURRENT DRIVE IN TOKAMAKS

In this work the effect of ray stochasticity on the multipass absorpti on of lower-hybrid waves, used to drive current in tokamaks, is consid ered. In toroidal geometry, stochasticity arises as an intrinsic prope rty of the Hamiltonian ray trajectories for lower-hybrid waves. Based on the wave kinetic equation, a diffusion equation is derived, with da mping and sources, for the wave energy density in the stochastic layer . This equation is solved simultaneously with the electron Fokker-Plan ck equation to describe the quasilinear flattening of the electron dis tribution function and the subsequent modification of the wave damping . Steady-state solutions of this system (obtained numerically) indicat e that the spectral gap is filled in a self-regulating manner, so that the boundaries of the diffused wave spectrum are independent of the l evel of ray stochastic diffusion. This allows the development of a sim ple (semianalytic) model for the self-consistent wave spectrum and the radial profile of absorbed power.