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

In this work the effect of ray stochasticity on the multipass absorption of lower-hybrid waves, used to drive current in tokamaks, is considered. In toroidal geometry, stochasticity arises as an intrinsic property of the Hamiltonian ray trajectories for lower-hybrid waves. Based on the wave kinetic equation, a diffusion equation is derived, with damping and sources, for the wave energy density in the stochastic layer. This equation is solved simultaneously with the electron Fokker-Planck equation to describe the quasilinear flattening of the electron distribution function and the subsequent modification of the wave damping. Steady-state solutions of this system (obtained numerically) indicate that the spectral gap is filled in a self-regulating manner, so that the boundaries of the diffused wave spectrum are independent of the level of ray stochastic diffusion. This allows the development of a simple (semianalytic) model for the self-consistent wave spectrum and the radial profile of absorbed power.