ENERGY-TRANSPORT BY LOCALLY TRAPPED ELECTRONS DURING ELECTRON-CYCLOTRON HEATING OF STELLARATOR PLASMA

An increase in the energy transport by the locally trapped electrons i n the active electron-cyclotron heating stage is considered as a possi ble channel of fast input-power losses. The two-dimensional Fokker-Pla nck equation is used to describe the evolution of the trapped electron distribution function due to microwaves and Coulomb collisions. Expre ssions for the quasi-linear diffusion coefficient are obtained for qua si-transverse propagation of both the ordinary wave at the fundamental harmonic of the cyclotron frequency and the extraordinary wave at the second harmonic. These expressions incorporate the magnetic field inh omogeneity along the force lines, relativistic corrections, inhomogene ity of the microwave intensity associated with the finite electromagne tic field width, and spatial wave damping. Numerical calculations are performed using the approximation of the steady-state distribution fun ction and assuming that there is only a radial drift of the trapped el ectrons. The calculations show that the effect under consideration can substantially increase the input-power losses when the single-pass co llisionless wave damping is not too large.