CHERENKOV ABSORPTION OF SHORT-WAVELENGTH MAGNETOHYDRODYNAMIC WAVES BYELECTRONS IN A TOKAMAK

Cherenkov absorption of short-wavelength (k(A)a much greater than 1, w here kA = omega/v(A)) magnetohydrodynamic waves (Alfven and fast magne tosonic waves) by electrons in a tokamak plasma is studied in the quas ilocal approximation, which is valid when k(\\)qR much greater than 1, where k(\\) is the longitudinal wavenumber, q is the safety factor, a nd R is the major radius of the tokamak. In the range of fairly high f requencies (x(0) = (omega root k(\\)qR)/(k(\\) v(Te) root epsilon(t)) much greater than 1), the longitudinal velocity of the electrons that absorb the wave is close to the local value of the longitudinal phase velocity of this wave, v(\\) approximate to omega/k(\\). In the course of interaction between these electrons and the wave, the velocity v(\ \) Of an electron moving along the magnetic held lines changes only sl ightly, and efficient energy exchange between the electrons and the wa ve occurs over short distances along the electron trajectories. In thi s case, the wave damping rates caused by circulating and trapped elect rons are derived separately The total damping rate is found to be the same as that in a uniform magnetic field. In the low-frequency range ( x(0) less than or similar to 1), wave damping is governed by the trapp ed electrons that interact resonantly with the wave near the turning p oints and the damping rate is of the same order of magnitude as that i n a uniform magnetic field.