Role of the relativistic mass variation in electron cyclotron resonance wave absorption for oblique propagation

The role of the relativistic mass variation on wave absorption in the electron cyclotron range of frequencies is investigated. It is first shown that the validity of the nonrelastivistic linear dispersion relation for a Maxwellian plasma is restricted by the conditions N2∥≫Te/mc2 and N2∥≫‖1−ω2c/ω2‖. A numerical investigation of wave damping in a plasma slab located in an inhomogeneous tokamak-like magnetic field shows that for most angles of practical interest the latter condition is easily violated and, therefore, the nonrelativistic dispersion relation yields inaccurate results. The problem of the validity of the nonrelativistic quasilinear equation for oblique propagation is also discussed. Using a quasilinear model equation, it is shown that the inclusion of the relativistic mass variation in the diffusion coefficient results in a basic change of the wave–particle selective interaction compared to the nonrelativistic approximation for any value of Te or N∥.