Lower-hybrid wave absorption in the presence of simultaneous density and magnetic field gradients

Abstract

It is shown that for the lower-hybrid wave (the slower of the two cold-plasma modes propagating at an angle to the static magnetic field), the WKB requirements are satisfied in almost the entire region from the antenna up to and including the ion-cyclotron-harmonic resonance. Expressions for the electric field and the energy density in the WKB approximation are derived. Both the collisionless and collisional damping become significant near the resonance, and the wave attenuation due to the two processes is estimated. From the differential equation for the electric field valid near the resonance, it is found that the wave energy would be completely assimilated even in the absence of a damping mechanism. The effect of impurities, although negligible near the plasma edge, becomes important following wave conversions and a significant fraction of the wave energy may end up heating the impurity ions in the plasma interior. The implications of using the ’’local’’ dielectric tensor approximation in the above context are examined. An alternative scheme for heating at the ion-cyclotron frequency using the lower-hybrid (quasi-shear) wave instead of the usual compressional wave is pointed out.