Linear interaction of electron cyclotron waves with a suprathermal electron tail

The analysis of power absorption and current generation by electron cyclotron waves at the first cyclotron harmonic, interacting with a suprathermal tail, has been performed using the relativistic linear theory and the adjoint technique. A one-sided tail has been considered and the comparison between upshifted resonance conditions, corresponding to coinjection with respect to the tail, and downshifted (corresponding to counterinjection) has been made. The results are explained by means of the behavior, along the resonant curve, of the absorbed power density in momentum space and of the single-particle current drive efficiency. The ratio between the transverse linear dimension of the resonant curve and the transverse average momentum of the tail is found to play a determinant role in the interaction process. Upshifted resonance conditions are found to be generally more favorable than the downshifted with respect to both power absorption and current drive; this advantage tends to vanish as the perpendicular temperature of the tail is increased.