Influence of a superthermal tail upon ECRH in the TJ-II flexible heliac – a theoretical study

A superthermal tail is simulated by adding a drifted Maxwellian distribution to a bulk Maxwellian distribution, f a = (l−μ)f b + μf t . The values of the tail temperature (T t ), drift velocity (v 0 ) and weight coefficient (μ) are chosen so as to reproduce the experimental residual current observed in the Wendelstein VII-A stellarator. An analytical expression for the resulting dielectric tensor coefficients is obtained. It is shown that, even for small tails, the global absorption coefficient increases for both the first harmonic (ω = 28 GHz) and the second harmonic (ω = 56 GHz) propagation. The effect is more dramatic for wave propagation in the first harmonic. The spatial bandwidth of the absorption region increases sensibly with respect to the pure Maxwellian distribution in both harmonics.