Absorption of electron cyclotron radiation in tokamak plasmas with a superthermal tail in the electron velocity distribution

A monotonically decreasing superthermal tail in the distribution of electron velocities parallel to the tokamak magnetic field is represented by adding a drifted Maxwellian distribution to a bulk Maxwellian distribution. This combines a high degree of flexibility in the set of tail parameters available with the possibility of obtaining exact analytic expressions for the dielectric tensor elements. Calculations have been carried out in the nonrelativistic limit, which restricts its applicability to the extraordinary mode away from normal incidence. A fully three-dimensional ray tracing code which includes the effects of toroidal geometry is employed. The results indicate that small tails (tail fraction ∼ 0.5%) can significantly alter the wave absorption characteristics. These include the integrated absorption, spatial deposition profile, and velocity space deposition profile. Due to the presence of tail electrons, significant heating above the high density cutoff is found for high field side louching with W = ωc (central), together with strong absorption at low density for low field side lounching with W << ωc (central). (author)