On estimating the role of diffraction in electron cyclotron absorption at the periphery of the plasma column

The propagation and absorption of microwave radiation at the periphery of a tokamak plasma under ECR conditions are considered. For microwaves propagating in a quasi-transverse direction, the range of plasma parameters is determined in which the effective plasma permittivity can be approximated by a piecewise linear function. With this approximation, it is possible to obtain analytic solutions to the wave equation and to use them to estimate the width of the power deposition region for different modes of microwave launching. A detailed analysis is given of tangential launching—the propagation of microwaves along a tangent to the resonance magnetic surface at which they begin to be absorbed under ECR conditions. Using the ITER tokamak as an example, it is shown that this launching method is most efficient in providing the narrowest power deposition profile at the plasma periphery. The results obtained are of interest for the problems of suppressing tearing-mode instabilities by localized ECR heating.