Electromagnetic wave scattering from magnetic fluctuations in tokamaks

Abstract

Cross sections are calculated for electromagnetic wave scattering and mode transformation from magnetic and density fluctuations in the vicinity of the principal electron cutoffs and resonances in a homogeneous plasma. For the special case of scattering perpendicular to the magnetic field, density fluctuations scatter ordinary to ordinary and extraordinary to extraordinary modes -- but cannot transform these modes. On the other hand, magnetic fluctuations perpendicular to the field can transform modes but cannot scatter on a single branch. For incident frequencies on the order of the electron plasma frequency or gyrofrequency, the cross sections for scattering and transformation due to field and density fluctuations have a similar value. Estimates are given for scattering in a tokamak plasma with special emphasis on the question of how to detect and localize magnetic field fluctuations. Toroidal effects are handled by ray tracing. Ray tracing calculations and estimates of practical limitations on detection technique show that magnetic fluctuations can be detected and localized by this method -- even when the magnetic fluctuation levels are orders of magnitude lower than the density fluctuations. 33 refs., 12 figs.