EFFECT OF MODE-MIXING ON MICROWAVE REFLECTOMETRY IN THE TOKAMAK AND HELICAL DEVICES

One-dimensional full wave analysis simulating microwave reflectometry in tokamak and helical plasmas has been carried out in order to investigate the polarization change in an inhomogeneous plasma with magnetic shear. Numerical results show that the mode-mixing process through the nonlinear coupling between the ordinary (O->) mode (or extraordinary (X-) mode) wave and the strong magnetic shear, especially in the helical plasma devices, inevitably occurs near the plasma boundary. Simultaneous reflections from both the O-mode and X-mode cutoff layers in the frequency range where both the cutoffs exist, will significantly affect the accuracy of the phase measurements in microwave reflectometry. The effects of mode-mixing are also numerically examined for microwave reflectometry in tokamak plasmas. It is shown that the reconstructed density profile disagrees with the true profile in the peripheral plasma region, which is due to the simultaneous reflection resulting from the polarization mismatching between the incident wave field and the magnetic field line at the plasma boundary.