Spectrum broadening and degradation of the O-X mode coupling efficiency due scattering of a microwave beam on plasma density fluctuations

摘要

Linear transformation of the ordinary (O) and extraordinary (X) electromagnetic modes in the vicinity of the plasma cutoffs in magnetized plasma is a basis for advanced schemes of microwave heating and diagnostic of overdense plasmas (where the central density exceeds the cutoff) in magnetic fusion research. A significant influence of density fluctuations, inherent of hot magnetized plasma in toroidal magnetic traps, on the process of linear transformation was first noted in [1], where a simple phenomenological model of this phenomenon was considered. Recently, by a more rigorous analysis, this picture has been substantially revised [2]. As a result of solving the truncated Maxwell equations in the linear interaction region in the presence of density fluctuations, it was established that the dominant effect in the O-X coupling efficiency is due to the small-angle scattering of the wave beam on the fluctuations along the propagation path towards the cutoff, at least fluctuations for the parameter range corresponding to modern experiments. Outside the coupling region, the scattering modifies the incident O-wave beam, which affects its subsequent transformation and reflection. At this, the so-called enhanced scattering, characteristic of the plasma cut-off, does not appear due to peculiar features of wave propagation near the point of polarization degeneracy (where the X and O modes can not be treated as independent) [3]. In this case, perturbations of the averaged coupling efficiency due to the fluctuations inside the localized coupling region are usually negligible.