Role of ponderomotive density expulsion in ion Bernstein wave coupling to the core plasma

Abstract

When the density at the ion Bernstein wave (IBW) antenna is relatively low, mode transformation of the electron plasma wave to the IBW is sensitive to the density gradient scale length, and hence to ponderomotive effects. A second-order nonlinear ordinary differential equation that describes mode transformation at the lower-hybrid layer, including self-consistent ponderomotive density profile modification, is solved for the rf electrostatic potential in front of the IBW antenna, for the particular case of heating just below the second harmonic of the deuterium cyclotron frequency. The complex antenna impedance and a local reflectivity are calculated, assuming vacuum within the antenna box. These calculations reveal diminished antenna coupling to the IBW with increasing ponderomotive density expulsion, as compared to the linear prediction. The ponderomotive force increases the density gradient in the edge plasma, thus enhancing reflection and lowering the loading resistance. The model also describes the direct launch of IBWs in high edge density regimes, lacking a lower-hybrid layer, where the impedance is found to be much smaller than in the low density regime.