Geometric Optics of Lower Hybrid Waves in Nonuniform Plasmas

The propagation of lower hybrid waves in the presence of strong density and field gradient-produced refractive effects is studied in the geometric optics limit. Attention is focused on the electrostatic wave and the modification of the parallel wavenumber k¿ due to gradients. It is found that in cases where the gradients occur along the direction of the magnetic field the resulting decrease of k¿ gives rise to a cusp or reflection of the ray at the resonance layer. An asymptotic series for the field amplitudes is developed which is valid in the short wavelength limit everywhere except near the cusp point. It is found that the cusp represents a singularity where the geometric optics assumption is invalid; however, the inclusion of thermal effects removes the singularity by linear mode conversion (LMC). The analysis is applied to toroidal geometry and is extended to include distributed sources.