Lower hybrid penetration across a steep density gradient

Abstract

Flat density profiles in tokamak plasmas are characterized by a sudden density rise at the plasma periphery, which may prevent LH waves from penetrating deeper into the center. The reasons may be the deviation of the ray trajectories toward the plasma edge or the lack of validity of the linear optics approximation (WKB). In this paper we solve the wave equations in full wave limit to analyze if outward reflections take place. The results indicate that unless the density rises as a very sharp jump, the reflection is negligible and the linear WKB wave equations are a valid tool to describe the wave ray tracing. These trajectories are deviated across the density rise but their deviation depends on the central density value and scarcely on the rise steepness.