An Assessment Of Full-Wave Effects On Maxwellian Lower-Hybrid Wave Damping

TL;DR

This paper compares full-wave and raytracing simulations of lower-hybrid wave damping in tokamaks, revealing that discrepancies are mainly due to numerical issues, and shows that accurate agreement is achievable in realistic scenarios.

Contribution

It provides the first direct comparison of full-wave and raytracing simulations using consistent plasma models in realistic tokamak conditions.

Findings

Differences in previous work were due to simulation inconsistencies.

Good agreement can be achieved with proper numerical treatment.

Full-wave effects are significant near caustics and reflections.

Abstract

Lower-hybrid current drive (LHCD) actuators are important components of modern day fusion experiments as well as proposed fusion reactors. However, simulations of LHCD often differ substantially from experimental results, and from each other, especially in the inferred power deposition profile shape. Here we investigate some possible causes of this discrepancy; "full-wave" effects such as interference and diffraction, which are omitted from standard raytracing simulations and the breakdown of the raytracing near reflections and caustics. We compare raytracing simulations to state-of-the-art full-wave simulations using matched hot-plasma dielectric tensors in realistic tokamak scenarios for the first time. We show that differences between full-wave simulations and raytracing in previous work were primarily due to numerical and physical inconsistencies in the simulations, and we demonstrate that good agreement between raytracing and converged full-wave simulations can be obtained in reactor relevant-scenarios with large ray caustics and in situations with weak damping.