An assessment of full-wave effects on Maxwellian lower-hybrid wave damping

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 quantitative agreement between raytracing and converged full-wave simulations can be obtained in reactor relevant-scenarios and qualitative agreement can be obtained in situations with weak damping.

Verifying raytracing/Fokker–Planck lower-hybrid current drive predictions with self-consistent full-wave/Fokker–Planck simulations

“…In order to obtain physically and numerically self-consistent results, we needed to not only implement the improved TORLH boundary condition and field solver convergence criteria from Frank et al. (2022), but we also had to implement significant improvements in the non-Maxwellian components of TORLH including: reformulation of the quasilinear diffusion coefficient, construction of a lookup table and the lookup table interpolation in TORLH. In the following sections we will describe the key modifications to the non-Maxwellian components of TORLH and develop convergence requirements for them.…”

“…As in Frank et al. (2022), we have used the IPS (Elwasif et al. 2010) to create matched raytracing simulations with plasma state files from the full-wave simulation to evaluate the importance of full-wave effects.…”

“…The GENRAY raytracing simulations here used the same source codes as those in Frank et al. (2022) in which the dispersion relation is based on the TORLH dielectric and includes hot plasma corrections to the component of the dielectric tensor. The GENRAY simulation of this shot used 400 rays, all with , spread evenly over four grills placed at and from the outboard midplane.…”

“…The boundary condition was matched to the raytracing simulations using the improved boundary condition in Frank et al. (2022), and an was launched. The CQL3D simulations coupled to full-wave simulations used 96 flux surfaces equi-spaced over to 0.95.…”

Radiative pulsed L-mode operation in ARC-class reactors

“…This allowed us to design a current drive actuator for our RPLmode scenario capable of delivering power to the q = 2 surface without encountering a cutoff. Raytracing/Fokker-Planck simulations like GENRAY/CQL3D should be accurate in ARC scenarios as turbulence and diffraction are of little importance when the LHCD is single-pass damped [91,92]. There should also be minimal power loss to parametric decay as the LHCD here has ω > 2ω LH in the edge, where ω LH is the lower-hybrid frequency [93,94].…”