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

Abstract

Raytracing/Fokker–Planck (FP) simulations used to model lower-hybrid current drive (LHCD) often fail to reproduce experimental results, particularly when LHCD is weakly damped. A proposed reason for this discrepancy is the lack of ‘full-wave’ effects, such as diffraction and interference, in raytracing simulations and the breakdown of the raytracing approximation. Previous studies of LHCD using non-Maxwellian full-wave/FP simulations have been performed, but these simulations were not self-consistent and enforced power conservation between the FP and full-wave code using a numerical rescaling factor. Here, we have created a fully self-consistent full-wave/FP model for LHCD that is automatically power conserving. This was accomplished by coupling an overhauled version of the non-Maxwellian TORLH full-wave solver and the CQL3D FP code using the Integrated Plasma Simulator. We performed converged full-wave/FP simulations of Alcator C-Mod discharges and compared them with raytracing. We found that excellent agreement in the power deposition profiles from raytracing and TORLH could be obtained, however, TORLH had somewhat lower current drive efficiency and broader power deposition profiles in some cases. This discrepancy appears to be a result of numerical limitations present in the TORLH model and a small amount of diffractional broadening of the TORLH wave spectrum. Our results suggest full-wave simulation of LHCD is likely not necessary as diffraction and interference represented only a small correction that could not account for the differences between simulations and experiment.

Optimization of the N∥ Upshift in the DIII-D high field side lower hybrid current drive experiment

“…If good agreement is reliably obtained in these single-pass discharges, GENRAY/CQL3D would be shown to properly model the LHCD physics. This would bode well for the use of GENRAY/CQL3D to predict HFS LHCD in a reactor, especially because a reactor-grade plasma will not require this mode conversion effect (therefore being simpler to model), and full-wave phenomena are unlikely to be important [15,22].…”

Nonlinear lower hybrid wave equations in collisional tokamak plasmas

“…Therefore, the eikonal hypothesis is not satisfied. An accurate modeling of the nonlinear LH physics in the SOL region thus requires a full-wave approach, as already performed concerning LHCD in the plasma core [69][70][71][72]. In addition, in the standard modeling of PI only four-wave interactions are considered, involving the LH pump wave, an upper and a lower LH sideband and an LF plasma quasi-mode, all of which are assumed to be monochromatic.…”