Impact of wave-filament scattering in the scrape-off layer during lower hybrid current drive

Abstract

Lower Hybrid Current Drive (LHCD) is an efficient means to control the current profile in a tokamak. The performance of LHCD is sensitive to the phase-space trajectory of the wave as it propagates and damps in the plasma. These trajectories are scattered by turbulent density fluctuations in the scrape-off layer (SOL), which is hypothesized to be the reason for poor agreement between LHCD experiments and simulations (which do not account for scattering). This thesis investigates the impact of scattering processes on LHCD, and attempts to bridge this discrepancy. Two novel scattering models are developed. The \emph{filament-refraction} model generalizes beyond the weak turbulence assumption made in previous works. The \emph{full-wave/statistical} model further generalizes beyond the Wentzel-Kramer-Brillouin (WKB) limit. This latter approach results in excellent agreement with experimental observations.

The filament-refraction model couples realistic SOL turbulence profiles to a ray-tracing code. Synthetic, 3-D turbulence profiles that mimic SOL turbulence are generated and coupled to the ray-tracing/Fokker-Planck codes GENRAY/CQL3D. In contrast with previous scattering models that employ the weak-turbulence approximation, this approach accounts for the spatial coherency of filamentary turbulence. As a result, the extent of scattering is shown to be greater in filamentary turbulence, leading to a significant modification of the current profile in the Alcator C-Mod tokamak.

The full-wave/statistical approach employs a Mie-scattering technique to treat a single wave-filament interaction in full-wave formalism. The radiative transfer approximation is employed to treat scattering from a statistical ensemble of filaments in a turbulent layer in slab geometry. This approach extends beyond ray-tracing, and retains all single-filament full-wave effects while remaining computationally inexpensive. Notably, it is found that LH waves can asymmetrically scatter in wave-number phase-space due to full-wave interactions with spatially coherent density fluctuations. Coupling to GENRAY/CQL3D reveals current profiles that are robustly monotonic and peaked on-axis, in much better agreement with experiment. In contrast, simulations without scattering result in current profiles that are non-monotonic and peaked off-axis.

The full-wave/statistical model is self-consistently coupled to GENRAY, allowing for LHCD simulations in the multi-pass regime. This multi-scale model couples local full-wave scattering physics to a global ray-tracing solver. Across multiple C-Mod discharges, simulations show that scattering plays a significant role in determining the current profile. Phase-space broadening due to scattering significantly broadens the power deposition profile, allowing for increased on-axis current and the mitigation of current valleys and off-axis peaks. These effects saturate for sufficiently intense SOL turbulence, and parametric scans suggest LHCD in C-Mod exists in this saturated regime. Furthermore, the asymmetric scattering effect is shown to significantly affect the current profile. At low and moderate densities, good agreement is found with experimental Motional Stark effect and Hard X-ray measurements. At high densities, the same general trends are found. However, uncertainties relating to Ohmic current, SOL collisionality, and parametric decay make comparisons with experiments challenging.