Ray-tracing/Fokker-Plack modeling of LHCD in the presence of SOL turbulence

Lower Hybrid waves are an efficient actuator to drive steady-state off-axis current in a tokamak. Wave propagation and current drive can be sensitive to plasma and antenna parameters, so an accurate model of this phenomenon is of interest. Presently, the standard ray-tracing/Fokker-Planck (RTFP) model is unable to match experiment in the weak-damping regime, and at high Greenwald fractions. In both cases, the interaction between the LH wave and the scrape-off-layer (SOL) is important. One possible SOL interaction (not accounted for by standard RTFP models) is turbulent scattering of the LH wave, which broadens the LH wave-spectrum, and perturbs the intended wave-trajectory.

SOL turbulence is made of coherent, intermittent structures called blobs. Several methods exist to account for turbulent scattering of RF waves, but only under the assumption of weak-turbulence. Here, we present a method to include realistic SOL turbulence in a ray-tracing code. This model predicts greater LH wave scattering than previous models. When ray-tracing simulations are coupled to the Fokker-Planck code CQL3D, the scattered and broadened wave-spectrum is shown to cause smoother current profiles that are robust to small changes in plasma density. An increase in on-axis power deposition also leads to better agreement with hard X-ray measurements in Alcator C-Mod. This demonstrates that turbulent scattering significantly improves RTFP models of LHCD in the weak-damping regime. Conversely, this effect is predicted to be less important in single-pass dam