A hybrid full-wave Markov chain approach to calculating radio-frequency wave scattering from Scrape-off Layer filaments

Radio-frequency (RF) wave scattering from turbulence modifies the wave-spectrum, and affects RF heating and current drive (CD) in tokamaks. For Lower Hybrid (LH) launch, scatter from scrape-off layer (SOL) density fluctuations may bridge the spectral gap at low density, and trigger a drop in CD efficiency at high density. Previous scattering models have been limited to weak turbulence treatments [1], or ray-tracing in more realistic, filamentary SOL turbulence [2]. Here, a new model is introduced which retains full-wave scattering effects from filamentary turbulence. A Mie-scattering technique models a single wave-filament scattering event [3]. Next, a Markov chain accounts for several of these events. The resulting wave-spectrum is asymmetrically broadened in wavenumber angle-space; this is not accounted for in previous LH scattering models. The broadened wave-spectrum is modeled in GENRAY/CQL3D to study its impact on CD. For a low density C-Mod discharge, this results in significantly increased on-axis current, and decreased off-axis peaks. This is attributed to a portion of the wave-spectrum that strongly damps on-axis during first-pass.

[1] Bonoli and Ott, Phys. Fluids 25, 359-375 (1982). 

[2] Biswas et al., PPCF 62, 115006 (2020). 

[3] Myra and D’Ippolito, PoP 17, 102510 (2010).