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

Abstract

High field side (HFS) lower hybrid current drive (LHCD) is one potential candidate for efficient non-inductive current drive in tokamak power plants, and the first test of this technology will occur on the DIII-D tokamak during the 2024 campaign. Previous LFS launch experiments operated in the multi-pass regime and relied on scrape-off (SOL) layer interactions to close the spectral gap. In the DIII-D experiment, single pass damping is achievable via an upshift in the parallel refractive index N|| caused by mode converting twice (slow → fast → slow). This mode conversion affects the ray trajectories and can lead to enhanced N|| upshift depending on where mode conversion occurs radially. Compared to multi-pass absorption experiments, the optimization of launched N|| and plasma parameters can be counter-intuitive: increased density may increase efficiency and smaller N||,launch tend to damp closer to the separatrix. A hard x-ray (HXR) camera installed to measure the bremsstrahlung (50-250 keV) radiation from LHCD-generated fast electrons is capable of verifying the trends reporting in this paper through comparison to the ray-tracing/Fokker-Planck codes GENRAY/CQL3D.