Characterization of the DIII-D High-Field Side Scrape-Off Layer and Implications for High-Field Side Lower Hybrid Current Drive

Abstract

High-field side lower hybrid current drive (HFS LHCD) is a potential candidate to provide efficient, non-inductive, off-axis current drive in tokamaks for steady-state operation, stability control, and/or access to advanced tokamak scenarios. The first experimental test of HFS LHCD has begun on the DIII-D tokamak. LHCD is known to be sensitive to scrape-off layer (SOL) conditions local to the coupler, and to characterize the HFS SOL in preparation for the high-power HFS LHCD experiments, a high-field side reflectometer was installed on DIII-D. The reflectometer operates in the 6-19 GHz range in O-mode polarization, corresponding to a measured density of 4.5e17-4.5e18 per cubic meter. A HFS SOL measurement database of over 600 DIII-D discharges has been constructed and includes HFS LHCD target plasmas, such as high q-min discharges. Furthermore, the effect of ELMs and magnetic configuration has been studied in detail. Machine learning techniques have also been applied to this database, allowing for the prediction of HFS SOL density profiles from global discharge parameters such as plasma current, plasma density, and plasma shaping parameters. Simulation-based inference (SBI) techniques were used to infer HFS SOL turbulence characteristics from reflectometer density profile measurements. Finally, experimental data, turbulence inferences, and machine learning regression enable accurate prediction and optimization of HFS LHCD coupler loading, which is calculated using 3-D finite element full-wave codes.