Upshift in the DIII-D High Field Side Lower Hybrid Current Drive Experiment*

Tokamak power plants will require some form of efficient, non-inductive current drive for current profile tailoring, instability mitigation, and/or pulse extension. High field side lower hybrid current drive (HFS LHCD) is one potential candidate, and the first test of this technology will occur on the DIII-D tokamak during the 2024 campaign. For HFS launch, single-pass absorption (SPA) is achieved by an n_|| upshift rather than multi-pass interactions with the scrape-off layer (SOL) to bridge the spectral gap. Compared to multi-pass absorption experiments, the optimization of launch spectrum and plasma parameters can be counterintuitive. For example, simulations show that decreasing the launched |n_||| improves SPA in some cases due to the modified ray trajectory leading to a faster upshift. A hard x-ray (HXR) camera has been developed to measure the bremsstrahlung (50-250 keV) radiation from LHCD-generated fast electrons. The measured HXR emissivity profiles provide a means to investigate the wave propagation dependence on plasma parameters and magnetic geometry through comparison to the ray-tracing/Fokker-Planck codes GENRAY/CQL3D. The latest simulations and diagnostic status will be presented.