Diagnosing Helicon and Lower Hybrid Wave Coupling with the Edge Plasma for Current Drive Optimization in DIII-D Using Laser Spectroscopy

Over the next several years, the operational space of two novel RF actuators designed for off-axis current drive will be extensively explored in the DIII-D tokamak. The goal of these programs is focused on evaluating the potential for efficient current drive in advanced tokamak scenarios. Previous experimental work on C-Mod and NSTX determined that wave coupling with the scrape-off-layer (SOL) plasma can result in substantial core power loss. However, recent computational studies indicate that the SOL plasma can be optimized to minimize the undesired wave/SOL-plasma coupling, A diagnostic based on Doppler-free saturation spectroscopy (DFSS) has been proposed for direct measurement of the wave's electric field vector (\textbf{E}$_{\mathrm{\mathbf{RF}}})$ in the edge plasma of DIII-D. The DFSS diagnostic was designed to provide a local measurement over a 2-D region with mm-scale spatial resolution and \textless 10 V/cm electric field resolution. The measured 2-D \textbf{E}$_{\mathrm{\mathbf{RF}}}$ data will be directly compared with 3-D full-wave simulations to quantitatively identify and characterize wave/SOL-plasma coupling. Utilizing 3-D full-wave simulations the expected \textbf{E}$_{\mathrm{\mathbf{RF}}}$ in DIII-D will be discussed and results from mock-up performance validation testing of the DFSS diagnostic at ORNL will be presented.