Investigation of EBW Coupling and Propagation in H-Mode Plasmas in NSTX

EBW emission (EBE) diagnostics and EBE modeling have been employed on NSTX to study oblique EBW to O-mode (B-X-O) coupling and propagation. Initial EBE measurements in H-mode plasmas exhibited strong emission before the L-H transition, but the emission rapidly decayed after the transition. EBE simulations show that EBW collisional damping prior to mode conversion (MC) can significantly reduce the measured EBE when T$_{e}$ $<$ 30 eV, explaining the observations. Lithium evaporation was used to significantly reduce EBW collisional damping near the MC layer. The lithium evaporation rate was increased from 0 to 19 mg/min to reduce n$_{e}$ and increase T$_{e}$ outside the LCFS in an H-mode plasma. With edge conditioning, an increase in T$_{e}$ near the fundamental MC layer from 10 eV (no Li) to $>$ 20 eV (with Li) was observed. As a result the measured B-X-O transmission efficiency increased from $<$ 10\% (no Li) to 60\% (with Li), consistent with EBE simulations.