Spectral Effects on Fast Wave Core Heating and Current Drive

RF heating and current drive is an essential component in most magnetic fusion devices, including ITER. Recent results obtained with high harmonic fast wave (HHFW) heating and current drive (CD) on NSTX strongly support the hypothesis that the onset of perpendicular fast wave propagation right at or very near the launcher is a primary cause for reduced core heating efficiency at long wavelengths that is also observed in ICRF heating experiments in numerous tokamaks. A dramatic increase in core heating efficiency was achieved in NSTX L-mode helium majority plasmas when the onset for perpendicular wave propagation was moved away from the antenna and nearby vessel structures. This was accomplished by reducing the edge density in front of the launcher with lithium conditioning and avoiding operational points prone to instabilities, resulting in efficient core heating in deuterium L mode and H mode discharges. These results indicate that careful tailoring of the edge density profiles in ITER should be considered to limit RF power losses to the antenna and plasma facing materials. Finally, in plasmas with reduced rf power losses in the edge regions, the first direct measurements of high harmonic fast wave current drive were obtained with the motional Stark effect (MSE) diagnostic. The location and radial dependence of HHFW CD measured by MSE are in reasonable agreement with predictions from both full wave and ray tracing simulations.