HHFW heating efficiency on NSTX versus B_φ and antenna k _∥

HHFW RF power delivered to the core plasma of NSTX is strongly reduced as the launched wavelength is increased - for B=4.5kG, heating is ∼1/2 as effective at k=-7m^-1 as at 14m^-1 and ∼1/10 as effective at -3m^-1. Measured edge ion heating, attributable to parametric decay (PDI), increases with wavelength but not fast enough to account for the observed power loss. Surface fast waves (FW) may enhance both PDI and also losses in sheaths and structures around the machine - FW fields propagate closer to the wall with decreasing B and k_∥ (onset n _e∝B×k_∥²). A dramatic increase in core heating efficiency is observed at -7m^-1 when B is increased to 5.5 kG - central T_e near 4 keV at P_RF=2MW. Also, the PDI losses are a weak function of B and k_∥, whereas the far-field RF poloidal magnetic field (at 5.5 kG) increases a factor of ∼3 when k _∥ is reduced from 14m^-1 to -3m^-1, suggesting a large increase in wall/sheath power loss and a major effect of surface fast waves on edge losses.