Observation of the Enhanced High-k Spectra during H-mode Phase on NSTX.*

A multi-channel collective scattering system was commissioned on NSTX to investigate the anomalous electron transport physics. A high power microwave source ($\sim $100 mW) at $\sim $ 1 mm wavelength was employed as the probe beam. The system consists of 5 discrete channels which primarily measure five radial wave-numbers up to k$_{r} \quad \sim $ 20 cm$^{-1}$ which corresponds to k$_{\bot }\rho _{e} \quad \sim $ 0.2 and $\sim $0.7 for the edge and core for the NSTX parameters, respectively. Initial tests were performed at the edge of the quiescent OH and NBI heated plasmas. In OH and L-mode plasmas, the variation of the scattered power spectra at each wave-number was monotonic. However, significant changes of the scattered power were observed during the L/H transition phase. Relative decrease of the scattered power spectra at lower wave-numbers is largely attributed to the refraction effect due the edge density gradient, but the increase of the power spectra at the high-k channels is under investigation. The details of the scattered spectra from two types of H-mode plasmas (double null and single null) and spatial dependence will be discussed with the relevant TRANSP calculations.