NSTX High-k Scattering System on NSTX: Status and Plan

A multi-channel collective scattering system was commissioned on NSTX to investigate anomalous electron transport physics related to the electron density turbulence. NSTX plasma parameters with a large gyro-radius ($\rho _{i} \quad \sim $1 cm) facilitate the investigation of the turbulence related physics in the high-k regime (k$_{r}\rho _{s}>$3) which is not readily accessible in other toroidal devices. The system employs a moderate power ($\sim $100 mW) source at $\sim $ 1 mm wavelength as the probe beam and has an excellent spatial and wavenumber resolution. The system consists of 5 discrete channels which primarily measure five radial wavenumbers up to k$_{r} \quad \sim $ 20 cm$^{-1}$ which corresponds to k$_{r}\rho _{e}\sim $0.2 and $\sim $0.7 for the NSTX edge and core parameters, respectively. Initial results from various operating regimes [edge and core of the quiescent OH, L/H modes of the RF and NBI heated plasmas] will be addressed in this paper. The observed high signal to noise ratio at the highest wave-number provides confidence in the future upgrade plan for even higher wavenumbers up to $\sim $50 cm$^{-1}$ (k$_{r}\rho _{e}\sim $2) employing a shorter probe beam wavelength in which the net S/N is comparable (optimum beam power and available detector sensitivity) to the present $\sim $1mm system. Upgrade plans for the tangential and poloidal high-k configuration that can share the same probe beam will be discussed.