Collective scattering system for transport study on NSTX

The problem of degraded energy confinement due to excessive transport across the magnetic field remains one of the critical issues for magnetically confined plasma. Radial transport of ion energy is relatively well understood, but that of the electrons has remained as anomalous, greatly exceeding the neoclassical prediction. It has been suggested that the anomalous electron thermal transport is explained by an electron gyro-scale turbulence driven by the electron temperature gradient (ETG) instability. A 280-GHz collective Thomson scattering system has been employed to address the electron gyro-scale fluctuations in National Spherical Torus Experiment (NSTX) plasmas. The spatial resolution of the targeting wavenumber is greatly affected by the configuration of the magnetic field since the radial fluctuation is perpendicular to the local magnetic field line. The effect of the toroidal field curvature and magnetic shear on the spatial resolution of the scattering system is investigated numerically. An absolute power calibration was performed to determine the power response of the heterodyne detection system. These spatial resolution studies and absolute power calibration were applied to estimate the normalized density fluctuations from the measured scattering signals in NSTX plasmas.