Density fluctuation measurement by FIReTIP for the Enhanced Pedestal H-mode on NSTX

The multi-channel Far Infrared Tangential Interferometry/Polarimetry (FIReTIP) system has been used to measure changes in the electron density fluctuation spectrum for the Enhanced Pedestal H-mode (EPH-mode) on the National Spherical Torus Experiment (NSTX). Data shows dramatic density fluctuation suppression as the EPH-mode is triggered, similar in nature to the turbulence reduction present at the conventional L$\backslash $H transition. Coherent fluctuations are observed by FIReTIP during the EPH-mode with frequencies greater than 10 kHz. Density fluctuation measurements from FIReTIP edge channels with different tangency radii during the EPH-mode are compared with L-mode and H-mode cases, and are presented together with a discussion of a possible EPH-mode triggering mechanism based on the gyro-center shift (GCS) theory.