Edge density fluctuation characterization in H-mode and polarimetry measurement via the FIReTIP system on NSTX

The edge interferometry channel of the far infrared tangential interferometer polarimeter (FIReTIP) is used to characterize the localized edge density fluctuations during the L/H transition in conjunction with other edge diagnostic systems on the National Spherical Tokamak Experiment (NSTX). With the high IF frequency employing a “Stark-tuned” laser as local oscillator, it is possible to measure the absolute level of fluctuations at high time resolution. Density fluctuation measurements using FIReTIP are feasible at both the high field side and the low field side boundary throughout the plasma shot. The normalized level of measured density fluctuations was in the range of 0.01–0.3 depending upon the channel location and the plasma confinement. The experimental results showed that the sampling rate of 500 KHz was sufficiently high to produce valuable frequency spectral information during the L/H transition, ELMs period and TAE/f.b.s activities. The improved polarimetry measurement was introduced for the para/diamagnetic study of the NSTX plasma. Multichannel operation of FIReTIP with high resolution density measurement provides promising opportunities as a density fluctuation diagnostic system for turbulence/transport and boundary physics studies as well as the robust density measurement for plasma operation. In this article, a brief description of the upgraded portion of the FIReTIP system will be given together with the experimental results of spectral analysis of the edge fluctuations and assessments on polarimetry signals.