Modeling and laboratory test of a prototype polarimeter for magnetic fluctuation measurements on NSTX

Polarimetry is a powerful technique capable of probing magnetic field fluctuations in fusion plasmas. [W. X. Ding et al. Phys. Rev. Lett. \textbf{90}, 035002 (2003).]~A 288 GHz polarimeter operating along a major radial chord in a retroreflection geometry is being developed for NSTX to determine magnetic fluctuations in the frequency range from 1$\sim $100 kHz. Modeling is used to investigate the sensitivity of this planned system to magnetic fluctuations resulting from tearing modes, Alfv\'{e}n eigenmodes, etc. Preliminary analysis suggests that measurement of relative magnetic fluctuation levels $\ge $ 0.1{\%} is feasible. Previous calculations based on NSTX plasma equilibria have revealed an interaction between Faraday rotation and Cotton-Mouton effects, which can complicate interpretation. [J. Zhang et al. RSI (Oct, 2010).] A prototype system is under laboratory test to determine measurement sensitivity and establish the optimum detector configuration for measurement of the full polarization state.