Implementation of millimeter-wave polarimetry on NSTX

Measurements of internal magnetic fluctuations on NSTX, a high-$\beta $ machine, are important to understand stability, fast-ion and thermal transport. A 288 GHz polarimeter operating along a major radial chord in a retroreflection geometry has been developed and installed on NSTX. This will provide the first direct measurement of internal magnetic fluctuations (1--100 kHz) in a high-performance spherical tokamak. Laboratory tests indicate $\le $ 1\r{ } phase resolution. Calculations using a simplified tearing mode model indicate the feasibility of measurement of magnetic fluctuation levels $\ge $ 0.2{\%}. The sensitivity of polarimetry to microtearing modes is assessed using gyrokinetic simulations. The polarimetry phase fluctuations are calculated using the predicted magnetic ($\le $ 1{\%}) and density ($\sim $ 1{\%}) fluctuations in addition to the input equilibrium profiles. It is shown that the system is primarily sensitive to magnetic fluctuations, as long as the propagation chord lies within $\pm $10 cm of the plasma midplane. Initial measurement results from NSTX will be presented.