Faraday-effect polarimetry diagnostic for NSTX-U spherical tokamak

A Faraday-effect polarimetry-interferometry diagnostic is under design for the NSTX-U spherical tokamak. Aiming to address issues such as model validation and plasma control, the diagnostic will provide internal magnetic field and magnetic fluctuation measurements associated with MHD instabilities, energetic particle driven modes, broadband magnetic turbulence, and 3D non-axisymmetric effects. The diagnostic will utilize 3-wave technique to accomplish simultaneous Faraday-effect and line-integrated density measurements with fast time response (0.1 µs) and low phase noise (0.01 degree). The probe beams will access the plasma along a horizontal chord via a mid-plane port, to directly detect internal radial magnetic fluctuations via the Faraday effect. Solid state sources and planar diode mixers at 930 GHz will be used to minimize contamination from Cotton-Mouton effect. Probe beam diameter of ~22 mm infers a corresponding upper bound wavenumber of measurable fluctuations at kρ_s~1, for NSTX-U plasmas with B_T=1 T and T_e=2 keV. Various optical designs are under consideration, including a dual-chord design with toroidal offset to measure toroidal mode number up to n=60 and a single-pass design to reduce feedback.