Design of NSTX-U Radial Polarimeter- Interferometer

A Faraday-effect polarimeter-interferometer for the NSTX-U spherical tokamak is designed to internally measure low-k (wave number perpendicular to probe beam less than ~2/cm), radial magnetic and density fluctuation in order to investigate MHD instabilities, energetic particle driven modes and broadband turbulence. The diagnostic will utilize the three-wave technique to accomplish simultaneous polarimetric and interferometric measurements with 10 MHz bandwidth and 0.01-degree phase noise. Probe beam wavelength is selected to be 321 µm (935 GHz) to minimize Cotton-Mouton effect. Two chords with 7-degree toroidal separation will be installed, allowing toroidal mode number measurement up to n=25 for instability identification. Engineering challenges are identified and solutions are developed, including customized, low-profile, one-inch diameter retro-reflector and extra-wide high-field-side graphite tile. Optical design with realistic configuration compatible with NSTX-U test cell environment is developed.