Conceptual design of NSTX-U Radial Polarimeter-Interferometer diagnostic

Conceptual design of a 935 GHz Radial Polarimeter-Interferometer Diagnostic (RAPID) for the NSTX-U spherical tokamak is presented. The diagnostic will utilize 3-wave technique to accomplish simultaneous Faraday-effect and line-integrated density measurements with ~0.1 µs time response and ~0.01 degree phase noise, to provide internal magnetic and density fluctuation measurements. The primary application of this diagnostic is to investigate MHD instabilities, energetic particle driven modes and turbulence for purposes of physics validation and plasma control. High-level diagnostic requirements are determined, such as implementation of two chords with 4.5 degrees toroidal separation, which will allow toroidal mode number measurement up to n=40 and is critical for physics validation. Engineering challenges are identified and possible solutions are developed, including fabrication of low-profile retro-reflectors with 1 inch diameter, modification of high-field-side graphite tiles and rearrangement of window positions on the low-field-side port flange. Optical design with realistic configuration compatible with the NSTX-U test cell is developed.

Work supported by US DOE under grant no. DE-SC0021140 and DE-AC02-09CH11466.