Radial Interferometer-Polarimeter for measurement of density and magnetic field fluctuations on NSTX-U

In fusion relevant plasmas, the density and magnetic fields fluctuate across a wide range of frequency and length scales. These fluctuations originate from various physical processes like turbulence, magnetohydrodynamic (MHD) instabilities and energetic particle (EP) modes. The understanding of these fluctuations is critical for the confinement and stability of the plasmas. We present the design of a radial interferometer-polarimeter (RIP) diagnostic based of the Faraday effect being developed for the NSTX-U tokamak. Solid state sources of frequency 935 GHz (wavelength 321 µm) will be used having output power of ~1 mW. Two probe beams will be launched from the low-field side of the tokamak and travel radially inwards on the midplane, providing simultaneous line-integrated density (interferometry) and line-integrated density-weighted radial magnetic field (polarimetry) measurements of both the equilibrium and fluctuating quantities at 10 MHz bandwidth. The fluctuating Faraday-effect signal from polarimetry will provide a direct measure of radial magnetic field fluctuations. Phase noise will be minimized by using cross-correlation technique and time delay optimization to 0.01 degrees, corresponding to sub-Gauss level resolution [1,2]. RIP will measure simultaneously at two toroidal locations with a separation of 7 degrees enabling measurement of toroidal mode numbers up to 25.

[1] J. Chen et al., Rev. Sci. Instrum. 92, 043502 (2021)

[2] G. Prabhudesai et al, Rev. Sci. Instrum. (under review)