Multifunctional interferometry-polarimetry for fusion plasma control and physics applications (invited).

Future fusion reactors require robust monitoring of plasma equilibrium and fluctuations to sustain and optimize energy gain, despite severely restricted machine access. We demonstrate a multifunctional interferometer-polarimeter on the DIII-D tokamak that utilizes only four radial chords to simultaneously measure: (a) the interferometric phase shift, (b) the Faraday effect, (c) the Cotton-Mouton effect, (d) density and magnetic fluctuations up to 10 MHz, and (e) fluctuation toroidal mode numbers and vertical wavenumbers. This is achieved via a three-wave, dual-sensor scheme incorporating four vertically and toroidally displaced chords. The Faraday effect measurements constrain the core magnetic equilibrium, directly determining the magnetic axis vertical position. The Cotton-Mouton effect measurement yields a robust line-integrated density measurement free of fringe jumps. Furthermore, fluctuation and wavenumber measurements enable mode localization, facilitating a spatiotemporal survey of multiscale fluctuations from the plasma core to the edge. These results validate a compact interferometry-polarimetry approach capable of delivering comprehensive equilibrium and fluctuation measurements under limited access, supporting the demands of fusion reactors.