A combined phase contrast imaging-interferometer system for the detection of multiscale density fluctuations on DIII-D

A heterodyne interferometer channel has been added to the DIII-D phase contrast imaging (PCI) system. Both measurements share a single 10.6 $\mu$m probe beam. Whereas the PCI excels at detecting medium- to high-$k$ fluctuations (1.5 cm$^{-1}$ $\le$ $k$ $\le$ 20 cm$^{-1}$), the interferometer extends the system sensitivity to low-$k$ fluctuations ($k$ $\le$ 5 cm$^{-1}$), allowing simultaneous measurement of electron- and ion-scale instabilities with sub-microsecond resolution. Further, correlating measurements from the interferometer channel with those from DIII-D’s pre-existing, toroidally separated interferometer ($\Delta\emptyset$ = 45$^{\circ}$) allows identification of low-$n$ modes. This new capability has been corroborated against magnetic measurements and may allow novel investigations of $core-localized$ MHD that is otherwise inaccessible via external magnetic measurements, with potential applications to fast particle transport and disruptions.