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

A combined phase contrast imaging (PCI) and heterodyne interferometer system has been implemented on DIII-D, extending the physics capabilities of the pre-existing PCI and acting as a prototypical fluctuation diagnostic for next-step devices. The combined PCI-interferometer uses a single 10.6 $\mu$m laser beam, two interference schemes, and two detectors to measure $\int \tilde{n}_edl$ over a large spatiotemporal bandwidth $\mbox{(10 kHz $< f < $2 MHz and 0 $\le k \le$ 20 cm$^{-1}$)}$, allowing simultaneous measurement of ion- and electron-scale instabilities. Further, time-correlating our interferometer's measurements with those of DIII-D's pre-existing, toroidally separated ($\Delta\zeta$ = 45$^{\circ}$) interferometer will allow novel studies of low-$n$ Alfv\'en eigenmodes. The combined diagnostic's small port requirements and minimal access restrictions make it well-suited to the harsh neutron environments and limited port space expected in next-step devices. Measurements from sound wave calibrations and DIII-D operations will be presented.