A combined phase contrast imaging and heterodyne interferometer system for multiscale fluctuation measurements

A novel combined diagnostic capable of measuring multiscale density fluctuations that extend from magnetohydrodynamic (MHD) to the lower range of electron temperature gradient (ETG) turbulence has been designed, installed, and operated at DIII-D. The combined diagnostic was constructed by adding a heterodyne interferometer to the pre-existing phase contrast imaging (PCI) system, both of which measure line-integrated electron-density fluctuations. The port-space footprint is minimized via use of a single 10.6-micron probe beam. With temporal bandwidths in excess of 1 MHz, the PCI measures high-k (1.5/cm < |k_R| <= 25/cm) fluctuations with sensitivity 3e13 m^{-2} / \sqrt{kHz}, while the interferometer simultaneously measures low-k (|k_R| < 5/cm) fluctuations with sensitivity 3e14 m^{-2} / \sqrt{kHz}. The intentional mid-k overlap has been empirically verified with sound-wave calibrations and allows quantitative investigation of multiscale effects, which are predicted to be significant in the reactor-relevant T_e ~ T_i regime. Further, via correlation with the primary DIII-D interferometer, the toroidal mode numbers of core-localized MHD can be measured.