Multiscale fluctuation measurements with a combined phase contrast imaging & interferometer diagnostic on DIII-D

A novel, combined phase contrast imaging (PCI) and interferometer diagnostic capable of measuring line-integrated electron density fluctuations that extend from magnetohydrodynamic (MHD) scales to the lower range of the electron temperature gradient (ETG) mode has detected evidence of cross-scale coupling in the turbulent density spectrum in DIII-D. In an attempt to elicit a multiscale response, the ETG drive a/L_Te was increased locally relative to the ion temperature gradient (ITG) drive a/L_Ti (while maintaining β_N=1.9), and the PCI measured distinct "spectral flattening'" around kρ_s∼1, which has been found by gyrokinetic simulations to be a tell-tale signature of increased coupling between ITG and ETG scales. The measured spectral flattening is in qualitative agreement with predictions from the reduced model TGLF, but quantitative comparisons await completion of the corresponding multiscale, nonlinear gyrokinetic simulations. Additionally, the interferometer detects a low-k, electromagnetic mode driven unstable by collisionality and η_e, features reminiscent of the micro-tearing mode (MTM); linear gyrokinetic modeling is used to further constrain and identify this mode.