Disparate-scale coupling of turbulence in QH-mode plasmas on DIII-D

Analysis of incoherent fluctuations in quiescent H-mode (QH-mode) plasmas suggests nonlinear coupling between high- and low-frequency turbulence. In QH-mode plasmas with edge harmonic oscillations (EHO), transport levels are enhanced when incoherent fluctuations are present compared to QH-mode plasmas with only EHO. Furthermore, in some cases without EHO, the incoherent fluctuations alone can sustain QH-mode. Bispectral analysis of microwave imaging reflectometer (MIR) data indicates nonlinear 3-wave coupling among disparate spatial scales of the turbulence. The bicoherence is above noise levels for high-frequency (300 $<$ f $<$ 500 kHz), intermediate-scale ($k_\theta \approx$ 0.2 - 0.6 cm $^{-1}$) and low-frequency (f $<$ 50 kHz), large-scale ($k_\theta < $ 0.2 cm $^{-1}$) turbulence. Cross-phase analysis reveals that the high-frequency turbulence rotates in the electron diamagnetic drift direction, while the low-frequency turbulence rotates in the ion diamagnetic drift direction, suggesting coupling between different instabilities.