Nonlinear Coupling of Core Tearing Modes and Edge Density Fluctuations in QH-Mode Plasmas in DIII-D

Quiescent H-mode (QH-mode), which avoids ELMs by enabling particle transport via a coherent edge harmonic oscillation (EHO), is a leading naturally non-ELMing scenario. We consider a marginal QH-mode plasma in DIII-D which transitions to ELMy H-mode. During the QH phase, the EHO is observed using Faraday-effect polarimetry via the Radial Interferometer-Polarimeter (RIP). After the steady EHO vanishes, ELMs begin, two core tearing modes (TM) arise, and an intermittent inter-ELM mode emerges, all visible with RIP. One of the core TMs is never detected with external magnetic coils, while the intermittent mode appears weakly on a few coils. Bispectral analysis of RIP data reveals statistically significant nonlinear coupling between the two TMs and the intermittent mode. The intermittent mode is also detected by beam emission spectroscopy (BES) in the edge (ρ > 0.94), indicating that it is localized to the edge (BES does not detect the two TMs). The RIP polarimetry and BES signals are highly correlated, implying the intermittent mode comprises both density and magnetic field fluctuations. The intermittent mode frequency is near that of the previous EHO. We posit that nonlinear interaction between TMs and edge fluctuations may prevent reformation of a steady EHO and return of QH-mode.