Understanding the ELM trigger and suppression mechanisms in DIII-D negative triangularity plasmas with Electron Cyclotron Emission Imaging

First characterization of the Edge Localized Mode (ELM) dynamics and edge turbulence spreading for Negative Triangularity (NT) plasmas has been carried out with the Electron Cyclotron Emission Imaging (ECEI) instrument on DIII-D, which measures the 2-dimensional ECE fluctuations (0-200 kHz) at Ψ_N ~0.85-1.1. NT plasmas are typically in L-mode as the larger bad curvature region is theoretically predicted to make the edge susceptible to local ballooning modes. However, dedicated experiments on DIII-D found ELMs during H-mode and Limit Cycle Oscillations in NT plasmas at relaxed negative triangularities or low heating powers. It is thus highly desirable to understand the ELM behavior in these plasmas with direct experimental evidence. ECEI observed that the NT ELMs are triggered by a low-n (<5) MHD mode, which displays a clear energy exchange signature across the separatrix during ELM crashes. At δ=-0.2, this low-n mode is triggered at a critical pressure gradient. The energy exchange signature and critical pressure gradient at the ELM agree with an interchange mode driven by the pressure gradient. At δ=-0.4, the plasma first enters an ELMy phase followed by an ELM-free phase at an increased heating power. In the ELM-free phase, ECEI observed that the ELMs are replaced with low-n turbulence, which spreads to the SOL and causes transport across the separatrix. The edge turbulence spreading may also enhance the SOL transport, as a clear interELM divertor heat-flux-width broadening is observed after the turbulence amplitude increases with heating power at δ=-0.2. For both δ=-0.2 and δ=-0.4, the edge turbulence amplitude measured by ECEI and Beam Emission Spectroscopy correlates with the decreasing edge density gradient and rotation shear, suggesting their roles in turbulence enhancement, and the corresponding ELM suppression and divertor heat flux width broadening in NT plasmas.