ECE-Imaging characterization of edge magnetic islands affecting pedestal transport and stability in a net-zero torque plasma

Characterization of a m=5, n=1, and f~0.5-3 kHz rotating island at the pedestal top (psiN~0.89) of a net-zero torque low-collisionality, wide pedestal QH-mode is presented. The characterization is made with ECE-Imaging and other diagnostic systems at the DIII-D tokamak. The magnetic island is observed to exert a profound effect on the pedestal structure and the axisymmetric peeling-ballooning mode stability. Though the discharges have no Resonant Magnetic Perturbation (RMP) applied, the observations of edge island and peeling mode serve to improve the understanding of Edge-Localized-Mode (ELM) suppression with 3D fields. As the edge island grows, the pedestal pressure ceases increasing and holds relatively unchanged until a large ELM is triggered after ~ 100 ms. The island rotation also displays a clear modulation on the amplitude of the peeling mode, which eventually grows to a large amplitude and triggers the ELM. These observations suggest that an edge magnetic island plays a complicated role in RMP-ELM suppression. Though it enhances the pedestal transport that benefits ELM suppression, the rotation of the island also periodically excites a high frequency (~50 kHz) ELM-trigger peeling mode, which can be a result of the 3D field effect on the peeling mode stability boundary.