Progress in characterization of the pedestal stability and turbulence during the edge-localized-mode cycle on National Spherical Torus Experiment

Abstract

Progress in characterizing the edge stability and properties of the microinstabilities responsible for enhanced transport in the pedestal region is reported. The stability of the pedestal is characterized in high performance discharges on National Spherical Torus Experiment. These high performance plasmas are found to be ideal kink-peeling and ideal infinite-n ballooning unstable prior to the onset of edge-localized modes (ELM). The spatial structure of turbulence present during an ELM cycle in the pedestal region indicates poloidal spatial scales propagating in the ion diamagnetic drift direction at the pedestal top, and radial spatial scales . These propagating spatial scales are found to be poloidally elongated and consistent with ion-scale microturbulence. Both global and local gyrokinetic simulations have been performed to identify the microturbulence structure. The local gyrokinetic analysis indicates the presence of a linearly unstable hybrid kinetic ballooning mode and trapped electron mode with spatial scale and propagation direction consistent with experimental observations. In the global gyrokinetic analysis, the nonlinearly saturated potential fluctuations show radial and poloidal correlation lengths in agreement with experimental density fluctuation correlation length measurements.