Progress in Characterization of the Pedestal Stability and Turbulence During Edge-localized-mode Cycle On National Spherical Torus Experiment
Authors: Ahmed Diallo, et. al.
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 (NSTX). These high
performance plasmas are found to be ideal kink-peeling and ideal infi nite-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 kθpiped ~ 0.2 propagating in the ion diamagnetic
drift direction at the pedestal top, and radial spatial scales krpiped
~ 0.7. 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 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 uctuations correlation length
measurements.
Accepted to: Nuclear Fusion (May 2013)
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