Pedestal and edge turbulence characteristics from an XGC1 gyrokinetic simulation

Abstract

Understanding the multi-scale neoclassical and turbulence physics in the edge region (pedestal + scrape-off layer) is required in order to reliably predict performance in future fusion devices. We explore turbulent characteristics in the edge region from a multiscale neoclassical and turbulent XGC1 gyrokinetic simulation in a DIII-D like tokamak geometry, here excluding neutrals and collisions. For an H-mode type plasma with steep pedestal, it is found that the electron density fluctuations increase towards the separatrix, and stay high well into the SOL, reaching a maximum value of $\delta n_e / \bar{n}_e \sim 0.18$. Blobs are observed, born around the magnetic separatrix surface and propagate radially outward with velocities generally less than 1 km/s. Strong poloidal motion of the blobs is also present, near 20 km/s, consistent with $E \times B$ rotation. The electron density fluctuations show a negative skewness in the closed field line pedestal regions, consistent with the presence of "holes", followed by a transition to strong positive skewness across the separatrix and into the SOL. These simulations indicate that not only neoclassical phenomena, but also turbulence, including the blob-generation mechanism, can remain important in the steep H-mode pedestal and SOL. Qualitative comparisons will be made to experimental observations.