Kinetic instabilities that limit {\beta} in the edge of a tokamak plasma: a picture of an H-mode pedestal

摘要

Plasma equilibria reconstructed from the Mega-Amp Spherical Tokamak (MAST)have sufficient resolution to capture plasma evolution during the short periodbetween edge-localized modes (ELMs). Immediately after the ELM steep gradientsin pressure, P, and density, ne, form pedestals close to the separatrix, andthey then expand into the core. Local gyrokinetic analysis over the ELM cyclereveals the dominant microinstabilities at perpendicular wavelengths of theorder of the ion Larmor radius. These are kinetic ballooning modes (KBMs) inthe pedestal and microtearing modes (MTMs) in the core close to the pedestaltop. The evolving growth rate spectra, supported by gyrokinetic analysis usingartificial local equilibrium scans, suggest a new physical picture for theformation and arrest of this pedestal.