Local measurements of the pedestal magnetic field profile throughout the ELM cycle on DIII-D

Abstract

New high speed localized measurements of the pedestal magnetic field during the edge localized mode (ELM) cycle of a DIII-D High confinement mode (H-mode) discharge indicate a temporally and spatial complex redistribution of the edge current density profile, jedge. The measurement technique extracts the magnetic field magnitude, B, via the spectral separation of Stark-split neutral beam radiation in the pedestal. Single spatial channel measurements from a novel spatial heterodyne spectrometer are validated in discharges with core current profile changes. The technique measures Stark-splitting changes that imply B changes as small as 1 mT with high time resolution (50  μs). At normalized poloidal flux [Formula: see text], B appears saturated in the inter-ELM period and then rapidly decreases in <200 μs by [Formula: see text], before edge recycling emission begins to increase. Radially inboard of jedge, B increases at the ELM crash. The behavior is consistent with a rapid collapse of jedge at the ELM crash and subsequent pedestal recovery. In some discharges, at [Formula: see text], changes in B are observed throughout the ELM cycle. In others, B recovers and is relatively stable until a few ms leading up to the next crash. Measurements of B during the H-mode transition show a large increase at [Formula: see text] with little change at [Formula: see text], consistent with the formation of the edge bootstrap current density peak. The [Formula: see text] spectrum is complicated by predicted changes to the Stark component intensities with density at the L–H transition.