New High-Speed Measurements of Pedestal Magnetic Field during the ELM cycle on DIII-D and Implications for Modeling*

New high speed pedestal localized measurements of the plasma magnetic field during the ELM cycle of a low input power DIII-D H-mode discharge imply a temporally and spatial complex redistribution of the edge current density profile (j[r]) and a magnitude that is inconsistent with theory. A novel spatial heterodyne spectroscopy technique extracts |B| (specifically the magnitude of the magnetic field perpendicular to the neutral beam velocity) from the Stark-split neutral beam radiation and enables these high time resolution (200 µs) measurements. Measurements across the pedestal, from normalized poloidal flux Ψ~0.9 to 1.0, have been made in repeated discharges. At Ψ=0.975±0.016, |B| appears saturated in the inter-ELM period and then rapidly decreases (in 200 µs) by ~1% at the crash. |B| then gradually recovers to the inter-ELM nominal value over ~10 ms. Inboard of the pedestal, |B| increases slightly after the ELM crash. This behavior is consistent with a rapid collapse of j[r] at the ELM crash and subsequent pedestal recovery. In some discharges, at Ψ>0.96, changes in |B| are observed approximately 5-10 ms before the ELM crash. Measurements of |B| during the H-mode transition show a large increase at Ψ =1 with little change at Ψ =0.9, consistent with the formation of the edge bootstrap current density peak. The measured change in |B| from Ψ =0.9 to 1 is ~2x larger than a predictive model of the edge current density based on a kinetic equilibrium reconstruction and Sauter bootstrap current model. While the increase is consistent, the discrepancy in the magnitude of the change across the pedestal region could have implications for pedestal stability, the ELM drive, and models of the edge current density.