H-mode pedestal characteristics in ITER shape discharges on DIII-D

Characteristics of the H-mode pedestal are studied in Type I ELM discharges with ITER cross-sectional shape and aspect ratio. The scaling of the width of the edge steep gradient region, δ, which is most consistent with the data are with the normalized edge pressure, (β_POL^PED)^0.4. Fits of δ to a function of temperature, such as ρ_POL, are ruled out in divertor pumping experiments. The edge pressure gradient is found to scale as would be expected from infinite n ballooning mode theory; however, the value of the pressure gradient exceeds the calculated first stable limit by more than a factor of 2 in some discharges. This high edge pressure gradient is consistent with access to the second stable regime for ideal ballooning for surfaces near the edge. In lower q discharges, including discharges at the ITER value of q, edge second stability requires significant edge current density. Transport simulations give edge bootstrap current of sufficient magnitude to open second stable access in these discharges. Ideal kink analysis using current density profiles including edge bootstrap current indicate that before the ELM these discharges may be unstable to low n, edge localized modes.