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

Characteristics of the II-mode pedestal are studied in Type I ELM discharge s with ITER cross-sectional shape and aspect ratio. The scaling of the widt h of the edge steep gradient region, delta, which is most consistent with t he data are with the normalized edge pressure, (beta(POL)(PED))(0.4). Fits of delta to a function of temperature, such as rho(POL), are ruled out in d ivertor pumping experiments. The edge pressure gradient is found to scale a s would be expected from infinite n ballooning mode theory; however, the va lue of the pressure gradient exceeds the calculated first stable limit by m ore 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 curre nt density. Transport simulations give edge bootstrap current of sufficient magnitude to open second stable access in these discharges. Ideal kink ana lysis using current density profiles including edge bootstrap current indic ate that before the ELM these discharges may be unstable to low n, edge loc alized modes. (C) 1999 Elsevier Science B.V. All rights reserved.