Scaling studies of the high mode pedestal

Abstract

The structure and scaling of the H-mode (high mode) pedestal are examined for discharges in the DIII-D tokamak [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159]. For typical conditions, the pedestal values of the ion and electron temperatures Ti and Te are comparable. Measurements of main ion and C6+ profiles indicate that the ion pressure gradient in the barrier is 50%–100% of the electron pressure gradient for deuterium plasmas. The magnitude of the pressure gradient in the barrier often exceeds the predictions of infinite-n ballooning mode theory by a factor of 2. Moreover, via the bootstrap current, the finite pressure gradient acts to entirely remove ballooning stability limits for typical discharges. For a large dataset, the width of the pressure barrier δ is best described by the dimensionless scaling δ/R∝(βpolped)0.4 where (βpolped) is the pedestal value of poloidal beta and R is the major radius. Scalings based on the poloidal ion gyroradius or the edge density gradient do not adequately describe overall trends in the data set and the propagation of the pressure barrier observed between edge-localized modes. The width of the Ti barrier is quite variable and is not a good measure of the width of the pressure barrier.