Progress in understanding the enhanced pedestal H-mode in NSTX

This paper describes the enhanced pedestal (EP) H-mode observed in the National Spherical Torus Experiment (NSTX). The defining characteristics of EP H-mode are given, namely (i) transition after the L- to H-mode transition, (ii) region of very steep ion temperature gradient, and (iii) associated region of strong rotational shear. A newly observed long-pulse EP H-mode example shows quiescent behaviour for as long as the heating and current drive sources are maintained. Cases are shown where the region of steep ion temperature gradient is located at the very edge, and cases where it is shifted up to 10cm inward from the plasma edge; these cases are united by a common dependence of the ion temperature gradient on the toroidal rotation frequency shear. EP H-mode examples have been observed across a wide range of q95 and pedestal collisionality. No strong changes in the fluctuation amplitudes have been observed following the EP H-mode transition, and transport analysis indicates that the ion thermal transport is comparable to or less than anticipated from a simple neoclassical transport model. Cases are shown where EP H-modes were reliably generated, though these low-q95 examples were difficult to sustain. A case where an externally triggered edge localized mode (ELM) precipitates the transition to EP H-mode is also shown, though an initial experiment designed to trigger EP H-modes in this fashion was unsuccessful. © 2014 IAEA, Vienna.

Duke Scholars

Author David R. Smith Electrical and Computer Engineering