Progress In Understanding The Enhanced Petestal H-mode In NSTX
Authors: Stefan Gerhardt, et. al.
Abstract: The 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 behavior 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 10 cm 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 t hermal transport is comparable to or less than anticipated from a simple
neoclassical transport model. Cases are shown where EP H-modes were reliably generated, through these low-q95
examples were difficult to sustain. A case where an externally triggered 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
successful.
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Published in: Nuclear Fusion 54 083021 (26 June 2014)
doi:10.1088/0029-5515/54/8/083021
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Download PPPL-5031 (pdf3.43 MB 39 pp)
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