Characterization of Small, Type V ELMs in the National Spherical Torus Experiment
Authors: R. Maingi, M.G. Bell, E.D. Fredrickson, K.C. Lee, R.J. Maqueda, P. Snyder, et al.
Abstract
There has been a substantial international research effort in the fusion community to identify tokamak operating regimes with either small or no periodic bursts of particles and power from the edge plasma, known as edge localized modes (ELMs). While several candidate regimes have been presented in the literature, very little has been published on the characteristics of the small ELMs themselves. One such small ELM regime, also known as the Type V ELM regime, was recently identified in the National Spherical Torus Experiment [M. Ono, S.M. Kaye, Y.-K.M. Peng, et. al., Nucl. Fusion 40, 557 (2000)]. In this paper, the spatial and temporal structure of the Type V ELMs is presented, as measured by several different diagnostics. The composite picture of the 2
Type V ELM is of an instability with one or two filaments which rotate toroidally at ~5–10 km/sec, in the direction opposite to the plasma current and neutral beam injection. The toroidal extent of Type V ELMs is typically ~5m, whereas the cross-field (radial) extent is typically ~10 cm (3 cm), yielding a portrait of an electromagnetic, ribbon-like perturbation aligned with the total magnetic field. The filaments comprising the Type V ELM appear to be destabilized near the top of the H-mode pedestal and drift radially outward as they rotate toroidally. After the filaments come in contact with the open field lines, the divertor plasma perturbations are qualitatively similar to other ELM Types, albeit with only one or two filaments in the Type V ELM vs. more filaments for Type I or Type III ELMs. Preliminary stability calculations eliminate pressure driven modes as the underlying instability for Type V ELMs, but more work is required to determine if current driven modes are responsible for destabilization.
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Published in: Physics of Plasmas 13, 092510 (Sep 2006)
doi: 10.1063/1.2226986
Copyright (2006) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
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Presented at the Fifth International Conference on Atomic and Molecular Data and Their Applications (ICAMDATA), 15–19 October 2006, Meudon, France.
Download PPPL-4198 Preprint (Dec 2006) (1.9 MB)