Characterization of small, Type V edge-localized modes in the National Spherical Torus Experiment

R. Maingi, M. G. Bell, E. D. Fredrickson, K. C. Lee, R. J. Maqueda, P. Snyder, K. Tritz, S. J. Zweben, R. E. Bell, T. M. Biewer, C. E. Bush, J. Boedo, N. H. Brooks, L. Delgado-Aparicio, C. W. Domier, D. A. Gates, D. W. Johnson, R. Kaita, S. M. Kaye, H. W. KugelB. P. LeBlanc, N. C. Luhmann, J. E. Menard, D. Mueller, H. Park, R. Raman, A. L. Roquemore, S. A. Sabbagh, V. A. Soukhanovskii, T. Stevenson, D. Stutman

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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 Type V ELM is of an instability with one or two filaments that rotate toroidally at ∼5-10 km/s, in the direction opposite to the plasma current and neutral beam injection. The toroidal extent of Type V ELMs is typically ∼5 m, 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 versus 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.

Original languageEnglish
Article number092510
JournalPhysics of Plasmas
Volume13
Issue number9
DOIs
StatePublished - 2006

Funding

This research was supported by the U. S. Dept. of Energy under Contracts No. DE-AC05-00OR22725, No. DE-AC02-76CH03073, No. DE-FC02-04ER54698, No. W-7405-ENG-48 and Grants No. DE-FG02-99ER54524, No. DE-FG02-04ER54767, No. DE-FG03-99ER54518, No. DE-FG02-04ER54767, No. DE-FG03-99ER54519, No. DE-FG03-99ER51069, and No. DE-FG02-99ER54523. The filter scope data shown in Fig. 7 were enabled through a diagnostic collaboration between NSTX and DIII-D. We gratefully acknowledge the contribution of the NSTX technical staff and neutral beam operations staff.

FundersFunder number
U. S. Dept. of EnergyDE-FG03-99ER51069

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