Quiet Periods in Edge Turbulence Preceding the L-H Transition in NSTX
Authors: S. Zweben, R.J. Maqueda, R. Hager, K. Hallatschek, S.M. Kaye, T. Munsat, F.M. Poli, A.L. Roquemore, Y. Sechrest, D.P. Stotler
Abstract:
This paper describes the first observations in NSTX of ‘quiet periods’ in the edge turbulence preceding the L-H transition, as diagnosed by the GPI diagnostic near the outer midplane separatrix. During these quiet periods the GPI Dα light emission pattern was transiently similar to that seen during Hmode, i.e. with a relatively small fraction of the GPI light emission located outside the separatrix. These quiet periods had a frequency of ~3 kHz for at least 30 msec before the L-H transition, and were correlated with changes in the direction of the local poloidal velocity. The GPI turbulence images were also analyzed to obtain an estimate for the dimensionless poloidal shearing S =(dVp/dr)(Lr/Lp)τ. The values of S were strongly modulated by the quiet periods, but not otherwise varying for at least 30 msec preceding the L-H transition. Since neither the quiet periods nor the shear flow increased significantly immediately preceding the L-H transition, neither of these appears to be the trigger for this transition, at least for these cases in NSTX.
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Submitted to: Physics of Plasmas (April 2010)
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Download PPPL-4510 (pdf 191 KB 46 pp)
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