PPPL-3764 is available in pdf format (2.6 MB).

Configuration and Heating Power Dependence of Edge Parameters and H-mode Dynamics in National Spherical Torus Experiment (NSTX)

Authors: C.E. Bush, M.G. Bell, R.E. Bell, J. Boedo, E.D. Fredrickson, S.M. Kaye, S. Kubota, B.P. LeBlanc, R. Maingi, R.J. Maqueda, S.A. Sabbagh, V.A. Soukhanovskii, D. Stutman, D.W. Swain, J.B. Wilgen, S.J. Zweben, W.M. Davis, D.A. Gates, D.W. Johnson, R. Kaita, H.W. Kugel, D. Mastrovito, S. Medley, J.E. Menard, D. Mueller, M. Ono, F. Paoletti, S.J. Paul, Y-K.M. Peng, R. Raman, P.G. Roney, A.L. Roquemore, C.H. Skinner, E.J. Synakowski, and G. Taylor, for the NSTX Team

Date of PPPL Report: January 2003

Presented at: the 44th Annual Meeting of APS/DPP, in Orlando, Fla., November 11-15, 2002. Submitted to Physics of Plasmas.

Edge parameters play a critical role in H-mode (high-confinement mode) access, which is a key component of discharge optimization in present day toroidal confinement experiments and the design of next-generation devices. Because the edge magnetic topology of a spherical torus (ST) differs from a conventional aspect ratio tokamak, H-modes in STs exhibit important differences compared with tokamaks. The dependence of the NSTX (National Spherical Torus Experiment) edge plasma on heating power, including the L-H transition requirements and the occurrence of edge-localized modes (ELMs), and on divertor configuration is quantified. Comparisons between good L-modes (low-confinement modes) and H-modes show greater differences in the ion channel than the electron channel. The threshold power for the H-mode transition in NSTX is generally above the predictions of a recent ITER (International Thermonuclear Experimental Reactor) scaling. Correlations of transition and ELM phenomena with turbulent fluctuations revealed by Gas Puff Imaging (GPI) and reflectometry are observed. In both single-null and double-null divertor discharges, the density peaks off-axis, sometimes developing prominent "ears" which can be sustained for many energy confinement times, tE, in the absence of ELMs. A wide variety of ELM behavior is observed, and ELM characteristics depend on configuration and fueling.