H-mode threshold and dynamics in the National Spherical Torus Experiment

Journal Article · Wed Jan 01 00:00:00 EST 2003 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.1567288· OSTI ID:1017366

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

Edge parameters play a critical role in high confinement mode (H-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 National Spherical Torus Experiment (NSTX) [C. Neumeyer , Fusion Eng. Des. 54, 275 (2001)] edge plasma on heating power, including the low confinement mode (L-mode) to H-mode (L-H) transition requirements and the occurrence of edge-localized modes (ELMs), and on divertor configuration is quantified. Comparisons between good L-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 International Tokamak Experimental Reactor (ITER) [ITER Physics Basis Editors, Nucl. Fusion 39, 2175 (1999)] scaling. Correlations of transition and ELM phenomena with turbulent fluctuations revealed by gas puff imaging 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, tau(E), in the absence of ELMs. A wide variety of ELM behavior is observed, and ELM characteristics depend on configuration and fueling. (C) 2003 American Institute of Physics.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE
• Contributing Organization: NSTX Team
• DOE Contract Number: DE-AC05-00OR22725; AC02-76CH03073; W-7405-ENG-36; FG02-99ER54524
• OSTI ID: 1017366
• Journal Information: Physics of Plasmas, Vol. 10, Issue 5; ISSN 1070-664X: PHPAEN
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English