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

• Published in: Physics of Plasmas 2003-05, Vol.10 (5), p.1755-1764
• Main Authors: 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.
• Format: Article
• Language: English
• ISSN: 1070-664X; 1089-7674
• DOI: 10.1063/1.1567288

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 et al., 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, τ E , in the absence of ELMs. A wide variety of ELM behavior is observed, and ELM characteristics depend on configuration and fueling.