Evolution and Termination of H-modes in NSTX
Authors: C.E. Bush, R. Maingi, M.G. Bell, R.E. Bell, E.D. Fredrickson, D.A. Gates, S.M. Kaye, S. Kubota, H.W. Kugel, B.P. LeBlanc, R. Maqueda, S. Medley, J.E. Menard, D. Mueller, F. Paoletti, S. Paul, S.A. Sabbagh, V.A. Soukhanovskii, D. Stutman, G. Taylor, S.J. Zweben, D.W. Johnson, R. Kaita, M. Ono, Y.-K.M. Peng, A.L. Roquemore, and E.J. Synakowski
Date of PPPL Report: January 2002
Presented at: the 8th IAEA Technical Committee Meeting on H-mode Physics and Transport Barrier Physics in Toki, Japan, September 5-7, 2001.
The dynamic evolution of the first National Spherical Torus Experiment (NSTX) H-modes [high-confinement modes] will be discussed. The H-modes were obtained in lower-single null divertor discharges with various forms of plasma heating. The exact timing of divertor formation and also the neutral-beam injection (NBI) power level affects both whether or not the discharge exhibits ELMs [edge localized modes] and the duration of the ELM-free phase. The ELM-free discharges had energy confinement as high as 120 ms, whereas the few discharges with ELMs had confinement times approximately 50-70 ms. Buildup of a steep edge density gradient and formation of "ears" on the density profile were observed within a few ms of the L-H [low to high] transition, yielding broader density and pressure profiles. The L-H transition was marked by a decrease in edge visible light and simultaneous increase in electron Bernstein wave emission, reflecting a steepening of the edge density gradient. Gas puff imaging (GPI) of He-I light during the H-mode phase showed rapid formation of a narrow emission layer approximately 2 cm wide in the H-mode phase, which returned within 20 microseconds at termination of the H-mode phase to a broader turbulent emission layer. All of the H-modes were terminated by an MHD reconnection event. The first power threshold (Pth) study showed the neutral-beam injection power (Pb) component of Pth to be less than or equal to 0.84 MW, higher than that predicted by the ITER [International Thermonuclear Exprimental Reactor] database scaling.