H-Mode Turbulence, Power Threshold, ELM, and Pedestal Studies in NSTX
Authors: R. Maingi, C.E. Bush, E.D. Fredrickson, D.A. Gates, S.M. Kaye, B.P. LeBlanc, J.E. Menard, H. Meyer, D. Mueller, N. Nishino, A.L. Roquemore, S.A. Sabbagh, K. Tritz, S.J. Zweben, M.G. Bell, R.E. Bell, T. Biewer, J.A. Boedo, D.W. Johnson, R. Kaita, H.W. Kugel, R.J. Maqueda, T. Munsat, R. Raman, V.A. Soukhanovskii, T. Stevenson, and D. Stutman
Date of PPPL Report: October 2004
Presented at: the 20th IAEA Fusion Energy Conference, 1-6 November 2004, Vilamoura, Portugal. The papers will be published by the IAEA as unedited proceedings in electronic format on CD-ROM and on the IAEA Physics Section web site as soon as possible after the conference.
High-confinement mode (H-mode) operation plays a crucial role in NSTX [National Spherical Torus Experiment] research, allowing higher beta limits due to reduced plasma pressure peaking, and long-pulse operation due to high bootstrap current fraction. Here, new results are presented in the areas of edge localized modes (ELMs), H-mode pedestal physics, L-H turbulence, and power threshold studies.
ELMs of several other types (as observed in conventional aspect ratio tokamaks) are often observed: (1) large, Type I ELMs, (2) "medium" Type II/III ELMs, and (3) giant ELMs which can reduce stored energy by up to 30% in certain conditions. In addition, many high-performance discharges in NSTX have tiny ELMs (newly termed Type V), which have some differences as compared with ELM types in the published literature.
The H-mode pedestal typically contains between 25-33% of the total stored energy, and the NSTX pedestal energy agrees reasonably well with a recent international multi-machine scaling.
We find that the L-H transition occurs on a ~100 μsec timescale as viewed by a gas puff imaging diagnostic, and that intermittent quiescent periods precede the final transition. A power threshold identity experiment between NSTX and MAST shows comparable loss power at the L-H transition in balanced double-null discharges. Both machines require more power for the L-H transition as the balance is shifted toward lower single null. High field side gas fueling enables more reliable H-mode access, but does not always lead to a lower power threshold e.g., with a reduction of the duration of early heating. Finally the edge plasma parameters just before the L-H transition were compared with theories of the transition. It was found that while some theories can separate well-developed L- and H-mode data, they have little predictive value.