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(EX/C2-5) H-mode Research in NSTX

R. Maingi1), M.G. Bell2), R.E. Bell2), C.E. Bush1), E.D. Fredrickson2), D.A. Gates2), T. Gray3), D.W. Johnson2), R. Kaita2), S.M. Kaye2), S. Kubota4), H.W. Kugel2), C.J. Lasnier5), B.P. LeBlanc2), R.J. Maqueda6), D. Mastrovito2), J.E. Menard2), D. Mueller2), M. Ono2), F. Paoletti7), S.J. Paul2), Y-K.M. Peng1), A.L. Roquemore2), S.A. Sabbagh7), C.H. Skinner2), V.A. Soukhanovskii2), D. Stutman8), E.J. Synakowski2), T. Tan9), J.B. Wilgen1), S.J. Zweben2)
1) Oak Ridge National Laboratory, Oak Ridge, USA
2) Princeton Plasma Physics Laboratory, PO Box 451, Princeton, NJ, USA
3) North Carolina State University, Raleigh, NC, USA
4) University of California at Los Angeles, Los Angeles, CA, USA
5) Lawrence Livermore National Laboratory, Livermore, CA, USA
6) Los Alamos National Laboratory, Los Alamos, NM, USA
7) Columbia University, New York, NY, USA
8) Johns Hopkins University, Baltimore, MD, USA
9) West-Windsor/Plainsboro High-School North, Plainsboro, NJ, USA

Abstract.  The National Spherical Torus Experiment is a medium-sized device whose mission includes the evaluation of confinement and stability at low aspect ratio. This paper will discuss various characteristics of NSTX H-mode discharges, including transport, scaling of the threshold power, macroscopic stability and investigations of the H-mode edge region with probes and spectroscopic imaging diagnostics. In summary, both ELM-free and ELMy H-modes have been triggered on NSTX, with either neutral beam or radio-frequency auxiliary heating. Energy confinement time as high as 120 ms was obtained transiently in ELM-free H-mode, which is 50% above the ITER98 Hpby(2) scaling. Both giant and ``grassy'' ELMs have been observed. Wall conditioning has been found to play a pivotal role in obtaining good H-mode discharges. Access to H-mode was first enabled by boronization of the plasma facing components. The first long pulse H-modes were obtained after a 350 deg. C bakeout of the plasma facing components, followed by boronization. A reduction in the intrinsic non-axisymmetric error field has helped to extend the duration of the H-mode phase by reducing the occurrence of locked modes.

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IAEA 2003