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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 H_pby(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