Confinement and Local Transport in the National Spherical Torus Experiment NSTX
Authors: S.M. Kaye, F.M. Levinton, D. Stutman, K. Tritz, H. Yuh, M.G. Bell, R.E. Bell, C.W. Domier, D. Gates, W. Horton, J. Kim, B.P. LeBlanc, N.C. Luhmann Jr., R. Maingi, E. Mazzucato, J.E. Menard, D. Mikkelsen, D. Mueller, H. Park, G. Rewoldt, S.A. Sabbagh, D.R. Smith, and W. Wang
NSTX operates at low aspect ratio (R/a~1.3) and high beta (up to 40%), allowing tests of global confinement and local transport properties that have been established from higher aspect ratio devices. NSTX plasmas are heated by up to 7 MW of deuterium neutral beams with preferential electron heating as expected for ITER. Confinement scaling studies indicate a strong ΒΤ dependence, with a current dependence that is weaker than that observed at higher aspect ratio. Dimensionless scaling experiments indicate a strong increase of confinement with decreasing collisionality and a weak degradation with beta. The increase of confinement with ΒΤ is due to reduced transport in the electron channel, while the improvement with plasma current is due to reduced transport in the ion channel related to the decrease in the neoclassical transport level. Improved electron confinement has been observed in plasmas with strong reversed magnetic shear, showing the existence of an electron internal transport barrier (eITB). The development of the eITB may be associated with a reduction in the growth of microtearing modes in the plasma core. Perturbative studies show that while L-mode plasmas with reversed magnetic shear and an eITB exhibit slow changes of LΤe across the profile after the pellet injection, H-mode plasmas with a monotonic q-profile and no eITB show no change in this parameter after pellet injection, indicating the existence of a critical gradient that may be related to the q-profile. Both linear and non-linear simulations indicate the potential importance of ETG modes at the lowest ΒΤ. Localized measurements of high-κ fluctuations exhibit a sharp decrease in signal amplitude levels across the L-H transition, associated with a decrease in both ion and electron transport, and a decrease in calculated linear microinstability growth rates across a wide κ-range, from the ITG/TEM regime up to the ETG regime.
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*NOTICE: this PPPL Report is a preprint version of work that has been accepted for publication in Fusion Science and Technology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in:
Fusion Science and Technology
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Presented at the Twenty-First IAEA Fusion Energy Conference, 16–22 October 2006, Chengdu, China.
Conference Presentation (ppt)
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Download PPPL-4238 Preprint May 2007 (pdf 883 KB)