PPPL-3845 is available in <a href="PPPL-3845.pdf">pdf</a> format (1.0 MB).

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<b>Low- to High-confinement Mode Transitions in the National Spherical Torus Experiment</b>
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<b>Authors: </b> S.M. Kaye, C.E. Bush, E. Fredrickson, B. LeBlanc, R. Maingi, and S.A. Sabbagh
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<b>Date of PPPL Report:</b> July 2003
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<b>Published in:</b> Phy. Plasmas <b>10</b> (October 2003) 3953-3960.</p>


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Edge data from plasmas in the National Spherical Torus Experiment (NSTX) [S. Kaye et al., Fusion Technology <b>36</b> (1999) 16] have been compared to theories of transport suppression that have been used to develop a physics framework for low-confinement (L) to high-confinement (H) mode transitions. The NSTX data were obtained in low aspect ratio (R/a approximately equal to 1.3) discharges taken from a variety of discharge phases, including L-modes, L-H transitions, and H-modes with and without edge localized modes (ELMs). The comparisons show that the group of points taken just before the L-H mode transition are well mixed with the purely L-mode group to within the measurement uncertainties, indicating that changes in these parameters leading up to the transition are subtle. One of the theory parameters, <FONT FACE = "SYMBOL">a</FONT FACE = "SYMBOL"><sub>MHD</sub> = -Rq<sup>2</sup>d<FONT FACE = "SYMBOL">b</FONT FACE = "SYMBOL"><sub>t</sub>/dr, does show a clear threshold (<FONT FACE = "SYMBOL">a</FONT FACE = "SYMBOL"><sub>MHD</sub> = 1 to 2) between the H-mode grouping of points and those remaining in the L-mode or taken just prior to the transition. Additionally, there is no evidence for an edge temperature threshold necessary for transitioning into the H-mode. NSTX data indicate further a possible connection between L-H transitions and non-ambipolar beam ion losses.



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