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approvedku_abstract.pdf2017-03-17 12:25:58Seung-Hoe Ku

Abstracts

Author: Seung-Hoe Ku
Requested Type: Pre-Selected Invited
Submitted: 2017-03-17 12:24:02

Co-authors: C.S. Chang, G.R. Tynan, R. Hager, R.M. Churchill, I. Cziegler, M. Greenwald, A. E. Hubbard, J. W. Hughes

Contact Info:
Princeton Plasma Physics Laboratory
100 Stellarator Rd.
Princeton, NJ   08540
USA

Abstract Text:
Despite its critical importance in the fusion program and over 30 years of H-mode operation, there has been no fundamental understanding at the kinetic level on how the H-mode bifurcation occurs. We report the first observation of an edge transport barrier formation event in an electrostatic gyrokinetic simulation carried out in a realistic C-Mod like diverted tokamak edge geometry under strong forcing by a high rate of heat deposition. The results show that the synergistic action between two multiscale dynamics, the turbulent Reynolds-stress driven [1] and the neoclassical X-point orbit loss drive [2] sheared ExB flows, works together to quench turbulent transport and form a transport barrier just inside the last closed magnetic flux surface. The synergism helps reconcile experimental reports of the key role of turbulent stress in the bifurcation [3] with some other experimental observations that ascribe the bifurcation to X-point orbit loss/neoclassical effects [4,5]. The synergism could also explain other experimental observations that identified a strong correlation between the L-H transition and the orbit loss driven ExB shearing rate [6]. The synergism is consistent with the general experimental observation that the L-H bifurcation is more difficult with the ∇B-drift away from the single-null X-point, in which the X-point orbit-loss effect is weaker [2].
*Work supported by US DOE, mostly through the SciDAC-EPSI program.
[1] P. Diamond et al., Plasma Phys. Controlled Fusion, 47, R35 (2005)
[2] C.S. Chang, S. Ku, and H. Weitzner, Phys. Plasmas 9, 3884 (2002)
[3] G. R. Tynan et al., Nucl. Fusion, 53, 073053 (2013), and the citations therein.
[4] T. Kobayashi, K. Itoh et al., Phys. Rev. Lett. 111, 035002 (2013)
[5] M. Cavedon et al., Nucl. Fusion 57 (2017) 014002[6] D.J. Battaglia et al., Nucl. Fusion 53,113032 (2013); S. M. Kaye et al., Nucl. Fusion 51, 113109 (2011)

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