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epr_munaretto-1.pdf2014-08-15 09:19:12Stefano Munaretto
epr_munaretto.pdf2014-08-15 09:18:00Stefano Munaretto

Control and understanding of 3D equilibria in the RFP

Author: Stefano Munaretto
Requested Type: Consider for Invited
Submitted: 2014-05-30 13:55:13

Co-authors: B.E. Chapman, M.D. Nornberg, D. Brower, M. Cianciosa, D.J. Den Hartog, W.X. Ding, J. Duff, P. Franz, J.A. Goetz, J.D. Hanson, D. Holly, J.-H. Kim, J.J. Koliner, L. Lin, M. McGarry, L. Morton, E. Parke, D. Terranova, P.W. Terry, G.G. Whelan

Contact Info:
University of Wisconsin - Madison
1150 University Ave
Madison, Wisconsin   53726
USA

Abstract Text:
A resonant magnetic perturbation (RMP) technique has been developed to vary controllably the orientation of 3D equilibria in the MST RFP. This has provided much improved diagnosis of these states, leading to the observation of a transport barrier in the core and an improvement of 3D equilibrium reconstructions with V3FIT-VMEC. The temporal dynamics of the 3D state and its dependence on Ip has been captured in a predator-prey model in which the 3D equilibrium is characterized as a coherent vortex that is sustained by shear suppression of background magnetic turbulence. This 3D state is now common in the RFP, subject to sufficiently large Ip (and Lundquist number). It emerges when the innermost resonant m = 1 mode becomes much larger than the neighboring modes in the core. Despite its routine appearance, much work remains to understand the underlying physics. Diagnosis of this state is hampered by the fact that the helical structure is stationary but with an orientation that varies shot to shot. Application of an RMP, with br/Bp(a) ~ 10%, now allows alignment of the structure with key diagnostics, including Thomson scattering and an interferometer-polarimeter. An electron thermal transport barrier forms in the core, with an increase in stored thermal energy, but the barrier is intermittent. On the other hand, particle confinement appears to be enhanced with less intermittency. 3D equilibrium reconstructions are progressing rapidly, now including polarimetry for internal B and advancing toward the more realistic assumption of a free boundary in VMEC. A new theoretical framework has been developed for the 3D state in which the dominant mode is a coherent structure that produces both flow and magnetic shear. These shears suppress the other m = 1 modes and thus help to sustain the coherent structure. The strength of the shears increases with Ip. At relatively low Ip, the structure is intermittent, but at sufficiently large Ip, the structure persists in steady state.

Characterization: 3.0

Comments:

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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