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Quasi-single helicity states in RFPs: studies of the mean field and of the flow

Author: Luis Chacon
Requested Type: Poster Only
Submitted: 2006-12-13 13:37:05

Co-authors: Gian Luca Delzanno, John M. Finn

Contact Info:
Los Alamos National laboratory
PO Box: 1663
Los Alamos, NM   87545

Abstract Text:
We present a systematic study of single helicity (SH) states and quasi-single helicity (QSH) states in RFPs. We begin with cylindrical paramagnetic pinch equilibria with uniform resistivity, characterized by a single dimensionless parameter proportional to the toroidal electric field, or the RFP toroidal current parameter Theta. For sufficiently high Theta, there are several unstable m=1 ideal MHD instabilities, typically one of which is nonresonant, with 1/n just above q(r=0). We evolve these modes nonlinearly to saturation for low Hartmann number H. We then obtain the m=k=0 mean-field profiles, which typically have toroidal field reversal, and study their stability. For typical cases, these profiles may remain unstable to tearing modes, but only for sufficiently high H. For lower H these states are stable. We show results indicating the proximity of these thresholds to the thresholds between SH and QSH behavior in 3D simulations. Next, we analyze fully three-dimensional runs and repeat the analysis based on the m=k=0 mean-field profiles. We conclude that the existence of single-helicity states is only qualitatively related to the stability properties of its mean-field and it is due to the helical structure of these states. Also, we show the importance of the radial paramagnetic pinch flow at low Hartmann number and how it gives unstable electrostatic (hydrodynamic) modes.

Characterization: A3


University of Maryland

Innovative Confinement Concepts Workshop
February 12-14, 2007
College Park, Maryland

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