Abstract Details

Single helicity and quasi-single helicity states in RFPs

Author: Gian Luc Delzanno
Submitted: 2005-12-21 18:12:29

Co-authors: L. Chacon, J. M. Finn

Contact Info:
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). (There are typically several resistive modes as well.)
We evolve these modes nonlinearly to saturation for low Hartmann
number H. We then obtain the m=k=0 quasilinear 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. The results show that these thresholds in H are fairly close
to the thresholds in H between SH and QSH behavior determined by a
fully nonlinear 3D code. The similarity of these threshold values for
H indicate that the stability of the quasilinear profiles plays a
role in the SH - QSH transition, but other factors such as mode
coupling are comparable.

Characterization: A2


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