51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009;
Atlanta, Georgia
Session PI2: Tokamak Physics: Edge, Scrape-off Layer and Wall
2:00 PM–5:00 PM,
Wednesday, November 4, 2009
Room: Centennial I
Chair: Andrea Garofalo, General Atomics
Abstract ID: BAPS.2009.DPP.PI2.1
Abstract: PI2.00001 : ELMs, Magnetic X-points, and Chaotic Fields*
2:00 PM–2:30 PM
Preview Abstract
Abstract
Author:
Linda E. Sugiyama
(M.I.T.)
Edge Localized Modes (ELMs) in a magnetically confined plasma are
shown to be a new type of nonlinear plasma instability, in which
a coherent plasma structure couples to part of a chaotic magnetic
field. Numerical simulation using the M3D code [1] provides a
detailed picture. Toroidal magnetic fields can be described as
Hamiltonian systems. Under small perturbations, a plasma boundary
magnetic surface containing an X-point splits into
two, defined asymptotically by the limits of the field lines
traced infinitely in each direction. The limiting surfaces
overlap to form a homoclinic tangle [2]. The steep pressure
gradient near the edge of an H-mode fusion plasma drives
ballooning-type plasma instabilities, aligned along the
equilibrium magnetic field. These can couple nonlinearly to
the ``unstable'' magnetic surface that ripples the
equilibrium boundary. Fingers of plasma grow outwards, above and
below the midplane on the outboard side, and the alternating low
density regions may penetrate deep into the plasma.
The field becomes chaotic over the affected region. Field lines
are contained for many toroidal transits, but develop significant
radial excursions. Many are eventually lost through the X-point
region. The original magnetic boundary is preserved.
Over several hundred Alfv\'en times the plasma heals back towards
the original shape, but with relaxed profiles of density and
temperature. A complex nonlinear interaction between
the plasma instability and a magnetic homoclinic tangle
leads to distinct stages in the ELM crash, that are similar to
experimental observations. This new picture may help to explain
the large range of ELM and ELM-free behavior seen in experiments
and suggests a re-examination of H-mode edge confinement and the
L-H transition.\\[4pt]
[1] W. Park, et al., \emph{Phys. Plasmas} \textbf{6}
1796 (1999).\\[0pt]
[2] T. Evans, et al,, \emph{J. Phys. Conf. Series}
\textbf{7} 174 (2005).
*Work supported by the U.S. Department of Energy.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.DPP.PI2.1