53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011;
Salt Lake City, Utah
Session CI2: Pedestal Control With 3D Fields
2:00 PM–5:00 PM,
Monday, November 14, 2011
Room: Ballroom BD
Chair: Jon Menard, Princeton Plasma Physics Laboratory
Abstract ID: BAPS.2011.DPP.CI2.3
Abstract: CI2.00003 : Enhanced Superbanana Transport Caused by Chaotic Scattering across an Asymmetric Separatrix*
3:00 PM–3:30 PM
Preview Abstract
Abstract
Author:
Daniel Dubin
(Univ. of California, San Diego)
This talk discusses a novel ``chaotic'' form of superbanana
transport,
and compares theory to experiments on nonneutral
plasmas.\footnote{D. Dubin
and Y. Tsidulko, Phys. Plas. {\bf 18}, 062114 (2011); A.A.
Kabantsev {\it et al.},
Phys. Rev. Lett. {\bf 105}, 205001 (2010).}
Magnetically-confined plasmas often have one or more
locally-trapped particle
populations, partitioned by separatrices from one another and
from passing particles.
Strong superbanana transport is caused by particles that cross these
separatrices in the presence of field ``errors'' (such as
toroidal magnetic
curvature), since trapped and passing particles respond to the
field error
differently.\footnote{H. Mynick, Phys. Plasmas {\bf 13}, 058102
(2006);
H. Mynick, Phys. Fluids {\bf 26}, 2609 (1983).}
Collisional scattering (at rate $\nu$) is one mechanism driving the
separatrix crossings; theory predicts a collisional boundary
layer at the separatrix
energy, and collisional transport that scales as
$\nu^{1/2} B^{-1/2}$.
The chaotic transport of interest here occurs when the separatrix is
``ruffled'' in the direction of plasma drift; then,
collisionless particle orbits (tp orbits)
cross the separatrix, giving essentially random trapping and
de-trapping, with
transport scaling as $\nu^0 B^{-1}$.
Prior theory assumed a symmetry such that these tp orbits become
trapped and
detrapped on the same flux surface, thereby giving zero chaotic
transport
and reduced collisional transport.$^3$
Here, we characterize chaotic transport without the assumed
symmetry, and find
quantitative agreement with pure electron plasma experiments and
simulations
in cylindrical geometry.
A global field error
consisting of a small tilt of the trap magnetic field is applied,
to play the role of large-scale curvature in tokamaks or
stellarators.
Also, a separatrix with
two trapped particle populations is produced by applying a
``squeeze potential''
to the middle section of the plasma column.
When the separatrix is $\theta$-symmetric,
radial transport is observed to scale as $1/ \sqrt{B}$ in
agreement with
standard $\sqrt{\nu}$ superbanana theory.
When the separatrix is not $\theta$
symmetric, some particles transit chaotically from trapped to
passing
and back as they ExB drift
in $\theta$ (the tp orbits).
Typical field errors then cause tp orbits to trap and
detrap on different flux surfaces, and enhanced transport
scaling as
$1/B$ is observed in the experiments,
in quantitative agreement with our theory and simulations.
*Supported by NSF PHY-0903877 and DOE DE-SC0002451.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.DPP.CI2.3