51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009;
Atlanta, Georgia
Session NI3: Plasma Rotation and Helical Equilibria
9:30 AM–12:30 PM,
Wednesday, November 4, 2009
Room: Centennial II
Chair: Eric Fredrickson, Princeton Plasma Physics Laboratory
Abstract ID: BAPS.2009.DPP.NI3.5
Abstract: NI3.00005 : Transport barriers in helical equilibria: structural change in the reversed field pinch*
11:30 AM–12:00 PM
Preview Abstract
Abstract
Author:
Piero Martin
(Consorzio RFX - Associazione EURATOM-ENEA sulla fusione, Padova, Italy)
Self-organization of RFX plasmas in single helical axis
equilibrium, with
$m$=1,$n$=7 helicity, is a structural change for the reversed
field pinch (RFP)
[Lorenzini et al., Nature Phys 2009 doi:10.1038/nphys1308]. This
happens at
high plasma current (I$>$1 MA) while axisymmetric boundary
conditions are
enforced: the helical state has almost conserved magnetic flux
surfaces,
interpreted as ghost surfaces [Hudson{\&}Breslau PRL 2008],
leading to
strong core electron transport barriers. Electron temperature Te
reaches
1.3keV @1.7MA. Ion temperature Ti is $\sim $(0.5-0.75)Te,
consistent with
collisional ion heating. The core barrier extends up to $\sim
$0.65r/a.
Magnetic surfaces quality improves with Lundquist number $S$,
thanks to the
simultaneous decrease of magnetic chaos and increase of the
helical field
strength. Helical equilibria are reconstructed by the 3d code
VMEC. The
(1,7) helicity acts to hold the core safety factor almost flat
and below
1/7. The barrier foot coincides with a zero magnetic shear
region, where
density of rational surfaces is minimum, as in other configurations.
Plasma-wall interaction is smoother. Main gas particle
confinement time
improves in pellet-fuelled plasmas, with record value $\sim
$10ms. No core
impurity accumulation is evident in Laser Blow Off experiments,
which is
consistent with numerical simulation results.
High current sets a transition also for the edge, where robust Te
gradients
are observed with a pedestal of $\sim $1keV in $\sim $3cm,
possibly due to
improved magnetic topology and synergic with core barrier.
As persistence and quality of these improved helical states
increase with
current, the likelihood of achieving steady helical multi-MA RFPs
can be
inferred. RFX experiments allow a study of the beneficial effects of
non-axisymmetric shaping and may provide a platform for a more
general
validation of theoretical tools developed for stellarators.
Moreover, these
results are transformational in supporting the RFP as a
low-external field,
non-disruptive, ohmic approach to fusion, exploiting
self-organization and
technological simplicity.
*In collaboration with the RFX-mod team.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.DPP.NI3.5