51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009;
Atlanta, Georgia
Session UI3: Basic Plasmas: Novel Plasmas in Laboratory and Space
2:00 PM–5:00 PM,
Thursday, November 5, 2009
Room: Centennial II
Chair: Matthew Stoneking, Lawrence University
Abstract ID: BAPS.2009.DPP.UI3.1
Abstract: UI3.00001 : Measurements of Correlation-Enhanced Collision Rates*
2:00 PM–2:30 PM
Preview Abstract
Abstract
Author:
C. Fred Driscoll
(University of California at San Diego)
This talk presents the first detailed experimental measurements
of the
Salpeter collisional enhancement factor $g ( \Gamma )$ in
strongly correlated
plasmas.
This factor is predicted to enhance the nuclear reaction rate
in dense strongly-correlated plasmas, such as
in giant planet interiors, brown dwarfs and degenerate
stars;\footnote{E.E. Salpeter
and H.M. Van Horn, Astrophys. J. {\bf 155}, 183 (1969).}
and recent theory establishes that it also applies to the
perpendicular-to-parallel collisions in magnetized plasmas
described here.\footnote{D.H.E. Dubin,
Phys. Rev. Lett. {\bf 94}, 025002 (2005).}
The enhancement is caused by plasma screening of the repulsive
Coulomb potential
between charges, allowing closer collisions
for a given particle energy.
The enhancement factor is predicted to be large
when the plasma correlation parameter
$\Gamma \equiv e^2 /aT$ is larger than unity, scaling as
$g ( \Gamma ) \sim e^\Gamma$.
The perp-to-parallel collision rate is then
$\nu_{\perp \|} = n \overline{\mathrm{v}} b^2 \, I (
\overline{\kappa} ) \, g ( \Gamma )$,
where $I ( \overline{\kappa} )$ decreases precipitously below
$( 8 \sqrt{\pi} / 15 ) \ln \Lambda$
in the highly magnetized regime of $\overline{\kappa} \equiv
\sqrt{2} \, b / r_c \gg 1$.
$\bullet$
Our measurements\footnote{F. Anderegg {\it et al.}, Phys. Rev.
Lett. {\bf 102},
185001 (2009); F. Anderegg {\it et al.}, Phys. Plasmas {\bf 16},
055705 (2009).}
of $\nu_{\perp \|}$ in Mg$^+$ pure ion plasmas are consistent
with the
predicted Salpeter correlation enhancement, with the comparison
limited mainly by systematic spatial variations in the plasma
temperature.
The plasma temperatures are controlled over the range
$4 \times 10^{-6} < T < 1$eV,
with the outer radii being up to 2$\times$ hotter.
Bulk-averaged collision rates of
$1 < \nu_{\perp \|} < 2 \times 10^4$ sec$^{-1}$ are measured by
2 techniques: for slow collisions, $T_\|$ is heated or cooled,
and the subsequent relaxation is directly observed;
for rapid collisions, sinusoidal modulation of the plasma length at
frequency $f_{\mathrm{mod}}$ gives maximal heating
when $f_{\mathrm{mod}} = \nu_{\perp \|} / 2 \pi c (\Gamma)$, where
$c ( \Gamma )$ is the specific
heat.
Two densities are used, 2.0 and $0.12 \times 10^7$ cm$^{-3}$;
the lower density has $ \sim 2.5 \times$ less
correlation at any temperature.
Experiments clearly show the expected $\nu_{\perp \|} \propto
T^{-3/2}$
regime at high temperatures, and show the strong $I (
\overline{\kappa} )$ suppression of
$\nu_{\perp \|}$ for $b / r_c \gg 1$.
At low temperatures and high density, the measured
$\nu_{\perp \|}$
is enhanced by up to $g \sim 10^{12} $ over the uncorrelated
prediction, consistent
with the Salpeter-enhanced
prediction.
At low (uncorrelated) densities, no enhancement is observed.
Future experiments may be able to image ``burn fronts''
propagating from hot regions
to cold regions.
*Supported by NSF grant PHY-0903877 and NSF/DOE grant PHY-0613740.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.DPP.UI3.1