49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007;
Orlando, Florida
Session YI2: HEDP and Plasma Simulations
9:30 AM–12:00 PM,
Friday, November 16, 2007
Rosen Centre Hotel
Room: Salon 3/4
Chair: Bedros Afeyan, Polymath Research Inc.
Abstract ID: BAPS.2007.DPP.YI2.1
Abstract: YI2.00001 : High-Intensity-Laser--Solid Interactions in the Refluxing Limit
9:30 AM–10:00 AM
Preview Abstract
Abstract
Author:
P.M. Nilson
(Laboratory for Laser Energetics, U. of Rochester)
Small mass targets are of interest in high-intensity-laser--solid
interactions due to their unique fast-electron transport
properties.\footnote{ S. P. Hatchett \textit{et al}., Phys. Plasmas \textbf{7}, 2076
(2000).}$^{,}$\footnote{ R. A. Snavely\textit{ et al}., Phys. Rev. Lett. \textbf{85}, 2945
(2000).} Electron refluxing in solid-density matter by the Debye sheath
fields that are set up at the target surfaces provide a unique environment
for the determination of the conversion efficiency of laser energy into fast
electrons.\footnote{ W. Theobald\textit{ et al}., Phys. Plasmas \textbf{13}, 043102
(2006).} Previous measurements of the absolute K$_{\alpha }$ yield from
copper foils as a function of laser intensity demonstrate excellent
agreement with electron refluxing models.$^{3,}$\footnote{ J. Myatt\textit{ et al}., Phys.
Plasmas \textbf{14}, 056301 (2007).}$^{ }$In particular, fast-electron
conversion efficiencies of around 10{\%} to 20{\%} have been inferred by
fitting the absolute K$_{\alpha }$ yields to semi-analytical modeling. It is
well known that ionization of the M shell during volumetric heating within
such small mass copper targets can cause a deviation in the ratio of the
number of emitted K$_{\beta }$ and K$_{\alpha }$ photons below the cold
material limit.$^{4}$ This is a direct consequence of bulk target heating
due to fast-electron energy loss. Such a deviation could provide a useful
code benchmarking parameter on the energy content of the fast electrons and
a consistency check on the laser-electron conversion efficiency. This
consistency check, however, has proven elusive experimentally. We
demonstrate here for the first time the consistency between the
fast-electron conversion efficiencies predicted by these two methods using
small mass targets. It is demonstrated that a 3.5$\times $ reduction in the
ratio of the number of emitted K$_{\beta }$ and K$_{\alpha }$photons is
achievable below the cold material limit using 20 $\times $ 20 $\times $ 2
\textit{$\mu $}m copper targets at laser intensities of 2 $\times $ 10$^{19}$ W cm$^{-2}$.
These results provide a comparison in preparation for the higher
energy-density environments that will be accessible using the future OMEGA
EP Laser Facility.\footnote{ C. Stoeckl\textit{ et al}., Fusion Sci. Technol. \textbf{49},
367 (2006)} This work was supported by the U.S. Department of Energy Office
of Inertial Confinement Fusion under Cooperative Agreements
DE-FC52-92SF19460 and DE-FC02-04ER54789. Contributors: W. Theobald, J.
Myatt, M. Storm, O.V. Gotchev, C. Mileham, C. Stoeckl, R. Betti,$^{\ast }$
D.D. Meyerhofer,$^{\ast }$ and T.C. Sangster. $^{\ast }$Also at the Fusion
Science Center for Extreme States of Matter and Fast Ignition.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2007.DPP.YI2.1