Systematic Study of Fast Electron Transport in Solid and Warm DensePlasma
Farhat Beg, Toshi Yabuuchi, and Hiroshi Sawada,Center for Energy Research, University of California San Diego, San Diego, California

Mingsheng Wei and Richard Stephens
General Atomics, San Diego, California

Sean Regan, Ken Anderson, Christian Stoeckl, Wolfgang Theobald and Riccardo Betti
Laboratory for Laser Energetics, Rochester, New York

Pravesh Patel, Andrew Mackinnon, Harry McLean, Scott Wilks and Mike Key
Lawrence Livermore National Laboratory, Livermore, California.
We report on experiments to study fast electron transport in a large volume (~300 µm) of warm dense
plasma, which was created by shock heating of 200 mg/ cm3 CH foam by 1.2 kJ, 3.5 ns laser and wascharacterized with a Sm x-ray backlighter. Radiation hydrodynamics simulations show about 40-50 eV
plasma with electron density of 1022 /cm3. Fast electrons created with OMEGA EP short pulse laser (1 kJ,10 ps) interacting with a Au foil on one side of plastic cube were collected on the opposite side on a Cu
foil to give information about the dynamics and energy deposition of fast electrons.

1. Introduction:
Understanding of relativistic electron generation and transport in solid and warm dense
plasma is a fundamental issue pertinent to relativistic high energy density plasmas and intense
particle beams. We have performed two experiments on the OMEGA EP laser to address thisimportant issue. The goals of the experiments were: i) to characterize plasma to be used as thetransport medium using the x-ray line absorption spectroscopy, and ii) to study the fast electrontransport in the characterized plasma.

2. Experimental Setup and Results:
In the first experiment, a shock-wave-heated foam target was used to create warm dense
plasma. The foam target package consisted of 200 mg/cm3 plastic foam doped with aluminum
inside a solid plastic container. A long pulse laser (1.2 kJ/3.5 ns in UV) irradiated onto a plastic
foil to launch a shock wave into the foam target. The other three lasers were focused onto asamarium (Sm) dot target to produce an x-ray point source for the absorption spectroscopy. The
Sm x-ray spectrum (1.4 – 1.6 keV) was transmitted through the Al-doped foam target andrecorded with an x-ray streak camera. The spectral analysis using a detailed atomic physics code
Prism SPECT [1] show the electron temperature of 40 ± 5 eV and the mass density of 30 ± 10mg/cm3 at 7 ns after the drive laser. Two-dimensional DRACO simulation [2] shows the foam

plasma peak temperature and density of 30 - 50 eV and 110-180 mg/cm3 respectively at about 7– 8 ns. The low density in experiments was due to reduced energy in driver laser beams.
In the second experiment, fast electrons generated by an OMEGA EP short pulse weretransported into the characterized foam plasma. The EP beam interacted with a gold foil on one
side of the package target to generate the fast electrons. The electron transport was studied bymeasuring 8.0 keV Kα x-ray from a Cu foil attached to the other side. The Kα x-rays induced by
the electrons were recorded with an x-ray spectrometer and a spherical crystal imager. Fig. 1
shows the Kα images from the three transport media: (a) a solid plastic, (b) an un-driven foam,and (c) the laser-driven foam plasma. A small Kα spot was observed in the cold plastic target;
however, no clear structure was found in the foam targets, indicating a large divergence of therelativistic electron beam in both driven and un-driven foam. The comparison of the total Kαyields in the foam targets shows a reduction of the yield by factor of 20 in the driven foam (Fig.
1d). Low signal in the warm foam target could be attributed to interface field effects andrelativistic electron beam instability exiting the gold layer into warm dense plasma.

Fig. 1. Cu Kα images observed with spherical crystal imager when the transport medium was:(a) 50 µm solid CH, (b) 250 µm cold foam, and (c) 360 µm driven foam. Integrated Kα x-ray
yield measured with the x-ray spectrometer for three cases (d).
The authors are grateful to B. Yaakobi and R. E. Bahr for diagnostic expertise, as wellas to the OMEGA EP crew for the operations of four EP UV beams and a short pulse beam. The
work was supported by NNSA/US DOE under Contracts DE-FG52-09NA29033 (NLUF), DEFC02-04ER54789 (FSC), and DEFG0205ER54834 (ACE).

References:
1. P. B. Radha et al., “Two-dimensional simulations of plastic-shell, direct-drive implosions onOMEGA,” Phys. Plasmas 12, 032702, 2005.
2. Prism Computational Sciences, Inc., Madison, WI, 53711(http://www.prism-cs.com/).