FAST ELECTRON DIVERGENCE AND TRANSPORT IN LASER-DRIVEN SHOCK HEATED WARM DENSE MATTER
F.N. Beg*,1, M.S. Wei1,2, T. Yabuuchi1, H. Sawada1, S. Chawla1,3, N. Nakani9,1, L. Jarrott1, D. Mariscal1, C.W. Murphy1, D. Higginson1,3, B. Westover1,3, B. Paradkar1, K. Akli2, J. Hund2, R.R. Paguio2, K.M. Saito2,R.B. Stephens2, A. MacPhee3, D. Hey3, S. Le Pape3, Y. Ping3, C.D. Chen3, H. Chen3, M. Foord3, H. McLean3, M. Key3, A. Mackinnon3, P. Patel3, S. Wilks3, R. Mishra4,Y. Sentoku4, H. Friesen5, H. Tiedje5, Y. Tsui5, R. Fedosejevs5, J. Pasley6, A. Morace7, D. Batani7, W. Theobald8, C. Steockl8, K. Anderson8, R. Betti8
1 University of California, San Diego, California, USA
2 General Atomics, San Diego, USA
3 Lawrence Livermore National Laboratory, Livermore, USA
4 University of Nevada, Reno, USA
5 University of Alberta, Alberta, Canada
6 University of York, York, UK
7 University of Milano, Italy
8 Laboratory for Laser Energetics, Rochester, New York, USA
Abstract. Understanding of fast electron source and transport is important for Fast Ignition Inertial Confinement
Fusion. Particularly, a detailed investigation of fast electron transport in warm/hot dense matter is
important pertinent to fast ignition conditions. We have performed experiments on the Titan laser at
LLNL and OMEGA EP laser at LLE to investigate electron source and transport into warm dense
matter (WDM) with varying densities and temperatures. On the Titan laser, WDM was created by a
long laser pulse (300 J, 3 ns, 600 µm spot) driven shock compression and heating of the low-density
foam with initial mass density of 150 mg/cm
3. At its maximum compression, a low-Z WDM with
approximately solid density and temperature of 5-10 eV was assembled beneath the Au foil. Transport
of the high intensity laser (150 J, 0.7 ps, Ipeak~1020 W/cm
2) produced relativistic electrons from the Au
foil (mimics the tip of the cone) through WDM, was characterized by measuring the K-shell x-ray
emission from the Cu fluorescence layer. A large angular spread (>100°) of fast electrons is observed
in the 2D spatial profiles of the Ka emission when fast electrons transport into WDM. In addition, 5x
increase in the number of escaped electrons at a large off-normal angle is seen compared to a case with
15 µm thick solid CH insulator as the transport medium, consistent with the observed large angular
spread. Collisional PIC simulations including dynamic ionization using the PICLS [1] code suggest that
the large angular spread is caused by the deformation of the laser plasma interaction surface due to
high laser ponderomotive pressure. The large source divergence is observed to be suppressed by the
self-generated fields at the ionization wave front when electrons propagate in the insulator medium.
On OMEGA EP, a series of shots have been carried out to, i) characterize the plasma and ii) to study
fast electron transport in a large volume of warm dense plasma, which was created by shock heating of
lower-density 200 mg/ cm
3 CH foam by 1.2 kJ, 3.5 ns laser. The large plasma (~300 µm) was
characterized with a Sm x-ray backlighter using the Al line absorption spectroscopy technique by
having 5% (atomic weight) Al doped in the foam. Radiation hydrodynamics simulations show about
40-50 eV plasma with electron density of the order of 10
22/cm
3. Fast electrons created with OMEGA
EP short pulse laser (1 kJ, 10 ps, Ipeak ~ 10
19 Wcm
-2) interacting with a Au foil on one side of plastic
cube were collected on the opposite side of the cube on a Cu foil to give information about the
dynamics and energy deposition of fast electrons. Experimental data shows that the Cu Ka signal is
reduced by a factor of 20 in shock heated WDM compared to undriven foam. The hybrid/PIC
modeling suggest that the magnetic field generated due to the Weibel instability plays an important
role in stopping the electrons in shock heated lower density WDM.
* This work was performed under the auspices of the U.S. DoE OFES under contracts DE-FC02-
04ER54789 (FSC), DEFG52-09NA29033 (NLUF) and DE-FG02-05ER54834 (ACE).
References:
[1] Y. Sentoku et al., J. Comp. Phys. 227, 6846 (2008).