Fast electron divergence and transport in laser-driven shock heated warm dense matter

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/cm3. 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/cm2) 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/ cm3 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 1022/cm3. Fast electrons created with OMEGA EP short pulse laser (1 kJ, 10 ps, Ipeak ∼ 1019 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. (author)