Fast electron transport in overdense laser-induced plasmas

Fast Ignition is a technique by which extremely high intensity lasers may be used to ignite precompressed inertial confinement fusion fuel capsules. Compared with conventional 'hot spot' ignition, this technique eases instability constraints, reduces the required energy inputs and increases gain. The current model for fast ignition requires the efficient conversion of laser light into suprathermal electrons which can propagate into the compressed target and heat the fuel to ignition. This fast electron propagation is not well understood.; Laser-solid interactions using multi-terawatt lasers were diagnosed using thermal imaging and tracer fluorescence. Thin foils of aluminum were sandwiched with fluor foils of titanium or copper. Rear surface images of inner-shell fluorescence of the fluor layers and of thermal radiation in the vacuum ultraviolet were recorded by CCDs in order to trace current paths. Bremsstrahlung was also observed, and used to determine the orientation of fast electron currents launched into the targets. The divergence and occasional breakup of electron currents was observed. A long-range (>100mum) divergence angle of 34 degrees was measured for the electron beam. Short-range (<100mum) spots were far larger than the laser beam spot. This study analyzes and interprets the results of these experiments.; Monte Carlo modeling of electron beams through targets similar to those used in experiment simulated binary electron-ion collisions and reflux. Comparison with experiment shows that fast electron dynamics in this experimental regime include significant collective or non-linear phenomena not included in the Monte Carlo model. However, long distance transport observed in experiment was consistent with the results of modeling using binary scattering.; The Weibel instability could induce spontaneous fast electron current filamentation. A linear analytical fluid model of the development of the linear Weibel instability was used to calculate expected growth rates for current perturbations in near term experiments as well as in the fast ignition regime. Significant instability growth is expected in both cases, but on scales too small for current diagnostics to resolve.