Short pulse, high intensity laser plasma interactions couple a significant portion of the laser energy into relativistic electrons at the plasma critical density. This interaction forms the basis for an igniter beam in the Fast Ignition (FI) approach to Inertial Fusion Energy. However, the details on how much laser energy is coupled into electrons, and the momentum distribution of these laser accelerated electrons are still not well known. This thesis contains experimental measurements of the specularly reflected light and electrons that escape the plasma, as well as numerical simulations of the energy spectrum of electrons escaping from a target struck by an ultra-intense laser pulse to investigate laser plasma interactions.
The electrons that escape the plasma to reach a detector in vacuum are modified by electromagnetic fields in the material and at the plasma-vacuum interface as electrons leave a previously charge neutral plasma. These dynamic fields have made the experimental connection between the electrons measured far from the plasma and the internal distribution difficult. The experimental spectrum recorded at the detector typically has two distinct regions, a strong low energy feature and a semilog feature at higher energies. The numerical simulations show that the measured electrons in this higher energy semilog feature contain information about the initial distribution but are modified by the fields due to target charging. The measured electrons in the low energy feature are due to a secondary process where the electrons accelerate ions and leave the plasma with them. These electrons do not directly contain information about the initial electron distribution. The experimental measurements when accounting for the results of the simulations show that the ponderomotive theory of laser plasma interactions is correct at high intensities and is independent of laser pulse duration.
The advanced fast ignition design uses a reentrant cone to protect the laser light from blowoff plasma during the implosion and has been proposed to provide a means to increase the focal intensity by using the cone walls to guide the laser. A novel specular imaging diagnostic was developed capable of measuring in-situ reflected laser pulses with greater than 100 J of laser light and oh-shot intensities greater than 1020 Wcm-2. This spatially resolved diagnostic for the first time presents both the spatial quality of the reflected light as well as the time-integrated reflectivity from the laser plasma interaction. These experimental results show that the laser light reflecting from the target has a much wider divergence than the initial laser beam, and the reflectivity of the plasma varies significantly with incident angle. This increased reflected divergence makes the use of the wall of the cone a poor choice for increasing laser intensity on target. Furthermore, the reflectivity varies from 2% to 50% in changing the incident angle of light from 20° to 75°, making multiple cone wall bounces at a further disadvantage for guiding light to the cone tip.