A study of fast electron transport in high-intensity laser-matter interactions through X-ray imaging and spectroscopy

During the interaction of short and ultraintense laser pulses with solids a copious amount of so-called “fast” electrons is generated. These electrons have a kinetic energy much greater (hundreds of keV to a few MeV) than the bulk plasma electrons. The study of the physics underlying the generation of the fast electrons and the propagation of the fast electron beam in the substrate target material is of a great interest for a variety of research fields including the Fast Ignition (FI) scheme to Inertial Confinement Fusion. In the FI scheme, a fast electron beam generated through the interaction of a PetaWatt laser beam with the pellet at the time of maximum compression, is envisaged to deposit its energy in the core of the fuel capsule thus igniting the fuel. The demonstration of the feasibility of inertial fusion energy using the FI scheme is the main goal of the High Power laser Energy Research Facility (HiPER), currently under preparation in Europe and in Italy. In this thesis, fundamental experimental studies of fast electron generation and transport phenomena carried out at the home CNR laboratory and at the large scale facility at the Rutherford Appleton Laboratory using spectroscopy and monochromatic imaging of the produced X-ray self-emission are presented. In particular, some of the key parameters of the system required for the study of feasibility of the FI scheme, such as the fast electron mean energy and the fast electron beam divergence are addressed. The main diagnostic techniques employed for the investigation of the fast electron transport phenomena are described. Moreover, a novel diagnostic technique based on an X-ray pinhole camera equipped with a CCD working in single-photon regime is presented. With this technique, monochromatic X-ray images can be obtained simultaneously at different photon energies in a wide photon energy range, to explore the dynamics of fast electron propagation in complex targets/systems.