Fast-electron source characterization and transport in high-intensity laser-solid interactions and the role of resistive magnetic fields

Thesis (Ph. D.)--University of Rochester. Institute of Optics, 2010.An electron transport regime is identified in which >1 MA, relativistic currents, generated by high-intensity, laser-target interactions, are collimated (or focused) by self-generated, resistive, magnetic fields within solid-density targets. The strongest magnetic fields appear at the edge of the electric current due to the radial shear in the current density. High-resolution coherent transition radiation (CTR) imaging of the target rear surface diagnoses the electrons that are accelerated from the target front surface by a laser that is focused to an intensity of I ∼ 1019 Wcm-2. CTR is emitted when an electron beam, with longitudinal electron density modulations, crosses a refractive index interface such as the target rear surface. A fast electron temperature of Thot = 1.4 ± 0.1 MeV is inferred from variations in the radiated CTR energy with target thickness. Variation in the CTR emitted energy with laser intensity obeys the power law ECTR ∝ I 5.7 ± 0.5. These results are consistent with the predictions of the ponderomotive scaling law. The high resolution images show the presence of bright, micron-scale structures in the CTR emission that indicate that the electron beam filaments. The micron scale structures are superimposed onto larger annular structures that suggest the electron beam hollows as it propagates. The variation in the spatial extent of the CTR emission with increasing target thickness gives an electron beam angular divergence of θ1/2 = 15.7 ± 0.9o. Three-dimensional, hybrid-particle-in-cell code simulations of the electron transport indicate a target rear-surface electron density distribution that reproduces the details of the CTR emission seen in the experiments. The variation of the electron density distribution with increasing target thickness resembles an expanding annulus that breaks into filaments due to the resistive filamentation instability. The radially expanding annular pattern results from the partial collimation of an initially divergent fast-electron beam by a self-generated, resistive, azimuthal, magnetic field that forms at the outside edge of the beam. A beam with an initial angular distribution of θ1/2 ~ 56o, full width at half maximum, is reduced to a beam with an angular distribution that is peaked at θ1/2 ~ 16o, as seen in the experiments. The self-generated resistive magnetic fields are responsible for the difference between the initial and measured divergence. These results are important for understanding fast-electron generation and transport in solid-density plasmas relevant to high-energy-density-physics, high-fluence x-ray source development, and advanced inertial fusion concepts, such as fast ignition