Measurement and Modelling of fast electron transport in solid materials

This thesis presents an investigation into the properties of relativistic electron beams generated by the interaction between an intense laser and a solid density target. Described are novel x-ray spectroscopic techniques for measuring the electron beam divergence, electron currents and to explore the electron distribution.
A key aim of this work is the constraint of fast electron beam parameters by the comparison of well characterised experimental data to computational plasma models. Multi-dimensional imaging of a single spectral line allows insight into heating and inferred spatially resolved temperatures, which are around 300eV, are compared to simulation. By comparison of imaging data and simulation, the electron beam is found to diverge with a half angle of approximately 70◦ ± 10◦, and is found to have a diameter greater than that of the laser focal spot.
Measured hollow ion spectra are shown, and calculations imply that their are two distinct ionisation mechanisms. The generation of hollow ion states is seen to be dominated by photoionisation in the the range of 7.2 ̊A−7.7 ̊A, and by collisional ionisation in the range 7.9 ̊A−8.1 ̊A. This implies the existence of an intense radiation field, and high fast electron flux within the target.
The velocity distribution of electrons is explored by observation of polarised x- ray emission. Using a pair of HOPG crystal spectrometers, each of the polarised components of the x-ray spectra are measured independently to obtain the degree of polarisation. The anisotropic population of magnetic sublevels results in a net polarisation in the spectral emission, and the polarisation degree measured is indicative of the degree of anisotropy in the velocity distribution of the return current. The large, positive degree of polarisation found implies that polarised emission is caused by the return current, and that it contains the necessary energy to excite ions into a hydrogenic state.