Experiments on Laser-Based Particle Acceleration : Beams of Energetic Electrons and Protons

Abstract: This thesis describes experiments involving laser-plasma-based acceleration of electrons and protons, using the techniques of laser wakefield acceleration (LWFA) and target normal sheath acceleration (TNSA). By using extremely high accelerating field strengths, up to the order of TV m-1, it is possible to reach high kinetic particle energies over very short distances.The multi-terawatt laser system at the Lund Laser Centre, with focused laser pulse intensities reaching over 1019 W cm-2, was used in these experiments. The laser pulses were focused on different types of targets, depending on the acceleration technique. When using LWFA, the target was usually a gas, which is instantly ionized. As the laser pulse propagates through the plasma, a plasma wave is induced that can be used to accelerate electrons. As the electrons are accelerated, they also oscillate about the central axis, which produces betatron radiation that extends to x-ray energies. In the experimental investigations presented in this thesis, both supersonic gas jets and gas-filled capillary tubes were used as targets. When using TNSA, the targets were usually aluminum foils, while some experiments were carried out on structured targets and very small hollow spheres. When the laser pulse hits a solid target, electrons from the front surface of the target are driven through the target. As these electrons exit the rear surface, they form an electron sheath, which creates very strong electrical fields, in which positively charged particles, such as protons, can be accelerated.In some of the LWFA experiments, electrons automatically enter the accelerating part of the plasma wave through a stochastic process called self-injection. This process was studied, and it was shown that temporal and spectral laser pulse self-compression and focal spot quality are important for electron injection to occur. A model predicting when self-injection occurs for certain parameters was also developed. In another study, it was found that the number density in supersonic gas flows depends on the choice of gas. To obtain better control over how the electrons are injected, density gradient injection was used, which resulted in electron beams with increased charge, decreased spatial divergence, and better shot-to-shot stability compared to electron beams relying on self-injection.Experiments using gas-filled dielectric capillaries showed an order of magnitude increase in x-ray fluence compared to supersonic gas jets. The acceleration and x-ray generation processes in capillary tubes were also studied in more detail, showing that the processes occurred over several millimeters.In two of the TNSA studies, double laser pulses were used. It was found that the spatial separation and relative intensities of the two pulses were important, and affected the spatial profile of the resulting proton beams. A laser pulse separation on the order of the size of the laser spot was found to result in elliptical proton beam profiles. Furthermore, the elliptical profile could be tilted by changing the relative intensities of the two laser pulses, as a result of the transverse expansion of the electron sheath. This sheath expansion was also utilized with the hollow spherical targets, where an increase in proton number was observed in the energy range 5.5 MeV to 6.5 MeV.Experiments on thin foil targets with very small surface structures showed that the spatial divergence of the proton beams was greatly affected by the structures on the rear surface.