Laser-driven particle acceleration - Experimental investigations

Abstract: This thesis describes experimental studies of laser-driven particle acceleration. With the focused intensity of today’s high-power lasers exceeding 10^18 W/cm2,extremely high-gradient accelerators are possible. A number of experiments have been performed using two different laser systems, the Lund multi-TW laser and the Vulcan laser at CLF/RAL, UK, to study aspects ranging from laser system optimization and acceleration physics to detection and analysis methods. In one series of experiments, electrons were accelerated in underdense plasmas. As an intense laser pulse propagates in the plasma, a large-amplitude plasma wave is driven up in its wake. The longitudinal electric field in this wake can exceed 100GV/m. Relativistic electrons, injected from the background plasma, surf on the wave. By carefully controlling acceleration conditions, such as plasma density, laser pulse duration and focusing, short bursts of ~ 100MeV electrons with a small energy spread) were generated. Details of the acceleration mechanism were elucidated by studying the dependence of the electron beam on laser and plasma properties. It was found that for a range of plasma density–laser intensity combinations, quasi-mono-energetic beams are reliably produced. Using linearly polarized laser radiation, the spatial profile of the electrons acquires an elliptical shape that can be controlled by the direction of the laser polarization. This implies that there is a direct interaction between the laser pulse and the bunch of accelerated electrons, and that the electrons are originating from the small region occupied by the laser. It was also found that the contrast of the laser has a significant impact on the stability of the electron beam. In a second series of experiments, protons and other ions were accelerated from solid foil targets, a few μm thick, irradiated by a high-power laser. Since the protons are not relativistic, a stationary potential is needed to accelerate them. This was accomplished by allowing electrons, heated by the laser, to establish an electrostatic sheath on the back surface of the target foil. The strength of this field amounts to TV/m, accelerating protons to MeV energies over a very short distance. The laser contrast was again found to be an important parameter in the acceleration. The pedestal of amplified spontaneous emission (ASE) preceding the main pulse launches a shock wave into the target that could potentially destroy the back surface. However, by ontrolling the level and duration of the ASE, a regime in which the back surface is plastically deformed by the shock wave can be established, leading to an energy-dependent deflection of the proton beam. Using a plasma mirror to improve the contrast further, targets as thin as 20nm were used for proton acceleration. The conversion efficiency and maximum proton energy were found to increase significantly for these thin targets. In another experiment, the scaling of the maximum proton energy was studied at extreme laser intensities. The maximum energy was found to have a square root dependence on the laser intensity, but with a much lower energy than predicted by previous 1D plasma expansion models. Heating the target removes the protons and leads to efficient acceleration of heavier ions. Properties of these accelerated ions, such as charge state, energy, etc., were experimentally characterized. While not strictly a particle acceleration experiment, the final section of this thesis concerns a soft X-ray laser generated by transient excitation of ions in a laser-produced plasma. A technique called GRIP was employed to efficiently transfer energy to the desired ion charge state. The process involves setting up a smooth plasma gradient with one pulse, and pumping the plasma to population inversion with another pulse, impinging on the plasma at a grazing angle of incidence. The output energy of the soft X-ray laser was optimized with respect to a number of parameters. The details of high repetition rate operation of this laser were also studied, and the importance of the pointing stability of the driving lasers was established.