Laser-Driven Plasma Waves for Particle Acceleration and X-ray Production

Sammanfattning: This thesis presents experimental results related to laser plasma accelerators. These rely on very different principles from conventional particle accelerators. They are able to accelerate particles over a very short distance and produce synchrotron x-rays at the same time, potentially providing a compact particle accelerator and x-ray source for many future applications. The laser pulses used in the experiments reported in this thesis were extremely powerful (TW) and when focused, intensities as high as 10^20 W/cm^2 were achieved. When matter is exposed to such high intensities, it becomes a plasma, and collective motion of electrons is possible. The laser pulse "ploughs" through the plasma creating a plasma wave behind it, very much like a boat at sea. The perturbation in electron density caused by the wave produces strong longitudinal electric fields, travelling at the same speed as the laser pulse. These fields can then be used to accelerate electrons to hundreds of MeV in only a few mm. In addition to longitudinal electric fields, transverse fields are also produced, which can make the electrons "wiggle" transversely and emit x-rays. In a related process, heavy ions can also be accelerated by high-intensity lasers, when interacting with a solid target. In the experimental work reported in this thesis, the use of dielectric capillary tubes was explored in order to increase the accelerating length by externally guiding the laser pulse and counteracting diffraction. Linear plasma waves over several centimetres were produced and characterised. Electron beams and x-rays produced in dielectric capillary tubes were also studied, where it was found that it was possible to trap electrons even at a low initial laser intensity. An active stabilisation system was developed in parallel to improve the pointing of the laser system, as the dielectric capillaries are very sensitive to pointing fluctuation. The laser focal spot was modified in a controlled way using adaptive optics. By adding coma aberration, the focal spot could be made asymmetric and the x-ray emission enhanced, as the electrons oscillate with a greater amplitude. Adding spherical aberration allowed the effect of the quality of the focal spot on the wavebreaking threshold to be studied. A simple model predicting whether or not the plasma wave breaks was developed by varying the laser energy and pulse duration. Two ways of increasing or modifying the proton energy distribution without increasing the laser power are also presented in this thesis. The absorption of the main laser pulse was improved and the proton energy increased by using a 100 fs long laser prepulse. Finally, hollow microspheres were used as targets, which allowed for "recycling" of the laterally spreading electrons to establish a new accelerating field that could accelerate the protons once more.