Laser-Produced Plasmas for Particle Acceleration
Abstract: This thesis describes experimental studies that aim to stabilise and optimise laser-based particle accelerators. The technique is called laser wakefield acceleration, where electric fields of the order of 102 GV/m accelerate electrons to high energies (⇠102 MeV) over mm-distances in laser-produced plasmas. Among the prerequisites for this acceleration technique to produce electron beams are laser intensities higher than 1018W/cm2 and sub-ps laser-pulse durations, both of which have seen rapid development since the invention of chirped pulse amplification. The laser is focused in a gas, which instantly ionises. In the created plasma, the propagating laser pulse creates a wave, which can accelerate injected electrons. Under the right experimental conditions, the injection mechanism is automatic, and is called self-injection. The conditions required for self-injection to occur are experimentally explored and presented in the thesis. In addition to the accelerated electrons, collimated beams of x-rays, called betatron radiation, are produced during the interaction. The thesis also discusses several ways to enhance important parameters, such as relative energy spread and divergence, of the resulting particle beams, which is important for future applications. By using smart target designs, it is possible to reduce both the spectral and spatial spread of laser wakefield accelerated electrons. In the experiment where density-downramp injection was implemented, relative energy spreads as low as 1% were achieved. During the experiment when the target consisted of a gas-filled capillary, the x-ray fluence was increased by a factor of ten when compared to betatron radiation generated in a supersonic gas jet. It is also shown in the thesis that the choice of gas is important, and increased stability is achieved if hydrogen is used as target gas instead of helium.
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