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Schmid, Karl (2009): Supersonic Micro-Jets And Their Application to Few-Cycle Laser-Driven Electron Acceleration. Dissertation, LMU München: Fakultät für Physik



This thesis covers the few-cycle laser-driven acceleration of electrons in a laser-generated plasma. The so-called \emph{laser wakefield acceleration} is a long-known concept that relies on strongly driven plasma waves for the generation of accelerating gradients in the range of several 100 GV/m. This value is approximately four orders of magnitude larger than the one attainably by classic accelerators, which is limited essentially by electrical breakdown in the accelerating structures to approximately 100 MV/m. Since the acceleration length necessary for obtaining a certain electron energy is inversely proportional to the accelerating field, this leads also to a drastic reduction of the size and of the price of the accelerator. Furthermore, the special properties of laser accelerated electron pulses, namely the ultrashort pulse duration, the high brilliance, and the high charge density, open up new possibilities in many applications of these electron beams. The laser system employed in this work is a new development based on optical parametric chirped pulse amplification and is the only multi-TW few-cycle laser in the world. It allows for the amplification of pulses with a duration of 8 fs up to a power of 6.5 TW. In the experiment, the laser beam is focused onto a supersonic helium gas jet which leads to the formation of a plasma channel. The laser pulse, having an intensity of 10^19 W/cm^2 propagates through the plasma with an electron density of 2x10^19 cm^(-3) and forms via a highly nonlinear interaction a strongly anharmonic plasma wave. The amplitude of the wave is so large that the wave breaks, thereby injecting electrons from the background plasma into the accelerating phase. The energy transfer from the laser pulse to the plasma is so strong that the maximum propagation distance is limited to the 0.1 mm range. Therefore, gas jets specifically tuned to these requirements have to be employed. The properties of microscopic supersonic gas jets are thoroughly analyzed in this work. Based on numeric flow simulation, this study encompasses several extensive parameter studies that illuminate all relevant features of supersonic flows in microscopic gas nozzles. This allowed the optimized design of de Laval nozzles with exit diameters ranging from 0.15 to 3 mm. The employment of these nozzles in the experiment greatly improved the electron beam quality. After these optimizations, the laser-driven electron accelerator now yields monoenergetic electron pulses with energies up to 50 MeV and charges between one and ten pC. The electron beam has a typical divergence of 5 mrad and comprises an energy spectrum that is virtually free from low energetic background. The electron pulse duration could not yet be determined experimentally but simulations point towards values in the range of 1 fs. The acceleration gradient is estimated from simulation and experiment to be approximately 0.5 TV/m. The electron accelerator is routinely operated at 10 Hz, which is a unique feature among laser based accelerators. The light amplification technique employed in the laser system in principle allows here improvements by several orders of magnitude.