Studies Of The Injection Conditions Of Vacuum Laser Acceleration

With the rapid advance of laser technology and the need for the efficient accelerators with low cost, how to utilize the laser's intense electromagnetic field to accelerate particle has received much attention. In recent years, accelerating particles by intense laser-plasma interactions has got great achievements in experiments, and it is possible to get GeV monoenergetic beams of electrons. In the same period, laser induced particle acceleration beyond Laser/Plasma also has got rapid development. The simulation results show that, electron can be captured and accelerated to sufficient energy when it is injected sideways into the laser beam. The ejection energy is related to the laser field parameters as well as to the electron injection conditions. Based on the previous works, this thesis has studied deeply on the injection conditions for the electron in vacuum laser acceleration, especially the injection angle and the injection longitudinal and transversal momentum. This work doesn't only develop the vacuum laser acceleration theory, but also provide the forthcoming experiment with useful theoretical background.There are two models for vacuum laser acceleration: Ponderomotive Potential Model and Capture Acceleration Scenario. Because of the unsymmetrical distribution of the laser field, Ponderomotive Potential Model treats electron moving in the ponderomotive potential field. But when the laser intensity or electron's initial energy is high enough, this model will be disused. The physical mechanism of Capture Acceleration Scenario is that the diffraction effect of the focused laser beam leads to slowing down of the effective wave phase velocity along the captured electron trajectory, such that the electron can be trapped in the acceleration phase of the wave for a longer time and thus gain significant energy from the field.The injection angleθ_i and the phaseφthat the election feels when it is injected into the laser beam play important roles in vacuum laser acceleration scheme. With the fixed injection momentum and initial laser phase, the ejection energy is very sensitive to the injection angle. There are many small valleys in the ejection-energy/injection-angle curve. By numerical simulation and theoretical analyzing, we find that it is because of the fast change ofφ. And no matter whatφis, there is a large valley in the profile of the ejection-energy/injection-angle curve, it means that there exists an injection angle area that electron can get no energy with these injection angles.To study the correlation between the ejection-energy and the injection-angle thoroughly, we investigate the distribution of the maximum ejection energyγ_fm in the two-dimension space constructed by the transversal momentum p_xi and the longitudinal momentum p_zi. The area where electron could gain sufficient net energy is connected, and its profile is just like a fish head. In front of the head, there is a gap like an open mouth. This is an explanation for why a valley appears in the profile of the ejection-energy/injection-angle curve. And the valley is caused by the nonlinear system itself sensitivity to initial conditions in different range. The suitable injection condition and the influence of the laser beam width are also investigated.The laser field intensity for showing the prominent merits in CAS is too high to be achieved with present laser technology. To overcome this difficulty, we analyze the longitudinal electric field distribution of the slit laser beam and its phase velocity distribution. We may as well use parallel Flat-topped Gaussian laser Beams with a central distance of (2N + 1)w₀ + d to describe the field distribution near a slit. Use simulation, we find that the area with strong longitudinal electric field and low phase velocity is prominently affected by the distance d and the phase difference between these two laser beams.