dc.description.abstract | Free electron laser (FEL) is a new type of laser light source that uses high-quality electron beams as the medium to stimulate and amplify electromagnetic radiation in a periodic magnetic field. FEL requires high-quality and high-stable electron beams as driving sources, and the de- velopment of accelerators has become an indispensable part in the development of free electron lasers. Laser wakefield acceleration has been studied for more than 20 years. The electric accel- eration field in this accelerator is 1000 times that of the traditional RF cavity, being capable of accelerating electrons to several GeV within a few centimeters. This implies that conventional linear accelerators that few kilometers can be shortened nearly 1,000 times. Therefore, LWFA is widely believed to be a solution that can replace conventional RF accelerators. At present, laser- plasma accelerators at the centimeter level have been proven to accelerate electrons to 1 billion electron volts with small divergent angle and high stability, with great potential to be applied in free electron laser applications.This thesis presents the physical mechanism of accelerating shock wave injection in the laser plasma wakefield through two-dimensional Particle-In-Cell simulation.
In order to achieve the electron energy spread required by free electron lasers below 0.1 %, this thesis focuses on controlling the injection and optimizing the quality of accelerated electrons and study the characteristics of this injection. The first part is using the supersonic phenomenon, the front of the shock wave, to stimulate instantaneous injection.
The accelerated electrons are optimized to a monoenergeitc beam by adjusting the laser plasma parameters, and a high-density region is added to the simulation to fine-tune the laser intensity to obtain the optimized parameters, then discuss how each parameter can effectively reduce the energy spread of the accelerated electron beam.
In the second part, it is found that the injection method under certain conditions is different from the shock front injection method of most studies, that is, it can capture electrons passing through the plasma wave boundary, and the initial position distribution of these electrons is also more concentrated, which means that the energy spread is also smaller. In order to compare these two injections, we will reproduce the commonly studied shock front injection, and compare the difference in the accelerated electron properties of the two injections through trajectory tracing. Utilizing such an injection mechanism with shock waves with a tilted angle can produce obvious asymmetric injection, and can also slightly increase the amplitude of electrons in the accelerator, which is expected to become a method to enhance betatron radiation.
The result shows that the optimized electron beam at dephasing has an energy spread of less than 1%, which is a decrease of 80% compared with before optimization. In addition, using this injection mechanism combined with the shock front at a tilted angle can produce apparent asymmetric injection, which may make the betatron radiation polarized.
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