Study On Description Of Real Ultra-Intense Laser Beam And Characteristics Of Laser Particle Acceleration

Since the gradient in conventional radio frequency (RF) accelerators is limited by the breakdown effects in the accelerating cavities, the scale (and the cost) of modern high energy accelerators have become nowadays at the edge of practicality. A new field of research was born and grew up around the fascinating and always tempting idea of finding new and better methods of accelerating particles, commonly termed Advanced Accelerator concepts. The progress of the laser capable of creating electromagnetic fields with very high energy density has long attracted researchers in accelerator physics seduced by the possibility of transferring that energy to a particle beam. In our previous works, a unique vacuum laser acceleration scheme, Capture and Acceleration Scenario (CAS) was proposed, which has received wide attentions because of the potential of being developed into a tabletop laser particle accelerator. CAS has been well developed theoretically in the last decade. However, it's still quite difficult to experimentally test the scheme since the theoretical results were mainly obtained in a Standard Gaussian Beam (SGB), i.e., TEM mode Gaussian beam, while the real ultra-high intensity laser beam is always of a flat-topped profile. Also, the CAS can work only when the incident angle is small (tanθ～0．1) . Concerning these difficults with aiming at experimentally testing of CAS, this thesis presents the theoretical descriptions for some kinds of laser beams, explores their propagation features, and studies the electron dynamics in these laser fields in connection with the CAS Scheme.By using the superposition of N suitably weighted Laguerre-Gaussian (LG) beams, the analytical expressions of all six electromagnetic field components of Flattened Gaussian Beams (FGBs) and focused FGBs are obtained in the Lorentz gauge, which are advisable for the simulation of laser particle interaction since they can greatly reduce CPU time while holding high precision.The phase velocity distributions of the field near the focus of FGBs propagating in vacuum are investigated. There exists a subluminous wave phase velocity region surrounding the laser beam axis. We further apply this focused FGB to vacuum laser acceleration. As with the focused SGB, electrons injected into the focused FGB can be captured in the acceleration phase and then violently accelerated.When a flat-topped laser beam is focused, there are diffraction rings around the main spot. To be effectively accelerated, electrons have to go through the rings to enter the high intensity field region. The presence of diffraction rings around the main spot of the focused flat-topped laser beam seriously affects the electron initial conditions, i.e., the optimum initial momentum and incident angle, required for CAS to work. These conditions are quite different from those for a focused TEM mode Gaussian beam. Because of the diffraction rings, there exist three typical CAS channels. An interesting one of them is that electrons injected with large incident angle and penetrating the ring have the characteristics of high capture fraction and quasi-monoenergetic distribution.The electromagnetic field intensity distribution as well as the phase velocity characteristics of FGB is analyzed. Electron dynamics in FGB is investigated using three-dimensional (3D) test particle simulations. Compared with SGB, FGB has higher longitudinal electric field intensity and a larger diffraction angle. Electrons can be more favorably captured and accelerated in FGB, namely, the electron incident angle (relative to the laser beam direction) is larger (tanθ＞0．2) , and the electron energy gain is higher than that in SGB.A scheme of proof-of-principle test for CAS at Laser Fusion Research Center of CAEP is proposed. Requirements for the experiment are investigated. Experimental setup and diagnostics are proposed, some underlying holdbacks are discussed, and simulation results for several cases are presented.In summary, the analytical expressions of all six electromagnetic field components of FGB and focused FGB are presented; CAS can work in a focused FGB, which is more close to real laser beam; electron with incident angle larger than tan 0．1 can still be captured and accelerated in FGB and focused FGB; a proposalof proof-of-principle test for CAS is presented. These results not only are important from the basic research view, but also can shed some light on the experimental design to test the CAS scheme.