Studies Of The Vacuum Laser Ponderomotive Acceleration Scheme And Properties

Rapid development of intense laser technology has excited a lot of interest in studying the interactions of ultra intense lasers with matter, especially laser acceleration of particles. Among many laser acceleration schemes proposed, we focus on research of the Ponderomotive Acceleration Scenario (PAS) scheme. Normally PAS refers to an interaction process where an intense laser pulse catches non-relativistic free electrons in the focal region, then interacts with and accelerates the electrons. In this thesis, the electron dynamic characteristics and acceleration properties of PAS have been investigated systematically, and the physics underlying the PAS scheme is also analyzed. The PAS has been demonstrated by experiments producing MeV electrons in vacuum, which is the only experimental data available now on vacuum laser acceleration. The PAS scheme is simple for experimental design to test the vacuum laser acceleration scheme. Hence, as an important branch of the vacuum laser acceleration research, studies on PAS scheme are of significance to both theory and application development.The physics underlying the PAS scheme is the asymmetry in the fields experienced by the accelerated electron during its acceleration and deceleration stages. When a laser pulse catches an electron in the focal region, the intensity experienced by the electron in the ascending front (corresponding to acceleration stage) is greater than that in the descending part (corresponding to deceleration stage), owing to the diffraction effect of the focused laser beam. As a result, the electron can gain net energy in the interaction process with the intense laser pulse.To study the physical properties of PAS and the detailed electron dynamics in the laser field, a three-dimensional test-particle simulation is used to solve the relativistic Newton-Lorentz equations of motion. It has been confirmed by our studies that the PAS can accelerate slow electrons to energies up to MeV, and these data are in consistent with experimental observation. Together with the capture and acceleration scenario (CAS) scheme, which deals with vacuum laser acceleration of fast electrons using the low wave phase velocity region of a laser beam, it is clearly demonstrated that far-field laser accelerations are available for either fast or slow electrons without applying any additional condition. At the same time, we discussed the ponderomotive potential models, including models under the non-relativistic and the relativistic circumstances, as well as their applicability. And the ponderomotive potential models have been used to interpret the related characteristics of PAS.The acceleration properties of PAS have been investigated systematically based on physical analyses and large amount of calculations and simulations. It has been found that electrons are scattered isotropically in the radial direction for moderate laser intensity. The output bunch exhibit relatively wide angular and energy dispersions and the fraction of electrons with high outgoing energy are rather small. Besides, the scaling law for accelerated electrons, a key problem in vacuum laser acceleration, has been studied deeply. The maximal electron-energy gain in the PAS regime is found to be approximately proportional to the laser intensity and the laser beam width, and inversely proportional to the laser pulse duration. Physical interpretations based on the ponderomotive potential model are presented. We have also compared the scheme and acceleration properties of PAS with those of CAS.Still, another crucial subject of vacuum laser acceleration ~ the correlation between the outgoing energy and scattering angle of accelerated electrons has been investigated. Bifurcation phenomenon in the energy-angular correlation spectrum of the vacuum laser ponderomotive acceleration has been observed with computer simulation. It can be seen that for focused laser pulse the classical single-valued energy-angle correlation for a plane wave is not only broadened to a band, which means electrons with the same outgoing energy will have an angular spread, but is also bifurcated, with the classical value lying in between the two branches. Analytic expression to describe the correlation has been derived and physical explanations based on the ponderomotive potential model and Lorentz-Newton force analyses are presented.The above studies are not only helpful in better understanding of ponderomotive acceleration scenario and further improving the studies on vacuum laser acceleration together with CAS, but also of significance for experimental design to test the vacuum laser acceleration schemes by providing theoretical reference as well.