Study On Interaction Between High Power Laser And Near Critical Density Plasma And Accelerated Charged Particle

The interactions between ultra-intense laser pulse and plasma has been intensively investigated in the recent decades since its wide applications. Impressive progresses have been achieved in this important and interesting realm.The broad applicability of laser plasma interaction has been spawned interest in the novel mechanisms of laser driven plasma accelerators. Generally, relatively low density plasmas are used to study the laser-electron acceleration, while overdense plasmas are used to investigate the ion acceleration. However, with the development of laser technology, more and more particle acceleration research are focused on the near critical density plasma. In this thesis, the following aspects are investigated based on the2.5-dimentional PIC simulations:(1) Electron Acceleration in the Wakefields excited by Ultra-intense Laser PulsesAccording to our studies, the plasma wakefields induced by ultra-intense laser pulses are regularly changed with the increase of plasma density. The maximum field is proportional to the root square of initial plasma density, while the width is inversely. The wakefield evolution in lower density is much steady than that in larger density cases. With the increase of the plasma density, the widths and amplitudes of the wakefields will oscillate intensively which will affect the beam quality of the LWFA electrons. Different from the bubble structure in underdense plasmas, the tail of the electron cavitation can not be closed in near-critical density plasma. The recoiled electrons are accelerated directly into the longitudinal wakefields by the strong charge separated fields. Since the initial density is near critical, the acceleration gradient reaches several tens GeV/cm which is much larger than that in underdense plasma cases. We first thoroughly analyzed the properties of the accelerated electrons and the wakefields excited by the interactions between ultra intense laser pulses and near critical density plasmas.(2) Ultra-intense Laser Guiding Plasma Channel Formation and the corresponding Criterion in highly Relativistic RegimeOn the plasma channel formation, we find when the plasma electrons disturbed by the strong ponderomitive force applied by the ultra-intense laser pulses, their velocities will soon approach to the light speed. In this case, the relativistic effects must be taken into consideration. Therefore, we rectify the plasma channel formation criterion and test it by PIC simulations. We find that our modified criterion effectively covers most of the parameter space. When the plasma density increases to near-critical or the laser intensity is too weak, the criterion deviates slightly from the simulation results. According to our criterion, a plasma channel induced by a laser with femtosecond pulse duration is possible if the laser is intense enough and the initial plasma density is close to critical. In our simulations, we are success in forming such the steady plasma channel which exists at least in several pico-seconds. Plasma channel can efficiently guide laser pulse and weaken its intensity spread effect even after propagation several Rayleigh lengths.(3) Large Quantity Ion Beam Generation and Collection by Persistent Coulomb Explosion in PlasmaOn the ion acceleration by plasma Coulomb explosion, we find the ejected ion beam is collimated and contains large charge quantity. It is easy to acquire the backwards-accelerated ion beam because the difficulties associated with particle-laser beam separation do not exist in this regime. According to the Coulomb sphere model and cylindrical model, we prove that the acceleration ion energy and charge quantity are both proportional to the initial plasma density. We also show the electron cavitation condition for the laser intensity to the corresponding plasma density.(4) Ion Acceleration Mechanism induced by High Energy LWFA Electron BeamA new mechanism of ion acceleration by LWFA electron beam is proposed. Different from the hot electrons generated in TNSA and BOA regime, the LWFA electrons have the beam quality with high energy and good collimation. Therefore, in this regime, the ions are accelerated in a long distance and period. The ion bunch is also well collimated and the energy reaches GeV. Based on this regime, we propose the double-layer plasma target in which the two layers have different densities. This structure enhances not only the accelerated ions’ energy and charge quantity, but also the laser-particle energy transfer efficiency.In summary, based on the PIC simulations, we focus on investigating the phenomena on ultra-intense and ultra short laser pulse interacting with near critical density plasma. We find this parameter spaces are effective on both the acceleration of electrons and ions and have widely potential applications.