| The wakefield excited by an ultrashort and ultrahigh laser pulse propagating through tenuous plasma can accelerate electrons to extreme high energy. This can be used as a mechanism to design minitype electron accelerator. This thesis is devoted to studying those issues relevant to the wakefield accelerator,namely the laser wakefield accelerator (LWFA),laser wakefield accelerator driven by multiple pulses (MP-LWFA),plasma beat wave accel-erator(PBWA) and self-modulated laser wakefield accelerator(SM-LWFA). We focused our discussions on the mechanism of saturation of the wakefield and the electron parametric instabilities which affect the process of the wakefield generation and electrons acceleration.We developed a 2-Dimension distributed parallel PIC code under MPI parallel environment and got a good speedup ratio tested on YH-IV and PC computer groups. We also improved some algorithms of PIC simulation as follows:1. We analyzed in detail the "absorption boundary condition" of the electromagnetic wave and generalized the "2D PIC Lindman absorption boundary condition" to adapt to all kinds of polarization modes and 3D case.2. We made a correction of the particles' velocities due to the Coulomb collision among electrons and ions(including electrons to electrons,ions to ions and electrons to ions ) with two-body collision model[76,77] in our code.3. An algorithm of initialization of the particle velocities to meet the 3-D Maxwell profile was proposed in our thesis. At the same time,an approximate algorithm of rest startup in multiple dimensional PIC simulation was also studied.With our code(PPICC),we studied the processes of the laser wakefield accelerator and got some meaningful results.1. In the LWFA simulation,we discussed mainly the influence of "Forward Stimulated Raman Scattering" on wakefield generation and electron acceleration. We found the "Forward Stimulated Raman Scattering" will be excited if the pulse length is greater than plasma wave length . The "Forward Stimulation Raman Scattering" decreases the phase velocity and the amplitude of the wake wave which will lead to the reduction of maximum kinetic energy of the electrons trapped. We modified the electron trapping theory to be consistent with our simulation results.2. In the MP-LWFA simulation,we studied the influence of multiple pulses with various arrangements of the position of the pulses. Firstly,we studied the relationship between the amplitude of the wakefield and pulse length and an optimum length exists which equal to LFWHM = 0.3 for Gaussian profile. Secondly,we compared the similarities and differences of two multiple pulse schemes,namely "optimum pulse spacing" and "fixed pulse spacing". It showed that the arrangement with both the spacing between pulses and the length of the single pulse increasing as the plasma wave becomes more nonlinear will drive wake wave effectively. In the PBWA simulation,we studied the main saturation mechanism of the wakefiled driven by multiple laser pulses. Thesaturation time in our simulation is consistent with the prediction of the theory. Furthermore,we also studied the propagation of two pulses with inverse phase. In this case,the wakefield excited by the first pulse will be absorbed by the second pulse which shifts to higher frequency. The optimum intervals between the two pulses is about 1.7 in our simulation which is much longer than the commonly cited value 1.5.3. In the SM-LWFA simulation,we turned our attention to the wakefield generation and electron acceleration driven by the "self-modulated instability" and "Side Stimulated Raman Scattering". In order to trap more electrons,"Triangle-shape pulse excite wakefield scheme" was proposed. The pulse has a slow rising wavefront which stimulates the "Raman scattering instability" to accelerate the electrons firstly and a sharp falling rear part which excites wakefield to trap electrons secondly. In the "Gaussian-shape pulse excite wakefield scheme" simulation,the laser pulse undergoes "self-modulation" and "self-focusin... |
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