Preliminary Research On The Generation Of High-charge Energetic Electron Beams By Optical Injection

Laser wake-field accelerator(LWFA), as a next generation compact source of high energy electrons, has been attracting more and more attention in the past decades. In LWFA, electron injection into laser wake-field is a key issue. To inject electrons into laser wake-field effectively, various injection schemes are proposed, such as self-injection, density gradient injection, ionization induced injection and optical injection.Colliding pulse injection(CPI) is a special case of optical injection schemes,which inject-s electrons into laser wake-field by the colliding of two counter-propagating pulses. The electron bunches generated by this scheme can have the stability and reproducibility that are required for ap-plications, in addition, the tunability of these electron bunches can be achieved using this scheme, so CPI is a desirable scheme for LWFA. However, except for the stability and tunability, the oth-er qualities of electron bunches are also required strictly for practical applications,such as high energy, quasi-monoenergetic, collimated and large charge. The generation of large charge, quasi-monoenergetic electron bunch in CPI scheme is the focus of this thesis. The optimal relations between laser parameters are analyzed by theory and numerical simulation. The work of this thesis in concrete is mainly as follows:(1) the variation of the charge of the electron beam with the increasing of injection pulse amplitude, under highly relativistic pump pulse, is studied by2D3V PIC code. Ultrashort pump pulse is employed in this paper, which is much less than the plasma wavelength. In addition, a nonlinear plasma bubble is excited by the highly relativistic pump pulse. It is found that the injected charge decreases as the injection pulse intensity increases when the amplitude of the injection pulse is greater than a threshold. The phenomenon is not favorable for generating large charge, high quality electron bunch. Therefore, it is very import to analyze the decreasing phenomenon and to adopt an effective method to avoid the decreasing. It is found that the stochastic motion of electrons, driving by the two colliding laser pulses, is the intrinsic reason for the charge decreasing phenomenon. To inhibit the stochastic motion of electrons, two circular polarization pulses are adopted. It is found that when using two circular polarization pulses, the decreasing of the injected charge does not appear, and that the charge of the electron beam generated by circular polarization case is much larger than that of linear polarization case.(2) the injected electron bunch with large charge and high quality, generated by using the optimal pump pulse parameters in CPI scheme, is studied. It is noted that there is an optimum relation between the amplitude and the spot size of the pump pulse. The electron bunch with large charge and high quality can be produced under the parameters at a certain pump power and we also find that the optimal plasma bubble can be excited under the parameters. It is demonstrated that the injected charge increases as the pump pulse amplitude increases and the injected charge decrease as the spot size of the pump pulse decreases, thus the transition of the dominant role between these two effects results in the optimum relation. The injection volume of the optimal bubble is the maximum and the optimizing effect of the beam loading appears in this case, so large charge and high quality electron bunch can be generated in the optimal bubble. Simulations for different pump power are also carried out. It is noted that there is an optimum relation between the amplitude and the spot size of the pump pulse of different pump power, and the relation satisfies the produce condition of the optimal bubble.(3) the injected electron bunch with large charge and high quality, generated by the beam loading in CPI scheme, is studied by2D3V PIC code. It is noted that the electron bunch with large charge and high quality can be produced by adjusting the duration of the injection pulse, when using two circular polarization pulses in CPI. It is demonstrated that the beam loading efficiency by electrons in nonlinear plasma wakes plays a key role in generating high quality electron beams. By using1D nonlinear plasma wave theory, we find that the peak density of electron beam plays a key role in the beam loading efficiency, while the duration and position of the electron bunch are invalid for beam loading. By2D3V PIC simulation, it is found that the increasing of the duration of injection pulse can only affect the duration and position of electron bunch and has no influence on the peak density of electron bunch, hence increasing the duration of injection pulse can not result in the disappearance of the optimum beam loading efficiency.(4) a new colliding pulse scheme using two pump pulses is proposed to generate large charge and high quality electron bunch. We find that the charge of the electron bunch generated by the two pump case is much larger than that of the single pump case, and the energy spread of the electron bunch by the two pump case is significantly better than that of the single pump. It is also found that the energy spread of the electron bunch generated by the two pump case is not sensitive to the injection pulse amplitude, thus we can tune the electron charge by adjusting the injection pulse amplitude and do not need to worry the energy spread of this electron bunch. The impact of the laser polarization on the electron bunch generated by the two pump CPI scheme is analyzed and the impact of the distance between the two pump pulse is also analyzed.(5) the collision of two counter-propagating bubbles is also studied by2D3V PIC code. The merger of two bubbles and the dynamics of electron are described on detail.