自1960年激光问世后，激光与等离子体相互作用的相关物理现象一直是人们 关注和研究的重点。尤其是啁啾脉冲放大技术的提出更是使得超短超强激光器得 到了快速的发展。将激光的能量提高到几百焦耳，且脉冲宽度也大大缩小到了飞 秒量级，目前正在建的ELI （Extreme Light Infrastructure）等激光器，更是有望将 激光的聚焦功率密度提高到1023 ? 1024 W/cm2 , 输出功率能达到几百拍瓦。如此高 的激光强度，更是为通过激光与等离子体相互作用的相关研究带来了更多新的物 理现象。例如，桌面型加速器、惯性约束聚变中的快点火、高能伽玛光辐射、正 负电子对产生以及量子电动力学（Quantum Electrodynamics，QED）级联等。 本论文主要研究了强激光与等离子体相互作用中的空泡加速机制下正电子的 加速动力学，惯性约束聚变中电子的输运与准直以及利用激光等离子体相互作 用的非线性Compton散射与Breit-Wheeler (BW)过程产生高能伽玛光与正负电子对 等，具体的研究内容如下： 1、研究了利用空泡前端电场区加速正电子时，由于正电子的引入引起的电 子-正电子-离子等离子体中空泡场的场修正以及这些场中正电子加速的动力学。 并且引入了一束对向的电子束以横向约束被加速的正电子。通过对正电子动力学 的数值模拟分析，我们得到了正电子的加速区与非加速区，并且讨论了其加速区 与非加速区在有质动力势下的不同分布，进而得到了正电子加速的最优初始相 位(ξ0, px0) 。除此之外，初始位置和动量在加速区中接近低边界的正电子能够被 更好地加速且能够得到更多的能量。这对实际应用具有重要意义。且需要说明的 是，虽然正电子动力学模型限制在单粒子，但数值模拟的结果是可以推广到多粒 子模型的。 2、我们参考了一种自生的窄高斯横向梯度磁场对快电子的准直效果，进 而引入了一种外加的横向宽高斯梯度磁场来提高快电子的准直效率。通过二维 的PIC 模拟结果可知，外加的横向宽高斯磁场比相同强度的窄高斯磁场更能有效 地约束快电子束。当不外加轴向磁场B0 时，快电子的能量密度随着横向磁场的增 大而增大，但此时电子发散角却很大。另一方面，当B0 = 30 MG 时，考虑到电 子的能量密度，最优外加横向磁场强度应为B max z = 200 ′ 300 MG，此时，电子发 散角能够被有效减小，这主要是由于自生磁场?Bx? 的三明治结构对电子的有效束 缚。稳定的横向自生磁场?Bz? 同样对于稳定准直快电子束也起到了重要作用。由 此可知，在外加的横向宽高斯磁场与轴向磁场共同作用下，快电子的能量密度能 够提高3 3 4 倍，且电子发散角能够适当降低。 3、我们采用了类金刚石材料的圆环靶以替代之前的平板靶来提高伽玛光 辐射和正负电子对的产生率。其相关的数值模拟采用了2D3V QED-PIC 模拟软 件EPOCH。两束相向激光束与等离子体靶相互作用时，圆环靶能够显著提高激 光与光子的能量耦合率。在20T0时，圆环靶产生的伽玛光子的密度与平板靶相 比能够提高约2 倍。当采用四束激光与圆环靶作用时，这四束激光的重叠会形成 一种稳定的网格状的光学势阱结构，进而提高激光强度。这种光学势阱能够有 效阻止由辐射压加速机制加速后的高能电子的横向逃逸。最终，通过激光非线 性Compton背散射会产生约7.5 × 1014 个伽玛光子，这与之前平板靶产生的光子数 目相比提高了一个量级。伽玛光子的最大密度能超过5000nc，这便可能为未来的 应用提供高亮稳定的伽玛光源。这些高品质的光子与激光束碰撞通过多光子BW 过程能够产生密度超过20nc 的致密正电子。随着时间的推移，平均能量230 MeV 的总的正电子的产量能够达到2.7 × 1011 个。另外，对于伽玛光辐射和正负电子 对产生来说，最优的圆环靶半径是不同的。当考虑伽玛光辐射时，最优靶半径应 该取5 μm。然而，当考虑产生更多的正电子数目时，靶的半径应该适当提高。在 最后，对实际应用中激光射入位置的偏差的影响我们也做了相关的讨论。研究发 现，当激光位置的偏差值控制在1 μm 之内时，基本对伽玛光辐射和正负电子对产 生没有影响。 4、我们采用等离子体双层靶（即通道靶与致密的固体铝靶）结构，利用激 光Compton散射来产生高能伽玛光。在第一层等离子体靶中，当用密度梯度靶代 替传统的近临界密度靶时，能够更好地聚焦激光束。通过PIC 模拟结果可知，我 们采用的台阶式密度梯度靶能够在短时间、短空间尺度内将激光束聚焦到高强 度，且此时激光的焦斑半径只有约2 μm。随着时间的推移，聚焦的激光束可以 在靶中稳定地传输而不散焦。这便有充分的时间和空间来加速电子束，其截止 能量能够比近临界密度靶产生的电子的截止能量提高近200 MeV。激光在穿过等 离子体通道靶后会被固体铝靶反射而与加速的高能电子束相互作用，通过非线 性Compton散射产生高能伽玛光。研究发现，台阶式密度梯度靶中产生的高能伽 玛光的发散角也很小，进而得到了空间分布较紧凑的高能伽玛光源。由于这种台 阶式密度梯度靶对等离子体通道长度的要求并不严格，因此对实验上产生高能伽 玛光很值得借鉴。

Since the advent of laser in 1960, the related physical phenomenon of the interaction between laser and plasma has always been the focus of people’s attention. In particular, the proposed chirp pulse amplification technology makes the ultra-short and ultra-strong laser develop rapidly. The energy of the laser is increased to several hundred J, and the pulse width is greatly increased to fs. The current laser device under construction in ELI and other lasers is expected to increase the focusing power of the laser to almost 1023 − 1024W/cm2 , and the output power can reach several hundred PW. Such a high laser intensity is even more important to research many new physical phenomenon of the interaction between laser and plasma. Such as: Tabletop accelerator, Fast ignition in Inertial Confinement Fusion, High Energy Gamma ray emission, Positrons and Electrons generation and QED cascade, etc. In the present thesis, we study the positron dynamics in the bubble acceleration in the interaction between laser and plasma、electron transport and collimation in inertial confinement fusion and tha high energy γ-ray emission and the positron and electrons generation through Nonlinear Compton scattering and Breit-Wheeler process. The details are summarized as follows: 1. The dynamics of positrons accelerating in electron-positron-ion plasma bubble fields driven by an ultraintense laser is investigated. The bubble wakefield is obtained theoretically when laser pulses are propagating in the electron-positron-ion plasma. To restrict the positrons transversely, an electron beam is injected. Acceleration regions and non-acceleration ones of positrons are obtained by the numerical simulation. It is found that the ponderomotive force causes the fluctuation of the positrons momenta, which results in the trapping of them at a lower ion density. The energy gaining of the accelerated positrons is demonstrated, which is helpful for practical applications. 2. A transverse gauss shape magnetic field with wide width is proposed for collimating fast relativistic electron beam in laser irradiating plasmas, which is highlighted by the two-dimensional particle-in-cell simulations, in particular the effects of this magnetic field on the production and transport of fast electron beam. When axial magnetic field is also present, it is found that the energy density of fast electrons can be enhanced greatly. For example, in presence of 30 MG axial magnetic field, it is enhanced by 3 − 4 times when the amplitude of the applied transverse magnetic field lies within the optimal regime 200 − 300 MG comparable to that without the transverse magnetic field. Meanwhile, the divergence angle of electron beam can be controlled and even decreased a little due to the better sandwich structure of the overall weakening magnetic field. The study imply that the proposed transverse magnetic field is helpful to obtain the high quality electron beam which is beneficial to the fast ignition in inertial confinement fusion. 3.A diamond-like carbon circular target is proposed to improve the γ-ray emission and pair production with lasers intensity of 8 × 1022 W/cm2 by using two-dimensional particle-in-cell simulations with quantum electrodynamics. It is found that the circular target can significantly enhance the density of γ-photons than plane target when two colliding circularly polarized lasers irradiate the target. By multi-lasers irradiate the circular target, the optical trap of lasers can prevent the high energy electrons accelerated by laser radiation pressure from escaping. Hence, high density beyond 5000nc γ-photons is obtained through nonlinear Compton back-scattering. Meanwhile, 2.7×1011 positrons with average energy of 230 MeV is achieved via multi-photon Breit-Wheeler process. Such ultrabright γ-ray source and dense positrons source can be useful to many applications. The optimal target radius and laser mismatching deviation parameters are also discussed in detail. 4.A double-layer plasma target (a plasma channel and a dense solid aluminum target) is proposed to produce high-energy γ-ray through the laser Compton scattering. In the first plasma target, the density gradient target is used instead of the conventional critical density target, thus the laser beam can be better focused. According to the PIC simulation results, the stepped density gradient target can focus laser beam to high intensity in short time and the laser spot radius can be only about 2 µm. As time goes on, the focused laser beam can be transported stably in the target without defocusing. Thus there are sufficient time and space to accelerate the electrons, and its cutoff energy can be increased by nearly 200 MeV, compared with the cutoff energy of electron produced by the critical density target. The laser passed the plasma channel can be reflected by the solid aluminum target and produces high-energy γ-ray through nonlinear Compton scattering. And the divergence Angle of γ-ray can be also small. Because of these, the stepped density gradient target does not have strict requirements for plasma channel length, so it is important to be applied in high energy γ-ray emission in experiment.