激光驱动的无碰撞冲击波产生及其离子加速

The study of laser-driven collisionless shocks is critical to understand astronomical phenomena such as supernova remnants and high-energy cosmic rays. Furthermore, ion acceleration by laser-driven collisionless shocks may provide energetic ion beams for various practical applications such as cancer radiotherapy. However, the underlying microphysics of collisionless shocks is not fully understood yet. In particular, it is still challenging to generate relativistic collisionless shocks in the laboratory with intense lasers. Moreover, the quality as well as the energy of ion beams accelerated by laser-driven collisionless shock still need to be greatly enhanced to fulfill the requirement of many practical applications. In this proposal, we will conduct a systematic study of laser-driven collisionless shock and try to solve the above mentioned issues. Firstly, we will characterize the underlying physics for collisionless electrostatic and electromagnetic shocks, respectively. The conditions for shock generation and their evolution laws in time and space will be studied with the help of numerical simulations. Secondly, we will explore the scheme to produce relativistic plasma flows by multi-petawatt lasers, and then use these relativistic plasma flows to generate and study relativistic collisionless shocks in the laboratory. Finally, we will design the ion acceleration scheme to generate high-quality energetic ion beams by laser-driven collisionless shocks. It is possible to simultaneously enhance the quality as well as the energy of ion beams via the use of near-critical-density targets or ultra-thin foils; therefore, the generated ion beams are particularly interesting for medical applications.

研究激光驱动的无碰撞冲击波对理解超新星遗迹和高能宇宙射线产生等天文现象至关重要；同时激光驱动的无碰撞冲击波离子加速有望产生癌症治疗等应用所需要的高能离子源。然而目前关于无碰撞冲击波的产生机制还没完全理解；如何利用强激光产生类似天文环境中的相对论冲击波更是富有挑战性；同时，如何设计无碰撞冲击波离子加速方案来高效地产生高品质的高能离子束依然是实际应用中亟待解决的问题。针对这些问题，我们首先将系统研究静电和电磁两种模式的无碰撞冲击波产生所需要的激光和靶参数以及它们的时空演化规律；接着，我们将探索数拍瓦强激光脉冲产生相对论等离子体束流的方案，并进一步利用相对论等离子体束流来产生相对论无碰撞冲击波并研究其特性；最后，我们将全面优化激光与近临界密度靶或者超薄靶相互作用时所激发的无碰撞冲击波对离子的加速过程，为实验上获得激光驱动的高品质高能离子束提供理论方案。

激光等离子体相互作用为实验室研究无碰撞冲击波产生等天体物理过程提供了便利。激光驱动的冲击波产生及粒子加速不仅与许多天体物理现象紧密相关，而且具有重大的潜在应用价值。在本项目中，我们研究了激光驱动的无碰撞冲击产生和离子加速。.首先，利用神光II号激光装置，研究了多路激光与一对固体靶作用所产生的对流等离子体流对撞过程中的冲击波产生过程。实验和模拟都表明在此情况下冲击波是无碰撞的且静电模式占主导作用。.其次，提出将近临界密度靶用于激光驱动的无碰撞冲击波产生。强激光脉冲在近临界密度靶中可以更快的速度推进，因此，可产生相对论速度的无碰撞冲击并将离子加速加速到非常高的能量。.第三，提出采用固体薄靶与近临界密度靶结合的混合离子加速方案。第一阶段，质子通过辐射压加速可达到亚相对论速度；第二阶段，这些质子可被近临界密度靶中的激光尾场进一步加速。基于此，利用目前10拍瓦的激光脉冲即可产生数GeV能量的质子束。.第四，首次发现了相对论诱导不透明度效应：即初始低密度的正负电子等离子体在相对论激光脉冲的照射下变得不透明。基于此，强激光脉冲可将正负电子对等离子体整体加速到极高速度形成高速等离子体喷流；所产生的正负电子对喷流在无碰撞冲击波产生等天文现象研究中具有重要应用。.最后，首次发现了纵向磁化等离子体中的极端法拉第效应：一束线偏振超短激光脉冲可在时空中分裂成两个圆偏振子脉冲。而基于磁双折射效应，横向磁化等离子体而可用作新型的等离子体波片。无论是极端法拉第效应还是磁双折射效应，都可用于产生10拍瓦级的高功率圆偏振激光脉冲，所产生的高功率圆偏振激光脉冲在激光驱动无碰撞冲击波产生及离子加速中具有重要的应用。