强激光与近临界密度等离子体相互作用中的无碰撞静电冲击波产生

Generation of collisionless electrostatic shock waves in interaction between strong intense laser and near-critical-density plasma

本文通过一维粒子模拟(particle-in-cell)方法研究了强激光与近临界密度等离子体相互作用中的弱冲击波和强冲击波产生,并讨论了非相对论和相对论光强以及等离子体密度分布区间对无碰撞冲击波形成的影响.非相对论的弱驱动光与等离子体相互作用产生的是弱冲击波.由于电子加热不充分,电子能谱呈现出双温分布.较低温度的电子对弱冲击波的形成以及质子反射加速有重要作用.弱冲击波加速质子的能谱呈连续分布.在等离子体密度上升沿区间较大时,可观察到后孤子结构向离子声波结构演化并进一步演化为弱冲击波结构的过程.在相对论的强驱动光强下,电子加热比较充分可达到相对论温度,且呈现出单温分布.进一步分析密度分布区间大小对冲击波形成的影响时发现:1)当等离子体密度上升沿区间较大时,离子声波的势垒易被热电子屏蔽且离子声波结构在传输的过程中容易被后续的激光破坏而无法演化为无碰撞冲击波;2)当等离子密度分布区间较小时,离子声波中加速电场的有效距离(即德拜长度)和持续时间更长,这导致其结构在传输过程中更加稳定.当离子声波中加速的质子与靶后鞘层场加速的质子之间的速度差满足无碰撞冲击波的离子反射条件时,离子声波进一步演化为强的无碰撞冲击波,同时产生了准单能的质子.

Weak and strong collisionless electrostatic shock wave(CESW)generated in the interaction between strong intense laser and near-critical-density plasma are studied by the one-dimensional particle-in-cell simulation in this work.And the effects of the ranges of plasma density profiles,non-relativistic and relativistic laser intensities on the generation of CESWs are also investigated.The non-relativistic weakly driven laser generates the weak CESW in the interaction between the laser and near-critical-density plasma.The electron spectra show double-temperature distribution because the non-relativistic driven laser cannot heat the electrons sufficiently.The low-temperature electrons have an important influence on the generation of weak CESW,and they can also cause the protons to be accelerated and reflected from the CESWs.The spectra of the weak CESW protons show a continuously distributed profile.When the range of plasma density up-ramp is large,the process can be observed that the post-soliton structure evolves into the ion acoustic wave and further into the weak collisionless electrostatic shock wave.When the driven laser intensity is relativistic,the electrons are heated sufficiently to a single relativistic temperature.The effect of the range of plasma density profile on the generation of CESW is further analyzed and it is found that 1)when the range of plasma density up-ramp is large,the potential barrier of ion acoustic wave is shielded by the hot electrons;2)when the range of plasma density up-ramp is small,the effective distance(i.e.the Debye length)of accelerating field is larger and the endurance time is longer than when the range of plasma density up-ramp is large.This makes the ion acoustic wave structure more stable in its forward propagation process.When the difference in velocity between the ion acoustic wave accelerating protons and the target normal sheath accelerating protons satisfies the proton reflection condition of CESW,the ion acoustic wave further evolves into the strong CESW,the monoenergetic protons generated at the same time.