强激光与亚临界密度等离子体相互作用中的近前向散射驱动光子加速机制

Mechanism of near-forward scattering driven photon acceleration in the interaction between an intense laser and under-dense plasmas

通过二维粒子模拟(particle-in-cell)方法研究了强激光与亚临界密度等离子体相互作用中的近前向光子加速机制.该机制利用强激光在亚临界密度气体传输过程中的电离效应产生在纵向和横向上密度分布不均匀的电子等离子体.在纵向上,入射激光电离氦气产生一个陡峭的电子密度前沿分布.在密度前沿处,入射激光与电子等离子体波作用发生近前向散射.散射光频率较激光频率增大,在频谱中产生了第一个特征峰.在横向上,密度不均匀造成电子等离子体波具有不同的相速度并与入射激光相互作用,使入射激光发生近前向散射,在频谱中产生了第2个特征峰.由于密度分布的不均匀性较电子等离子体波的密度扰动大得多,因此基于微扰理论的散射模型和色散关系,如受激拉曼散射,无法解释频谱中两个特征峰的出现.进一步研究发现:在密度不均匀的情况下,入射激光、电子等离子体波和散射光三者之间仍满足动量和能量守恒的三波耦合关系.这能够解释两个特征峰对应的频率和强度增长过程.该研究对于强激光在亚临界密度气体传输过程中的频谱演化具有重要参考意义.

The mechanism of photon acceleration driven by the near-forward scattering(NFS)in the interaction between an intense laser and under-dense plasmas is studied by particle-in-cell(PIC)simulation.This mechanism utilizes tunneling ionization effect to stimulate electron plasma waves when the intense laser pulse propagates in under-dense plasmas.The electron plasma density is inhomogeneous both in longitudinal and transverse direction.In the longitudinal direction,a steep ionized electron density front is generated by incident laser ionizing the helium gas.Around the ionization front,the incident laser interacts with electron plasma waves,thus generating the first kind of NFS waves.Compared with the frequency of laser,the frequency of NFS wave increases.This is the first characteristic peak in the frequency spectrum.In the transverse direction,the electron plasma waves have different phase velocities,which makes the incident laser pulse undergo NFS process and upshift its frequency.This is the second characteristic peak in the frequency spectrum.Owing to the fact that the electron density inhomogeneity is much larger than the electron density perturbation of electron plasma wave,the scattering model and dispersion relationships,which are based on perturbation theory like stimulated Raman scattering,are no longer applicable to this case.Our further study shows that the incident laser,electron density plasma waves and NFS waves still satisfy the energy conservation and momentum conservation that is,they still satisfy the three-wave coupling relationship of momentum and energy conservation under the condition of heterogeneous density,thus explaining the appearance of two characteristic peaks in the frequency spectrum and their growth in the wave-vector space.This study has significant reference to the spectrum evolution when the intense laser pulse propagates in under-dense plasma.