Sprangle P, Peñano J R, Hafizi B
Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2002 Oct;66(4 Pt 2):046418. doi: 10.1103/PhysRevE.66.046418. Epub 2002 Oct 30.
The propagation of short, intense laser pulses in the atmosphere is investigated theoretically and numerically. A set of three-dimensional (3D), nonlinear propagation equations is derived, which includes the effects of dispersion, nonlinear self-focusing, stimulated molecular Raman scattering, multiphoton and tunneling ionization, energy depletion due to ionization, relativistic focusing, and ponderomotively excited plasma wakefields. The instantaneous frequency spread along a laser pulse in air, which develops due to various nonlinear effects, is analyzed and discussed. Coupled equations for the power, spot size, and electron density are derived for an intense ionizing laser pulse. From these equations we obtain an equilibrium for a single optical-plasma filament, which involves a balancing between diffraction, nonlinear self-focusing, and plasma defocusing. The equilibrium is shown to require a specific distribution of power along the filament. It is found that in the presence of ionization a self-guided optical filament is not realizable. A method for generating a remote spark in the atmosphere is proposed, which utilizes the dispersive and nonlinear properties of air to cause a low-intensity chirped laser pulse to compress both longitudinally and transversely. For optimally chosen parameters, we find that the transverse and longitudinal focal lengths can be made to coincide, resulting in rapid intensity increase, ionization, and white light generation in a localized region far from the source. Coupled equations for the laser spot size and pulse duration are derived, which can describe the focusing and compression process in the low-intensity regime. More general examples involving beam focusing, compression, ionization, and white light generation near the focal region are studied by numerically solving the full set of 3D, nonlinear propagation equations.
从理论和数值上研究了短脉冲强激光在大气中的传播。推导了一组三维非线性传播方程，其中包括色散、非线性自聚焦、受激分子拉曼散射、多光子和隧道电离、电离导致的能量损耗、相对论聚焦以及有质动力激发的等离子体尾场等效应。分析和讨论了由于各种非线性效应导致的沿激光脉冲在空气中的瞬时频率展宽。推导了强电离激光脉冲的功率、光斑尺寸和电子密度的耦合方程。从这些方程中我们得到了单个光学等离子体细丝的平衡，这涉及到衍射、非线性自聚焦和等离子体散焦之间的平衡。结果表明，这种平衡需要沿细丝有特定的功率分布。发现在存在电离的情况下，自导光学细丝是无法实现的。提出了一种在大气中产生远程火花的方法，该方法利用空气的色散和非线性特性，使低强度啁啾激光脉冲在纵向和横向上都发生压缩。对于优化选择的参数，我们发现横向和纵向焦距可以重合，从而在远离光源的局部区域导致强度快速增加、电离和白光产生。推导了激光光斑尺寸和脉冲持续时间的耦合方程，它们可以描述低强度 regime 下的聚焦和压缩过程。通过数值求解完整的三维非线性传播方程组，研究了涉及光束聚焦、压缩、电离以及焦区附近白光产生的更一般示例。
