Numerical Study On Propagation Of Femtosecond Intense Lasers In Air And Underdense Plasma

With the development of the chirped-pulse amplification (CPA) technique, ultra-short and ultra-high intensity laser pulses of tens of femtoseconds and intensities1018～1022W/cm2have been produced. When such a laser propagates in a nonlinear medium, it can cause massive extraordinary physical phenomenon results from the intense nonlinear effects. Hundreds of meters even several kilometers long filaments created by high-energy femtosecond pulses propagating in air, potential applications of light filaments include Terahertz (THz) wave generation, ultra-wide-band lidar and laser induced lightning. The propagation of such ultrashort high intensity laser pulses in underdense plasma plays an important role in applications such as high-order harmonic generation, soft X-ray lasers and laser-driven particles acceleration.Firstly this thesis explains the physical mechanisms of laser propagating in plasma and air. The relevant problems of guiding and focusing when laser propagates in air and plasma are simulated and analyzed using numerical calculation. The main results in this thesis are presented as follows:1. Numerical simulations and analyses are done on two types of gas targets by using computational fluid-dynamics software. For a gas-filling capillary, it can produce a uniform density distribution when the gas filling is at a steady state. The capillary parameters such as inlet location and width have evident influence on the density profiles at the capillary’s ends. A conical gas jet can make the end-density profile steeper. While the capillary gas target produces a more stable gas flow, and the gas density distribution is also more uniform than that of the conical gas jet target.2. Simulations and analyses have been done on the impact of delayed Raman and high-order Kerr effects on the self-focusing, stable propagation length and spectrum when laser propagating in air. The delayed Raman effect has evident influence on the focusing location and the re-focusing times, and it makes the focusing intensity sensitive to the laser beam waist, and the center of the broadened spectrum move to the longer wavelength when the laser propagating further. High-order Kerr effect causes decrease of self-focusing peak intensity, and the spectrum is broadened symmetrically as the laser propagates in the air. When the incident laser power is fixed, the larger the initial pulse duration, the longer the self-focusing stable-propagating length is obtained in the whole model of delayed Raman and high-order Kerr effects both considered.3. The paraxial ray equation is deduced in detail for a Gaussian beam propagating in plasma in mildly relativistic regime, the effects of preformed channel, the density on the axis, laser power, and the initial spot size on the propagating are investigated. Preformed channel can guide the laser effectively. The larger density on the axis in the preformed channel, the smaller laser spot size and the higher intensity are obtained. The relativistic self-focusing becomes intenser when the incident laser power becomes larger. The laser beam with less initial spot diameter can propagate steadily with a smaller spot size variation; the laser beam with bigger spot size can be focused to a smaller size.