Study On The Formation Of Intense Femtosecond Laser-Induced Plasma And Its Kinetic Energy Structure

The interaction of an intense femtosecond laser pulse with atoms or molecules gives rise to a variety of nonlinear phenomena, including field-induced ionization, the filamentation in air, the formation of laser-produced plasma (LPP) channel, etc. In this thesis, the interaction of an ultraintense femtosecond laser field with atoms, the transient velocity of laser-produced plasma, and the kinetic energy structure of LPP in air are investigated. These researches can give some insights into the mechanisms of the formation of filamentation and LLP. They also have great potential in applications, such as laser distance measuring, optimizing the initial conditions of evolutions of plasma channel after the laser pulse. The main achievements are as follows:1. Phase-space analysis for ionization processesBased on the time-dependent Schrodinger equation, the ionization pro-cesses during laser-atom interaction are investigated in phase space using Ga-bor transformation, where the depletion of the whole system caused by the mask function is taken into consideration in the calculation of plasma densi-ty. We obtain the momentum distribution via the Gabor transformation of the escaping portions of the time-dependent wave packet at the detector-like points on the interior boundaries, from which the kinetic energies carried by the escaping portions are calculated.2. Transient velocity distribution of LPPThe ionization process of electrons freed due to field ionization and their velocity distribution is presented by using the single electron tracking method in the interaction of an intense laser pulse with noble atoms. We derive the velocity distribution by matching the time-varying velocities of LPP with the reference velocity. In order to smooth the distribution functions, we propose a partition scheme to calculate the velocity, in which the time-varying velocity is contributed by two adjacent grids with different weighting factors.The mean kinetic energy is calculated based on the velocity distribution.3. Kinetic energy structure of LPP in AirA method to calculate the fine structure of kinetic energy of laser-produced plasma is proposed, which bridges the two parts of researches of plasma channel usually studied independently of each other, i.e., the exten-sion of the length of plasma filament and the prolongation of the lifetime of plasma channel generated by the laser pulse. The kinetic energy structure of the plasma channel is calculated by solving the motion equation of ionized electrons and utilizing the ionization rate as the weighting factor. With the study on the laser intensity, the formation mechanisms of the kinetic energy structure is analyzed.The corresponding computer programs are written in FORTRAN 90, and the data are processed by MATLAB.