Numerical simulation of high intensity laser-plasma interaction.

摘要

In this work two different areas of high intensity laser-plasma interaction are considered. The first part of the dissertation describes the dynamics of laser-irradiated clusters. It addresses two different regimes of laser-cluster interactions. In the so-called Coulomb regime, the laser pulse removes a significant part of the electrons from the cluster. The remaining electrons form a cold electron core inside a positively charged ion shell. The ion shell expands due to its space charge. A different situation occurs in the so-called hydrodynamic regime. In this case, a two-component electron distribution is formed in the cluster due to stochastic vacuum heating. The cluster remains quasi-neutral and it expands due to the hot electron pressure. Understanding electron and ion dynamics in both these regimes is the main goal of the first part of the dissertation. Stochastic vacuum heating of the electrons is demonstrated in the hydrodynamic regime. Anisotropy in cluster expansion is predicted and the sign of the anisotropy is found to depend on the laser intensity. A model of harmonic generation in clusters is developed. Resonant enhancement of harmonic generation during cluster expansion is demonstrated. Our theoretical models are verified and extended via numerical simulations using a newly-developed particle-in-cell axisymmetric electrostatic code.; The second part of the dissertation deals with laser wakefield acceleration in the self-modulation regime seeded by a Raman shifted low amplitude laser pulse. Raman seeding provides means of coherent control of the excited wakefield. The energy threshold for pulse modulation in the diffraction limited regime is derived. The relative roles of the seed and the leading edge of the pulse in creating an initial perturbation are compared. One dimensional and two dimensional particle-in-cell simulations are employed to model the effects of the seed pulse. Examples of coherent control are demonstrated. Numerical simulations show that a 38 mJ Raman seeded pulse can generate relativistic bunches of ∼1 nC. Conventional (unseeded) self-modulated laser wakefield acceleration would require significantly more energetic pulses at relativistic intensities for generating similar electron bunches. Our results indicate that a pulse repetition rate of ∼1 kHz may be feasible with proper Raman seeding. The simulation also demonstrate the possibility of Raman-seeded acceleration by pulses of subcritical power (P = 1/2 Pc, 19 mJ) in a plasma channel.