Investigations On Nonlinear Phenomenon In Warm Dense Plasma

With the development of intense laser technology, the nonlinear interaction of lasers with warm dense plasma has been a subject of active research for decades due to its relevance to inertial confinement fusion (ICF) and particle acceleration. Beam self-focusiong, splitted beam intensity and electron acceleration in laser-plasma interaction are investigated. The research contents and the obtained results are as follows:1. Dielectric constant and laser beam propagation in an underdense collisional plasma are investigated, using the wave and dielectric function equations, for their dependence on the electron temperature. Simulation results show that, due to collision nonlinearity results in the distribution of electron density, and lead to modification of dielectric constant. The collision nonlinearity presents a weak-strong-weak variation trend by the variation of electron temperature, and lead to three kind various beam propagation phenomenon: steady defocusing, oscillatory defocusing and self-focusing.2. The phenomenon of splitted beam is researched in intense laser and overdense plasma interaction. Based on the modified nonlinear wave equation describing the interaction of intense laser and overdense plasmas in nonparaxial region, and first find a new parameter instability:three-splitted beam intensity profile. In the work, we analyze the reason of splitted intensity profile, and show that four-splitted and other intensity profile cannot be find in the system. The new parameter instability would enrich the nonlinear research of laser-plasma interaction.3. Based on Wentzel-Krammers -Brillouin (WKB) approximation and higher order paraxial ray theory, we investigate electromagnetic beam self-focusing in interaction between Gaussian electromagnetic beam and two axially exponential decay underdense inhomogeneous (include single and mixed type) plasma with completely opposite characters, respectively. The simulation results show that, the combination influence of relativistic nonlinearity and plasma inhomogeneity determine the variation of dielectric constant and propagation of electromagnetic beam in plasma. Especially, an interesting and important finding which the intrinsic mathematic (such as:monotonicity and extremum) qualities of plasma inhomogeneity may has many internal relations with electromagnetic beam propagation in plasma. On one hand, increasing plasma inhomogeneity and beam relativistic self-focusing and filamentation increasing, as well as bring to filamenation instability in ICF. On the other hand, if we can combine two influences of relativistic nonlinearity and plasma inhomogeneity, it is able to generate particularly intense and short pulses. The founding may be designed particularly intense and short pulses and new particle acceleration.4. The nonlinear interaction of a laser pulse with a homogenous unmagnetized underdense plasma, taking ohmic heating and the effects of ponderomotive force into account, is theoretically studied. Snce the ponderomotive force modifies the electrons density and temperature distribution, the nonlinear dielectric permittivity of plasma is obtained in non-relativistic regime. Furthermore, electric and magnetic fields, electron density, temperature distribution, and the effective permittivity variations are obtained in terms of plasma length by making use the steady state solutions of the Maxwell and hydrodynamic equations. It is shown that the oscillations wave length of electric and magnetic fields decreases when the laser intensity increases. At the same time, in this case, electron density oscillations become highly peaked. Also, the amplitude of the electron temperature oscillations increase and their wavelength decreases.5. In the paper, we examine electron acceleration by laser pulse in magnetized plasma. The laser pulse group velocity is less than the speed of light and hence electrons can resonantly interact with the pulse. The basic mechanism involves acceleration of electrons by the axial gradient in the ponderomotive potential of the laser. Research results show that, in the laser-plasma interaction, electron plasma frequency and cyclotron frequency can obvious effect the group of laser pulse. The electron acceleration depends on the ratio of laser frequency to electron cyclotron frequency, amplitude of the laser pulse and plasma density. Due to the influence of ponderomotive nonlinearity, lead to electron transverse oscillation, and the formation of intense plasma wave, as last results in electron acceleration and high energy gain. While in magnetic plasma, due to the influence of electron field of laser beam, result form the self-generated magnetic field, which it cause electron cyclotron at a frequency. When the frequency of laser beam and electron cyclotron frequency meetω=ωc and form resonance, self-generated magnetic field would strengthen the ponderomotive nonlinearity and increase electron transverse oscillation, finally further increase electron energy gain.