Nonlinear Propagation Characteristics Of Gaussian Laser Beam In Plasmas

Interactions of laser and plasmas have always been an important research fieldand attracted many researches’ attention. With the developments of laser technology,it has been reported that laser intensity overcomes10²² W/cm². When intense laserbeam interacts with matters, kinds of nonlinearites would be excited. Due to itsparticular components, electromagnetic response and entire mobility, plasma showsdistinctive physical properties. And plasma technique has been involved in plenty ofresearch frontiers as artificial plasmas become more and more practical.Self-focusing propagation characters of laser beam in plasmas have importanteffects on laser-driven fusion, electromagnetic radiation, laser-driven accelerationetc. Based on Gaussian laser beam propagation in plasmas, we mainly discuss theinfluence of nonlinear interactions and laser-plasma parameters in this dissertation.First, we study the electron temperature effects on propagation characters ofGaussian laser beam in slightly ionized cold plasmas. We employ the typical modelof the slightly ionized plasmas; consider Ohmic heating of electron temperaturemodulated by laser intensity. The mechanisms of collisional nonlinearity and beamdiffraction are considered in propagation problem. By WKB method, the waveequation could be converted into the second differential equation of dimensionlessbeam-width parameter, and solved numerically. We consider different initial electrontemperature distribution, including homogeneous distribuiton, linear variation anddifferent gradients respectively, to discuss the electron temperature and othereffects.Stemming from the ponderomotive nonlinearity, by adopting Drude model, wedefine the complex eikonal function to derive the propagation equation of Gaussianlaser beam. Ponderomotive force would redistribute the electron density as well asthe effective dielectric function, induce self-focusing nonlinearity. As an importantparameter of Drude model, electron collisional frequency would lead to dielectricloss. We consider the plasmas electron collisional frequency of spatial homogeneousdistribution and modulated by electron density respectively, discuss the propagationcharacters of laser beam-width and intensity, plasmas electron density andcollisional freqeucny.In terms of relativistic nonlinearity, we discuss the propagation characters ofGaussian laser beam in plasmas. The plasma frequency and electron collisionalfrequency are relativistically generalized. By laser intensity modulation, the plasmarefractive index distributes radially nonhomogenous, acting as positive lens. Thenonlinear mechanisms of relativistic nonlinearity, beam diffraction and dielectric loss by electron collision are involved in deriving propagation equation of Gaussianlaser beam. We consider the underdense, critical density and overdense plasmasconditions, and analyze the critical condition and laser beam-width for laser beam inoverdense plasmas.By employing the effective dielectric function for hot plasmas, we study thepropagation characters of Gaussian laser beam in hot plasmas. Due to the differentdielectric property and electron temperature effects from cold plasmas, we considerthe ponderomotive and relativistic nonlinearies respectively. We analyze the plasmasdielectric function componentε₂and critical curve for laser self-trapping regime, theponderomotive and relativistic nonlinearies would focus and defocus the laser beamdistinguished by critical electron temperature.