| Based on particle-in-cell simulations,the propagation of intense long pulse lasers in non-uniform plasma,and particularly,the formation of plasma density cavities caused by the nonlinear evolution of stimulated Raman scattering(SRS)near the quarter critical density,and its effects on parametric instabilities have been studied.The one-dimensional simulation shows that the stimulated Raman scattering instability developed near the quarter critical density leads to the trapping of scattered light and subsequent formation of a local electromagnetic solitary wave.Its amplitude increases with the development of the SRS instability,which pushes surrounding electrons and ions to form a quasi-neutral density cavity.When the first density cavity is formed,the plasma density evolves in such a way that more density cavities are formed during the laser interaction and subsequently the plasma is split into a few discontinuous portions.This new density profile finally tends to suppress the development of both SRS and the stimulated Brillouin scattering(SBS)instabilities considerably.In addition,the two-dimensional simulation result shows that the density cavities have been obviously anisotropic with different polarized incident laser.In the case where the incident laser is s-polarized,the density cavities generated by the SRS scattered light will suppress the generation and transmission of the electron plasma waves.Therefore,density cavities can inhibit the development of instability.In the case where the incident light is p-polarized,the incident light will decay into two electron plasma waves due to the influence of Two Plasma Decay(TPD)instability,and it suppresses the formation of electromagnetic solitons and plasma density cavities.The situation of plasma density cavities in real space and its effect on parametric instabilities need to be further studied by three-dimensional simulations. |
