| Laser-plasma interactions is important to a number of applications such as laserdriven acceleration and inertial confinement fusion etc.As the fundamental problem of the laser-plasma interaction,laser propagation in plasma has received widespread attentions.Due to the development of laser induced plasma technology,various s hapes of plasmas can be achieved by using pump beams with different intensity distribution s,which is helpful to study the effect of electron density distribution of plasma on the propagation of laser beam in plasma.In the paper,based on the propagation of Gaussian beam in inhomogeneous plasma,we mainly investigate the effect of plasma channel with Gaussian density profile on the generation of ring-shaped beam(RSB)in plasma channel.Firstly,the equation describing the propagation of low power laser beam in plasma with Gaussian density profile has been set up,and the evolution of the probe beam has been researched by the approximate solution,numerical simulation.In theory,the equation has been analyzed by a modified WKBJ method,the approximate solution shows that the probe beam can be transformed into RSB by the effect of plasma with Gaussian density profile and the produced RSB will become more obvious as the electron density and propagation distance increase or plasma width decrease;Numerical simulation shows that the influence of electron density,plasma width and propagation distance on the propagation of laser beam in plasma is similar to the theoretical analysis.Secondly,in experiment the RSB is generated by using a He–Ne laser to transmit through the plasma channel which is generated by the femtosecond pulse filamentation in carbon disulfide,and the dark spot size of the RSB can be controlled conveniently by adjusting the power of the femtosecond pulse.It’s found that the experimental results were in good agreement with the theoretical analysis.Lastly,a variational technique is used to analyze the propagation of high power laser beams in plasmas with Gaussian and parabolic profile,and results show that the evolution of the spot size and pulse length is related to the electron density and the initial intensity.Numerical simulations show that the laser spot continues to diverge,as the defocusing of the Gaussian plasma channel will further strengthen the diffraction.When the plasma channel width is far less than the laser spot size,the speed of the pulse compression and the laser spot divergence become small,due to the increment of the initial intensity and the electron density;In the plasma channel with parabolic profile,the cyclical evolution of the spot size own to balance between the diffraction and channel focusing.The speed of the pulse compression and the laser spot divergence become small,due to the increment of the initial intensity and the electron density.The balance between the diffraction and channel focusing will be destroyed when the pulse length gets smaller. |
