The Effect Of Tapered Plasma Channel On Laser Propagation And Harmonic Generation

The interaction between the intense laser and plasma has been a hot topic to the researchers,which has important applications in many aspects,such as x-ray sources,high-order harmonic,laser plasma accelerators and fast ignition.These applications often require the laser to be able to propagate over long-distance,which can be achieved by introducing a preformed plasma channel.In addition,most of the researches focus on the ordinary Gaussian laser beam,few studies are devoted to the focusing of the hollow Gaussian laser beam(HGB)in a plasma channel,particularly in a tapered plasma channel.A tapered plasma channel can be used to improve the performance of the laser wakefield accelerators.So in this paper,the influence of the tapered plasma channel on the propagation characteristics of the laser and the harmonic generation are studied.1.The propagation characteristics of a hollow Gaussian laser beam(HGB)in a tapered plasma channel are studied by simultaneously considering the effects of relativistic self-focusing,ponderomotive self-channeling,and preformed channel focusing.A differential equation of the laser width parameter is derived by using the paraxial like approximation,and the critical surface that classifies the evolution behaviors of the HGB into focusing and defocusing is obtained theoretically.For verification,some numerical results are also given.It is found that the focusing ability of the HGB is proportional to the density of the tapered plasma channel,but it weakens with the increase of the order of the HGB.Furthermore,we also find that for the Gaussian laser beam,when the transverse plasma density is homogeneous,its focusing ability is stronger than that of the HGB with n = 3,while the trend just reverses when the transverse plasma density is parabolic.2.The generation of the second harmonic of a Gaussian laser beam in a tapered plasma channel is studied by considering the ponderomotive self-channel and preformed channel focusing effects.A differential equation of the laser propagating in the tapered plasma channel is derived by using the paraxial like approximation,then considering the above non-linear effects,the dielectric function that determines the evolution of the laser is obtained.In addition,when the laser propagates in the plasma,the plasma wave with the same frequency as the pump wave can be excited,which has two components: one has its propagation vector supported by the hot plasma and the second has the same propagation vector as that of the pump wave.The excited plasma wave interacts with the incident laser beam and the second-harmonic can be produced.It is found that under some parameters,the plasma wave will propagate in the form of beat like wave.And in the tapered plasma channel,with increasing of the density of the tapered plasma channel,the intensity of the plasma wave and the second harmonic increases.