Laser Guiding And Vortex Terahertz Radiation In Laser-plasma Interaction

The propagation and radiation of the intense laser in plasmas have important theoretical significance and potential applications in the fields of laser driven plasmas acceleration,high order harmonic generation,X-ray source generation,and THz radiation,etc.Regarding the propagation of an intense laser in plasmas,previous studies have shown that the introduction of preformed plasma channels can effectively suppress the vacuum diffraction effect,thereby enabling laser to stably propagate over long distances.Currently,researches on the propagation of an intense laser in preformed plasma channels generally are designed to be parabolic in radial direction.However,due to factors such as the conditions and methods for the generation of preformed plasma channels,their transverse density distribution may deviate from the standard parabolic one and take on a more general power-law type.But researches on laser propagation in power-law plasma channels are still relatively rare.In the study of radiation generated by the interaction of intense lasers with plasmas,terahertz radiation has the characteristics of millimeter wave and far infrared spectrum,and can be applied to fields such as imaging,communication,and biomedicine,etc.Particularly,vortex terahertz radiation can couple orbital angular momentum,an additional degree of freedom,to matter,and has important applications in chiral matter manipulation,electron beam acceleration,and astrophysical source detection,etc.Previous studies have shown that using two-color lasers schemes and applying an external static magnetic field can effectively enhance the intensity of terahertz radiation.However,at present,the study of vortex terahertz radiation generated by the interaction of two-color laser pulses with plasmas under an external magnetic field has not received attention in depth.Therefore,the studies of the propagation of an intense laser in a radial power-law plasma channel and the generation of vortex terahertz radiation from the interaction of two-color laser pulses with plasmas under an external magnetic field have important scientific significance and wide application prospects.This thesis focuses on laser guiding and vortex terahertz radiation in laser-plasma interaction,and studies the propagation dynamics and stability of a Gaussian laser beam in a radial power-law plasma channel,as well as the generation of vortex terahertz radiation from the interaction of two-color Laguerre-Gaussian laser pulses with plasmas under an external magnetic field.Regarding the propagation of a Gaussian laser beam in a radial power-law plasma channel,it is found that with the increase of the radial power-law exponent of the plasma channel,the parameter region for stable laser propagation increases significantly.That is,the increase of the radial power-law exponent of the preformed plasma channel will be more conducive to laser propagation.In the study of vortex terahertz radiation generated by the interaction of two-color Laguerre-Gaussian laser pulses with uniform plasmas under an external magnetic field,it is found that the intensity and spatial distribution of terahertz radiation can be efficiently modulated by adjusting some parameters,such as the laser field,external magnetic field,and plasmas density.These conclusions provide theoretical and parametric guidance for the laser propagation over long distances and the generation and manipulation of vortex terahertz radiation sources.The research content and main results of this thesis are summarized as follows:In chapter 2,the propagation dynamics and stability of a Gaussian laser beams in a radial power-law plasma channel are studied.Starting from Maxwell’s equations,continuity equations,and motion equations,the coupling equations of the laser beam in a plasma channel are obtained.Using the variational method,the normalized evolution equation of the laser spot radius is obtained.Then,using the Sagdeev potential method,the propagation behaviors and parameter region of an intense laser in a power-law plasma channel are obtained.The effects of the power-law exponent,laser normalized amplitude,plasma channel radius,and initial plasma density on the stable region and the type of laser propagation are studied respectively.Finally,the theoretical results are verified numerically by using the finite difference method.The results show that with the increase of the power-law exponent of the plasma channel,the parameter region of the stable laser propagation increases significantly.At the same time,under the condition of a given power-law exponent,with the decrease of the laser normalized amplitude,and the increase of the radius of the plasma channel and initial plasma density,the parameter region for stable laser propagation increases.In addition,it is found that due to the influence of the numerical instability,the unstable propagation region increases in numerical simulation.That is,an unstable propagation transition region appears in the theoretically stable propagation parameter region,making the actual stable propagation parameter region smaller than the theoretical predication.In chapter 3,The generation of vortex terahertz radiation from the interaction of two-color Laguerre-Gaussian laser pulses with plasmas under an external magnetic field is studied.Firstly,motion equations and Poisson equations are combined to obtain the nonlinear velocity and perturbation density of the electron respectively,and then the nonlinear current is obtained.By introducing the nonlinear current into the wave equation of terahertz radiation,the analytical expressions of the vortex terahertz radiation field can be obtained.Then,the effects of laser parameters,applied magnetic field,and plasma density on the generation of vortex terahertz radiation are studied.Finally,this scheme is verified by two-dimensional PIC simulation.The results show that under the presenting of an external static magnetic field,not only can the intensity of vortex terahertz radiation be enhanced to a certain extent,but also the spatial distribution symmetry of vortex terahertz radiation field will be broken.Thereby,an external static magnetic field can effectively achieve the flexible control of the intensity and spatial distribution of vortex terahertz radiation.Meanwhile,with the topological charge number of the incident laser increasing,the orbital angular momentum of vortex terahertz radiation increases,while the intensity of vortex terahertz radiation decreases.In addition,it is found that the change of the initial plasma density can adjust the center frequency of vortex terahertz radiation.The increase of the initial plasma density will significantly enhance the vortex terahertz radiation intensity,before a saturation is taken.