The Characteristics Of An Intense Laser Beam Propagating In An Inhomogeneous Plasma Channel

The propagation of an intense laser beam in plasmas has drawn much attention since it is beneficial to various applications,such as laser-driven accelerators,harmonic generation,X-ray lasers,and advanced laser fusion schemes.Typically,for most of the above applications,propagating several Rayleigh lengths of a laser beam should be required.The characteristic distance for propagation of a laser beam in vacuum is about the Rayleigh length.Fortunately,in plasmas,the emergence of the relativistic effect will overcome the nature diffraction so that the distance for propagation of a laser beam will increase.However,when the laser power is smaller than the critical power,the relativistic self-focusing can not overcome the nature diffraction obviously.Hence,a preformed plasma channel with a radially parabolic density profile is usually used to reduce the beam diffraction and it has became a general method and technology in investigating the interaction between laser and plasmas.Recently,the study of the propagation of a laser beam in plasma channel mainly concentrates on the homogeneous plasma channel.The propagation behaviors of the laser spot size with focusing,defocusing,constant spot size and solitary wave are presented by theoretical research.But,some inhomogeneous plasma channels which have been experimentally created indicate that it will be benefical to the laser-driven accelerators.And,so far,the knowledge of these inhomogeneous plasma channels are relatively poor.So,it is meaningful to investigate an intense laser beam propagating in an inhomogeneous plasma channel.In this paper,we investigate the characteristics of an intense laser beam propagating in an inhomogeneous plasma channel.The content is mainly consist of two parts.One is the characteristics of an intense laser beam propagating in an corrugated plasma channel,the other is the characteristics of an intense laser beam propagating in an tapered plasma channel.The first chapter: The basic theory,research background,research status,and the research method of laser plasma physics are introduced in this chapter.The second chapter: In the presence of relativistic self-focusing,ponderomotive self-channeling and preformed channel focusing effects,the propagation of an intense laser beam in a corrugated plasma channel is investigated.By using the source-dependent expansion technique,an evolution equation of the laser spot size is derived.The behaviors including aperiodic oscillation,resonance,beat-like wave,and periodic oscillation with multipeak are found and analyzed detailedly by the method of Sagdeev potential and the nonlinear dynamics theory.The formula for the instantaneous wave numbers of these oscillations and the resonance condition of the laser beam are obtained.These theoretical findings are confirmed by the final numerical simulation.The third chapter: The characteristics of an intense laser beam propagating in a tapered plasma channel is presented in this chapter.By considering the effects of relativistic self-focusing,ponderomotive self-channeling and preformed channel focusing,and using the source-dependent expansion technique,the evolution equation of the laser spot size is derived.We also obtained the propagation characteristics of the laser spot size by approximately analyzing the instantaneous Sagdeev potential.We find when the longitudinal plasma density is increased linearly and quadratically,the propagation of the laser spot size including: Aperiodic focusing oscillation,Aperiodic defocusing oscillation and Catastrophic focusing are presented.And,the critical condition of the catastrophic focusing is obtained by theoretical analysis.Finally,we summarize the main results and give an outlook of the future in this field.