Theory And Experiment Of The Stimulated Raman Backward Scattering In Plasma

In the realm of laser technology,increasing the laser power and energy have always been the goal to be aimed at.High-power lasers are the most powerful tools in strong-field physics experiments in scientific research and play an important role in frontier researching fields such as particle acceleration,X-ray generation,and Inertial confine-ment fusion.As the laser power increases,damage to optical components can occur when its power density exceeds 10¹²W/cm².Therefore,using plasma as a medium to amplify the optical power of laser is the latest technological means.Among them,stimulated Raman backscattering amplification has attracted a lot of attention.In stimulated Raman backscattering amplification,many uncertainties are intro-duced due to various instabilities that occur in the plasma.Therefore experimental am-plification efficiency is difficult to reach the theoretical expectation at this stage.Sim-ulations through theory can help us further understand the amplification process and provide guidance for experiments.In this paper,a theoretical analysis and an experi-mental study of the amplification process of stimulated backward Raman scattering in plasma are carried out,and the following results are obtained.1.The process of scattering of pump light occurring in the plasma was simu-lated using a 1D Particle-in-cell simulation program.To investigate the factors influ-encing the change in the spectral spectrum of the scattered light,several simulations were performed by varying the light intensity of the pump light with the plasma density is 0.019)_cr,0.059)_cr,0.109)_cr,0.159)_cr和0.209)_cr.At the same time,the temperature and density changes of the plasma during the scattering process were analyzed.2.The femtosecond laser was focused directly in air to form the plasma.To mea-sure its density,a Nomarski interferometer was built to image the plasma with inter-ference fringes,and a density inversion algorithm was written to determine its density.The variation of the density of the plasma with time was analyzed,the plasma density variation within 1 ns was obtained.By focusing the laser whose energy is 4 m J and po-larization is linearly or circularly polazized in air,the maximum density of the generated plasma is the magnitude of 10⁹cm^-3and the length is up to 1.5 mm.3.Preliminary experiments of SRBS were conducted using a laser pulse with a single pulse energy of Joule magnitude,several sets of spectrum data were recorded while adjusting the energy of the pulse,varying the position of focal point in the gas target and the back pressure of the gas nozzle at an energy of 1.3 J and 500 m J.The scattering spectrum were also analyzed in relation to the light intensity of the pump light and the back pressure of the gas nozzle,and the scattering spectra can provide a reference for the selection of wavelength of seed light in the subsequent SRBS experiment.4.Spectral broadening experiments were carried out using a Herriott multipass cell,which can be used to generate the seed light in the SRBS experiments.The pa-rameters of input pulse are 100J,475 fs,and 1 k Hz.After performing the spectral broadening,the spectrum is broadened by a factor of 5 and the temporal contrast is increased by nearly one order of magnitude,and after performing the pulse duration compression,the output of 47 fs is obtained,the pulse duration is shortened by a factor of 10.