The Control And Ultrafast Applications Of Laser-driven Electron Acceleration And Radiations

Without a limitation of electric field breakdown threshold and extremely high acceleration gradient,plasma acceleration provides a possibility for the realization of a table-top particle accelerator.In recent years,with the development of chirped pulse amplification technology,plasma electron acceleration driven by ultra-intense and ultra-short laser pulse has been paid more and more attention.A secondary radiation can be emitted from the accelerated relativistic electrons by the transverse oscillations or inverse Compton scattering.These sources have obvious advantages in short pulse duration,high brightness as well as small source size,and offer attractive prospects for the vital and wide applications of the ultrafast research in physics,chemistry,biology and other fields.This dissertation mainly discusses the generation and control of the electron beam and radiation driven by ultra-intense laser pulse in plasma,and introduces the studies of ultrafast application on the developed ultrafast X-ray diffraction device.The dissertation is divided into the six chapters as following:Chapter 1 is the introduction.Firstly,the physical mechanism and development of laser wake-field electron acceleration are introduced.Then,the generation of the ultrafast radiation,including Betatron radiation,inverse Compton scattering and K?X-ray,driven by the laser plasma interaction is introduced.Finally,the applications of these three sources are introduced,including ultrafast pump detection,ultrafast absorption spectroscopy,phase contrast imaging and 3D CT scanning imaging.The second chapter presents the control of the laser electron beam direction.The principle of controlling the direction of laser electron beam and the evolution of the pulse front tilt in space are introduced.Then,a laser plasma electron accelerator is developed based on 20 TW Ti:sapphire femtosecond laser.The relativistic electron beams with good qualities with high charge,stable energy spectrum and pointing stabilities are obtained.Then,the pulse front tilt is introduced into femtosecond laser pulse,which controls the spatial direction of the electron beam linearly.The experimental results are in good agreement with the theory.Finally,the application of the laser beam steering in practice is discussed.The third chapter is focused on the control and promotion of the laser plasma X-ray source.(1).The high-energy hard X-rays are produced by the inverse Compton Scattering when the electron beams driven by the ultra-intense laser collides with the residual laser reflected by the plasma mirror.In the experiment,a high-Z pure nitrogen target with low back pressure and high transmittance is used to improve the laser energy transmission rate in plasma.Furthermore,the laser is focused on the back edge of the nozzle to improve the cross section of the electron laser collision,without sacrificing the quality of the electron beam.Finally,the yield of the X-ray source is improved.(2).Betatron X-rays are emitted from the transverse oscillation of relativistic electron beam accelerated by laser plasma wake field in plasma bubble.By introducing positive second-order dispersion into the pulse,stronger plasma wake wave is excited,which increases the transverse instability of the cavity and finally improves the critical energy of Betatron X-ray.(3).High-energy?-rays and positrons with yield of 1.0×10⁶(>30Me V)are produced through Bethe-Heitler process when the wakefield accelerated electron beam interacts with the high-Z metal material.The fourth chapter introduces the development of the ultrafast X-ray diffraction setup.The principle of ultrafast X-ray diffraction is introduced.Based on this principle,a time resolved X-ray diffraction system is designed.Then,the time-space coincidence of the pump detection light path,the flatness of the target surface,the generation and calibration of K?X-ray and the X-ray multilayer mirror are systematically optimized.Our experiment exhibits its ability of monitoring the transient structural changes in thin film SrCoO_2.5 crystal.Finally,a design scheme of single real-time ultrafast X-ray diffraction device is introduced,which can be used to measure the irreversible dynamic process of the sample in real time.The fifth chapter presents the application of ultrafast X-ray diffraction.The properties and preparation process of antiferromagnetic insulator SrCoO_2.5 are introduced.Then,the dynamic diffraction signal of SrCoO_2.5 has been obtained upon the laser illumination.And the structural phase transition of SrCoO_2.5 has been analyzed in combination with the femtosecond transient reflection signal of visible light.The tetrahedron structure of SrCoO_2.5 is excited and charge transfer is the main physical mechanism leading to lattice distortion upon 400 nm laser illumination.However,the octahedral structure is excited and thermal expansion is the main contribution upon 800nm laser illumination.Chapter 6 is the summary and prospect.The work during the doctoral period was comprehensively summarized and the follow-up work as well as the research prospects has been outlined.