Studies On Ultra-fast And Two-color X-ray Free Electron Lasers Based On Electron Beam Manipulation

Light source has been an indispensable tool in human beings' recognition of the world.And free electron laser(FEL)based on high quality relativistic electron beam is well known as the most advanced fourth generation light source.Comparing to the third generation synchrotron radiation light source,FEL can deliver radiation pulses with extraordinary characteristics such as ultra-high intensity,ultra-short pulse duration,fully spatial coherence,tunable wavelength and so on.They opened up new opportunities in the scientific research frontiers on physics,chemistry,material science and biology.The tunable wavelength of FEL can cover up the spectrum from terahertz to X-ray,and the studies of FEL are still flourishing.The recent FEL research tendencies are short wavelength,fully coherence,high intensity,high repetition rate,controllable pulse characteristics and so on.Among them the ultrafast two-color X-ray FELs are of great interests in the scientific community due to its unique properties and promising application prospects.Advanced short wavelength X-ray FEL can enable the study of atomic inner-shell electronic transitions.Sub-femtosecond ultra-short FEL pulses can be used to study the ultrafast electronic dynamics.Two-pulse two-color FEL with adjustable central wavelengths and time delays is suitable to conduct pump-probe experiments.Self Amplified Spontaneous Emission(SASE)is the most reliable scheme in present FEL facilities.SASE-FEL has been successfully operated to deliver hard X-ray radiation pulses with tens of GW peak power and a few femtoseconds pulse duration.However,since the initial radiation signal of SASE-FEL is from the random shot noise of the electron beam,the central wavelength and peak power of SASE-FEL are not very stable,and the temporal coherence is relatively poor.To improve the quality of SASE-FEL,many schemes have been proposed,such as the Self-Seeding scheme,the Enhanced SASE scheme,and some other seededFEL schemes,including the High Gain Harmonic Generation(HGHG)scheme,the Echo Enabled Harmonic Generation(EEHG)scheme and the recently proposed Phase-merging Enhanced Harmonic Generation(PEHG)scheme.On the basis of all these schemes,there are many techniques have been developed to generate ultrafast two-color X-ray FEL pulses,and some of them have been experimentally demonstrated in the world's famous labs,such as SLAC in the USA,SACLA in Japan and Fermi in Italy.The researches of FEL are also flourishing in China.In this paper,we will firstly give a comprehensive study of the PEHG-FEL,presenting the parameter optimization methods and start-to-end three-dimensional simulation results based on the Shanghai Soft X-ray Free Electron Laser(SXFEL)user facility.In this study,a new method based on the beam transport matrix is brought up to optimize the harmonic generation FEL.Using this method and considering comprehensively of the initial beam parameters and the practical layout configurations of the facility,we complete the beam transport matrix of the PEHG-FEL for the first time,and give out the optimization conditions.We find out that the initial beam emittance and the effective beamline length will work together to determine the bunching effects of the PEHG-FEL.During the parameter optimization and numerical simulation processes using the SXFEL facility,we adjust the parameters of the electron beam and modify the layout of the facility to satisfy the optimized conditions.We design a dogleg section which is nearly 23 m.Through optimization,the bunching factor of the 30 th harmonic of the seed laser is maximized to be about 0.09,and we use this prebunched electron beam to generate FEL radiation at 8.8nm.The results are in agreement with previous theoretical analysis,and demonstrate the possible performance of the PEHG-FEL.This study will also serve as an instruction to the conceivable PEHG-FEL experiment in the SXFEL facility.The second main research content of this thesis is that we propose a modified ESASE scheme to generate ultrafast two-color X-ray FEL pulses.By implementing an angular dispersion section into a conventional ESASE set-up,together with a few cycle seed laser to manipulate the electron beam,a significant enhanced current spike will be obtained,which can be used to generate ultrafast two-color X-ray FEL.This scheme bases on the laser-electron manipulation and takes advantage of the transverse-longitudinal phase space coupling,comparing to a conventional ESASE scheme,it can reduce the required seed laser power and improve the signal-to-noise ratio of the final FEL pulse.In the study,we present the schematic layout,theoretical analysis and formula derivation of the scheme,and demonstrate the parameter optimization,beam transport simulation,and FEL simulation using the Shanghai High Repetition Rate XFEL and Extreme Light Facility(SHINE),The results indicate that we can eventually obtain a FEL radiation pulse with 0.15 nm central wavelength,270 as pulse duration,35 GW peak power,and nearly 92% signal-to-noise ratio.Additionally,based on the laser-electron manipulation,we study the two-color soft X-ray experiment using EEHG scheme in the SXFEL,giving out the required parameters of the two-pulse two-color seed laser.We study the multi-color modelock FEL using the chirped electron beam and the laser beat frequency technique,giving out the needed energy chirp of the electron beam and the required see laser parameters.