Relevant Physical Studies On High-Gain Short-Wavelength Free Electron Lasers

High-power and short-wavelength Free Electron Lasers(FELs) is becoming more and more important as a modern scientific research tool, especially coherent x-ray source, which will lead the modern scientific research into a entirely new area. In present, seeded FEL and SASE are the two basic operation mode for short-wavelength FEL. However, they both have their own advantages and limitations. The SASE FEL can reach x-ray region, but longitudinal coherence of the radiation is not good; And the HGHG FEL and its improved schemes have a good longitudinal coherence, but the radiation wavelength is very difficult to reach x-ray region as limited to the wavelength of seeding laser. Therefore, full coherent short-wavelength FEL becomes one of research hotspots around the world. Under these circumstances, we started our work of studying the relevant physics for the seeded HG-FEL and SASE-FEL.In the dissertation, the history of FEL development, the operation modes and the interests of FEL community are reviewed and summarized. After an introduction of FEL basics, relevant physical researches on short-wavelength FEL are presented.Firstly, the parameter sensitivities of two new seeded schemes named enhanced high-gain harmonic generation (EHGHG) and echo-enabled harmonic generation (EEHG) were studied systematically, with the purpose of optimizing the properties and extending the radiation to shorter wavelengths.We introduced the basic theory of the EHGHG scheme and compared the FEL radiation with the HGHG scheme with the same input parameters. Then electron energy detuning was investigated with the goal of enhance the FEL power and a semi-quantitative formula for the best energy detuning was given. Next, the parameters sensitivities were studied, including the initial electron beam energy spread, the dispersive strength, the seeding laser power, and amount of the phase shift. These were also compared with the HGHG scheme with the same input parameters. The results showed that the EHGHG scheme has acceptable parameters tolerance requirements and is no more or even less sensitive to the system parameters than that of the HGHG scheme. Especially for the initial electron beam energy spread, the new scheme has a larger tolerance. We compared the radiation bandwidth of the two schemes, and the new scheme was shown to have a narrower bandwidth. These conclusions offer an important guidance on the parameters selection and optimization for designing an FEL facilty operating in EHGHG scheme.The basic principle of the EEHG scheme was elaborated. The effects of the facility parameters and electron beam parameters to the harmonic bunching factor and saturation power were studied. The results demonstrated that the bunching factor and saturation power are sensitive to the facility parameters, especially the strengths of the two dispersive sections. But the tolerances of these parameters are acceptable. As the strength of the first dispersive section is very strong, the growth of the energy spread and emittance due to the coherent synchrotron radiation (CSR) was estimated and simulated. It was shown that the growth of energy spread here would not distort the echo structure of the phase space. We also gived the radiation bandwidth of the EEHG scheme. These works deepen our understanding of the EEHG scheme and will help the parameters selection and optimization for FEL facilities operating in EEHG scheme. In addition, we simply investigated the convergence of the particle number when using 3D code GENESIS for simulation.Next, SASE FEL employing several kinds of parametron undulators was investigated in detail to improve the radiation spectrum and enhance the longitudinal coherence.We studied the radiation properties of several kinds of parametron undulators, including linear taper, square taper, step-taper and inverse taper. These undulators all can increase the radiation power with their own parameters optimization, and the square tapered undulator gives the most outstanding effect. However, only the linear taper can improve the radiation spectrum distinctly. In the spectrum of linear tapered undulator, the radiation in the center wavelength is greatly enhanced so that there is only one spike standing out, and the spectrum in sidebands are suppressed to a low level. Based on these researches, we proposed“step+taper”and sawtooth-tapered undulator, which were demonstrated that can improve the radiation spectrum obviously. With these two kinds of undulators, the whole spectrum concentrate to the center wavelength and the sidebands radiation in a very low level nearly disappeared. The improvement of the radiation spectrum will significantly enhance the longitudinal coherence of SASE FEL, and we take a step forward to the full coherent X-ray source.