Various Coherent Radiation Mechanisms And Relevant Physical Studies On Free Electron Lasers

Free electron Laser(FEL)has its unique characteristics such as good coherence and high peak power.Achieving bunched electron beam is the critical factor to produce coherent radiation in undulator.In principle,FEL can lase at arbitrary wavelength.Nowadays,FEL community has developed various kinds of coherent radiation mechanisms with different characteristics at different wavelengths.In this dissertation,the development history,status and interests of FEL community were reviewed and summarized.After the introduction of FEL theoretical basis,various kinds of coherent radiation mechanisms and simulation software,relevant physical researches were presented,including FEL Oscillator,Pre-bunched THz FEL,PEHG FEL and Multi-color FEL.Firstly,we introduced the basic theory of FEL Oscillator.The dependence of the small signal gain on undulator and electron beam parameter were analysed based on combination of theory and simulation.Utilizing the undulator gradient to enhance the tolerance on energy spread of the beam was investigated.The results showed that the gain and saturation power could be greatly improved by using the undulator gradient while the beam had a large energy spread.Then we investigated the effects of reflectivity,coupling hole size and cavity length detuning on the output characteristics.The maximum saturation power could be achieved when the cavity length detuning is smaller than the radiation wavelength.Then we studied the natural mode lock in an FEL Oscillator.It could be concluded that the comb structure is remarkable with a small cavity length detuning in the deep saturation region while the slippage length is much smaller than the electron bunch length.Then,the basic principle of pre-bunched FEL was elaborated,and the comparision of radiation power between pre-bunched FEL and other schemes were given in details.We also showed the distribution of bunching factor and energy spectrum.After that,we studied the radiation of the pre-bunched beam at the fundamental and up-conversion harmonic of the pre-bunched frequency,and the radiation bandwidth was investigated.The results demonstrated that the final radiation bandwidth is inversely proportional to the number of microbunches when it is much bigger than the number of undulator periods,on the contrary,the final radiation bandwidth is inversely propotional to the number of undulator periods.In addition,we studied the jitter and error introduced by the parameters of electron beam and undulator.Next,the PEHG FEL using the natural transverse gradient(NTG)of a normal modulator instead of the transverse gradient undulator(TGU)was analyzed.The corresponding optimization of high harmonic bunching factor and feasibility of NTG enabled PEHG were carried out.It was shown that NTG enabled PEHG can achieve the comparable performance with the original PEHG FEL.The effects of main system parameters on the bunching factor and radiation power were also analyzed.Moreover,a phase-reversed modulator was proposed to reduce the beam energy spread in PEHG,i.e.,Enhanced PEHG.The results demonstrated that EPEHG can effectively mitigate the energy spread to some extend.In addition,we investigated the feasibility of PEHG with a high repetition by using TGU or NTG in electron storage ring.Finally,we studied the generation of multi-color FEL pulses based on the HGHG scheme with tilted electron beam.The previous methods of producing two-color FEL and tilted electron beam were briefly reviewed.Then we proposed a new multi-color FEL scheme at EUV waveband by using a single tilted beam,a single seed laser and a single TGU radiator,and verified its feasibility through numerical simulations.The adjustability of the time and frequency separations between the two-color pulses was also investigated.In addition,with this scheme,we proposed to use a large-tilt electron beam and a large-gradient TGU radiator to make the beam satisfy three resonant conditions and then produce three-color FEL pulses.