Theoretical And Experimental Study On Polarization Control Of X-ray Free Electron Laser

Free electron laser(FEL)can produce lasers with ultra short pulses,ultra-high photon energy as well as radiation power and fully coherent radiation due to its unique principle,so it has been widely used in physics,chemistry,biology,materials and many other fields,especially in some frontier research fields.FEL has developed rapidly in the past few decades.It has not only developed from low-gain scheme to high-gain scheme,but also extended its radiation wavelength from visible and infrared bands up to terahertz(THz)and microwave bands as well as down to soft X-ray and hard X-ray bands.The world's first X-ray FEL facility is based on self-amplified spontaneous emission(SASE).Since SASE is simple and stable,most of the high-gain X-ray FEL facilities in the world choose SASE as their basic operation mode.In order to improve the longitudinal coherence of FEL radiation,self-seeding and external-seeding FEL schemes have been proposed.Nowadays,the high-gain FEL theory has gained enormous development and been verified by numerous experiments.Conventional planar undulator can only produce linearly polarized lasers,but due to the wide application of circular dichroism,X-ray FEL facilities with polarization control become popular.Generally speaking,a quarter-wave plate can be used to achieve the conversion between linear polarization and circular polarization.But for soft X-ray,which can be absorbed greatly by various materials,we must control the FEL polarization by means of the undulator.Based on Shanghai Soft X-ray FEL Facility(SXFEL)and the Shanghai High-Repetition-Rate XFEL and Extreme Light Facility(SHINE),the polarization control of high-gain FEL radiation is studied in this paper.For high-gain FEL,there are two methods to obtain circularly polarized radiation with undulators,which are crossed-planar undulator(CPU)and elliptically polarized undulator(EPU).The former is simple and costs less,but the polarization degree is not high;the latter has perfect polarization performance but costs more,and it is difficult to make an EPU with both short period and strong magnetic field.For soft X-ray,EPUs is often used to get circularly polarized radiation due to its better polarization performance.In this paper,the parameters of the upstream planar undulators are optimized to improve the polarization.The stability is analyzed according to the operation index,and the peak magnetic field of the EPU is calculated to determine the processing difficulty.Moreover,an additional electron beam deflection structure is designed to realize the fast switching of left/right circularly polarized lasers,and the demand of beam stability is analyzed.For hard X-ray,linearly polarized lasers with specific direction can be achieved by polarizer or special planar undulator.In this paper,we compare the radiation quality between these two methods,and finally choose the optical method due to its stronger power,higher polarization purity and lower cost.It is difficult for EPUs to operate at hard X-ray,so the feasibility of crossed-planar undulator(CPU)producing circularly polarized lasers based on self-seeding hard X-ray facilities is verified by simulation.By changing the transverse envelope of the electron beam,this method can be applied at different resonance wavelengths.In order to meet the demand of shorter undulator period as well as stronger magnetic field,an RF EPU is designed in this paper.Firstly,the simulation is done with Genesis to verify that the undulator parameters can effectively generate FEL pulses.Then,the cavity size of RF undulator is preliminarily determined according to them.With further optimization of the cavity size,CST simulation confirms that the efficiency of the structure is high and a stable periodic electromagnetic field can be stimulated in it.At last,the coupler is designed to get the required microwave mode.In this paper,EPU is also combined with the ultra-fast X-ray FEL.Firstly,the femtosecond fully coherent X-ray laser is obtained in simulation based on fresh-slice on SXFEL.Then,the sensitivity of femtosecond laser to the beam parameters is analyzed to verify the feasibility of the scheme.Finally,the EPUs are resonated at the second harmonic and placed downstream the radiation section.The fundamental femtosecond linearly polarized laser and the secondharmonic femtosecond circularly polarized laser,which are all fully coherent,can be obtained simultaneously.Finally,the terahertz(THz)source based on pre-bunched electron beams is studied on SXFEL.The THz radiation intensity of single bunch is evaluated by both numerical calculation and 3D simulation,and the influence of different beam parameters as well as undulator parameters is analyzed.Furthermore,the radiation intensity is optimized for Tsinghua's THz source based on nonlinear plasma wakefield modulation,which improves my understanding of multi-bunch THz radiation.In addition,the advantages of EPU in THz source are also found.Shorter gain length and stronger saturation power are very beneficial to enhance THz radiation.