Research On Generation And Application Of Microbunching In High-Gain Free Electron Laser

Free electron lasers(FEL) capable of providing high-power coherent short-wavelength radiation which are enabling forefront science in physics, chemistry, biology, medicine, material science etc, have witnessed an impressive development in the past few years. For nowadays, most short-wavelength(vacuum ultraviolet, soft and hard X-ray) FEL facilities around the world, such as FLASH in Germany, LCLS in US, SACLA in Japan and under-constructing SwissFEL in Switzerland, European-XFEL in Germany, PAL-FEL in Korea, LCLS-II in US are based on the self-amplified spontaneous emission(SASE) principle, which can provide extremely high-intensity, ultra-short light pulses with stable output pulse energy and good spatial coherence but limited temporal coherence due to its starting from electron beam shot noise. Recently, the "self-seeding" scheme, enhanced-SASE scheme(eSASE) and improved-SASE scheme(iSASE) have been demonstrated and researched.Alternatively, in order to improve the FEL performance and generate fully coherent radiation pulses, various seeded FEL schemes, such as high gain harmonic generation(HGHG), echo-enabled harmonic generation(EEHG) and phase-merging enhanced harmonic generation(PEHG) etc., have been proposed and studied around the world. Recently, a great success has been achieved at FERMI@Elettra, the first user facility based on cascaded HGHG principle, for providing coherent soft X-ray with the shortest central wavelength of 4 nm. The under-constructing Dalian Coherent Light Source(DCLS) and Shanghai Soft X-ray Free Electron Laser(SXFEL) will be China’s first FEL user facility based on HGHG scheme and cascaded HGHG scheme, which are great helpful to the research in FEL and other areas.As the advantages FEL could provide, it has a great requirement on the quality of the electron beam such as the energy, current, emittance and energy spread etc. To date, the growing user demands lead to the continuing enhancement of the facility capabilities, thus the continuing researching on the electron beam and microbunchings. In this dissertation, we focus on studying the generation, optimization and application of various kinds of microbunchings based on Shanghai Deep Ultraviolet Free Electron Laser facility(SDUV), Shanghai Soft X-ray Free Electron Laser facility and Dalian Coherent Light Source.The initial density modulation coming from the shot noise and the drive laser could accumulate the energy modulation because of the impedance driven by various effects, such as the longitudinal space charge(LSC), the coherent synchrotron radiation(CSR) and the linac wakefield when the beam is accelerated. The energy modulation introduced by those effects is converted into density modulation as the beam passes through a bunch compressor, which is usually much larger than the initial density modulation. This is so-called "microbunching instability" and will degrade the quality of the electron beam. Studies on the microbunching instability based on the SDUV facility and the SXFEL facility has been performed in this dissertation.The free electron laser facility is a big and complex engineering project, which requires the accelerating system, the bunch compressor system, the undulator system, the laser system and the vacuum system etc. coordinated to each other very well. Any jitter of those systems could introduce the jitter of the quality of the electron beam. At the same time, the SXFEL facility is a project based on the cascaded HGHG scheme, which is much more complex than the SASE scheme. One goal of this dissertation is to study the effects of various electron beam parameters jitters on the output pulse energy fluctuations of a two-stage cascaded HGHG, which is of great importance for the optimization of future seeded FEL facilities based on the cascading stages of HGHG.As has been widely used in radar, security, communication, medical imaging, etc., THz radiation has drawn a lot of attention all over the world. Many studies have been done on the generation of THz radiation. Accelerator-based free electron laser(FEL) technology, which can produce high-quality electron beam, might be a great candidate for the generation of THz radiation. The theory has been researched and the practical feasibility of this scheme has been demonstrated numerically with a start-to-end simulation using the beam parameters at the Shanghai Deep Ultraviolet Free Electron Laser facility(SDUV) as well.For nowadays, the growing user demands lead to the continuing improvement of the facility capabilities. Generation schemes of double pulses, which have been witnessed an impressive potential of application in pump-probe techniques, two-color X-ray free electron laser(FEL), high-gradient witness bunch acceleration in a plasma, etc., has been widely proposed and experimentally tested in many facilities such as LCLS and FERMI. In this dissertation, three methods for generating double pulses have been proposed and comparatively studied based on the SXFEL facility.Finally, we present the comparative study on three representative methods for producing the coherent multi-color Ultraviolet radiation in a seeded free electron laser based on the high gain harmonic generation, which may find its application on the four-wave-mixing experiments, the generation of isolated attosecond pulse(IAP) driven by multi-color gating, pump-probe experiments and other spectrum-based experiment. Realistic beam parameters obtained in 3D start-to-end simulations are used to compare the performance of each scheme based on Dalian Coherent Light Source(DCLS), which provides more choices for DCLS users.