The Studies Of Wakefield Effects In Free-Electron Laser

Free electron laser(FEL)holds the promise to generate a high power,full coherence and short wavelength radiation pulse in the frequency range from Terahertz to hard X-ray.And thus,the FEL can be applied to a wide range of scientific researches due to these properties,for instance,atom/molecular biology,material science,catalysis engineering and medical science.For an X-ray free electron laser facility,a high-quality electron bunch with low emittance,low energy spread and high peak current is indeed necessary.When the bunch experiences the phases of acceleration,bunch compression and transportation in the linear accelerator section,the energy loss caused by the induced longitudinal wakefields could be compensated by the feedback system of radio frequency structure.However,when the bunch traversing through the undulator line which consists of low-gap undulators,sophisticated undulator transition and chi-cane with different functions,the wakefields will lead to the energy loss and significantly degrade the beam quality and affect the FEL radiation process.Under this circumstance,to ensure the Shanghai soft X-ray free electron laser(SXFEL)facility provide stable and high flux radiation,the relevant studies of wake effects are always the important issue.Generally,the wake effects in the undulator section originated from three aspects according to the interaction approaches,which contains resistive wall wakefield caused by the conductive undulator vacuum chamber,surface roughness wakefield induced by the rough surface of chamber and geometric wakefield due to discontinuities.On the basis of this background,wakefields calculations of SXFEL test facility,user facility and possible measurements are mainly focused in this dissertation,and conduct a more thorough research to the wake effects.For the SXFEL test facility,first of all in this thesis,the systematic research of resistive wall wakefield induced in the elliptical cross section vacuum chamber have been done with the help of numerical simulation software,which provides the guidance for the future design of vacuum chamber size.And in the next step,the vacuum chamber resistive wall wakefield calculated by the analytical formulas and numerical simulation is compared,together with geometric wakefield computed by the 2-D simulation and 3-D numerical simulation is also compared.It is found that these two comparisons agreed well with each other,and the replacement of analytical formulas and 2-D simulation software could largely shorten the calculation time of undulator resistive wall,surface roughness and geometrics wake.Moreover,it could help overcome the restrictions brought from several billions meshes in the 3-D simulation,and close to the real situation.To ensure the electron bunch trajectory,the undulator section operated in the cascade HGHG/EEHG FEL type will become more complex due to the installation and insertion of the beam monitor and alignment devices.The total wake effects can be analyzed timely and locally on the FEL performance through dividing the whole undualtor line into several modules.Based on it,it is demonstrated that 44 nm output FEL at the first stage is almost not affected by the wakefields with the help of the simulation,while a total beam energy loss about 0.8 MeV degrades the peak brightness of the second stage 8.8 nm FEL.Besides one undulator line of SXFEL user facility upgraded and based on the test facility,there is also a total new line run in SASE mode installed in the tunnel to achieve the extreme high brightness 2 nm pulse throughput in the water region.The wakefields study for the SXFEL user facility is very similar to the test facility.Differently,the computed wakefields will be closer to the real situations under the circumstance of real bunch distribution tracked from injector to the undulator,and moreover,the wake impacts on the FEL quality could be analyzed more accurately.It is found that a total energy loss of 4 MeV along the SASE undulator system will cause the pulse energy drop more than seven-fold compared with no-wake case and the overall radiation power together with spectrum will also be strongly suppressed.Generally speaking,the FEL reduction can be compensated by adjusting the undulator magnetic field,in principle,if it is continued to increase the tapering coefficient,the larger pulse energy and radiation power will be obtained.In this sense,the SXFEL test facility and user facility could also achieve the expected FEL by adopting the taper technique,even could further improve the radiation efficiency.For the wakefields measurement,the numerical simulation of SASE undulator line of SXFEL user facility is mainly focused in this dissertation.As the good resolution is needed for the short bunch wakefield measurement simulation,thus the design and optimization of user facility diagnostic beamline is on the basis of the X-band RF deflecting cavity and bending magnet in this work.Through matching the beamline lattice,the optimized temporal resolution of 6.6 fs is obtained.This preferable resolution is greatly applied to the bunch longitudinal phase space and wakefields measurement.Based on it and through numerical simulation,the reconstructed bunch longitudinal phase space and distribution all have a good agreement with initial situations.Benefited from the alterable undulator gap and adopt the perturbation method for the electron bunch motion to avoid the FEL lasing,the total wakefields of undualtor section can be reconstructed and simulated.Besides,it is verified that X-ray pulse temporal distribution can be rebuilt in this diagnostic beamline,which provide another technique for the FEL lasing measurement.The works have been done are mainly wakefields numerical simulation and closely connected with SXFEL project,which will benefit the design,construction and operation of the SXFEL facility.