High Brightness Electron Beam Phase Space Manipulation Based On Wakefield

The shanghai high-repetition-rate X-ray free-electron laser and extreme light facility(SHINE)is the first device working at hard X band in China,which has irreplaceable advantages in coherence,ultra-short wavelength and so on.SHINE is mainly composed of linear accelerator part,which is used for providing free electrons with high energy and low emittance,and undulator part,which is utilized for the oscillatory luminescence of bunch in the undulator.The linear accelerator is composed of complex parts.In case of the loss of particles during transportation,there are different structures,pipes and magnets to ensure the quality of the bunch,such as: bunch length,particle distribution.When the tube radius and beam length are small,the wakefield generated by the interaction between the pipes and the bunch causes the six-dimensional phase space distortion within bunch,which affects the oscillatory luminescence during the undulator.Therefore,it is important to study the wakefield in free electron laser device,which can be divided into structure wakefield and resistive wall wakefield.The structure wakefield mainly includes the special structures and the pipe connections.Resistive wall wakefield mainly refers to the wakefield generated by pipes of different materials and radius.Considering that the X-ray free electron laser device requires a long device length,this paper mainly analyzes and calculates the influence in phase space by the wakefield of different pipes and structures at different positions.The three main works of this paper are concluded as followed.Since the wakefield of the superconducting accelerating cavity of SHIINE device is small,the linear energy spread introduced by the bunch compression cannot be eliminated.In this paper,we design the plate corrugated structure,without power source,suitable for SHINE device,and perfectly cancel the linear energy spread in the end of the linear accelerator.With properly optimization,the energy spread is well compensated,meanwhile the transverse wakefield effects are studied simultaneously,including the dipole and quadrupole effects.With two orthogonal dechirpers,the feasibility to restrain the emittance growth caused by quadrupole wakefield is confirmed and a more efficient method is proposed with another pair of orthogonal dechirpers.Then,time-resolved measurements of longitudinal wakefields of dechirper is presented and compared with calculation in average energy loss.The transverse beam deterioration has been detected during SXFEL energy compensation test with one-pairplates,which is typically caused by the quadrupole wakefield.Two-pair-orthogonalplates,a novel scheme,has been studied and further verified effectively to restrain the emittance growth.Finally,based on the theory of wakefield,we calculate the resistive wall wakefield from the Linear Accelerator(Linac)exit to the end of the undulator in SXFEL(Shanghai X-ray Free Electron Laser)with bunch traveling through the 245 m stainless steel transfer line and copper beamline in undulator.Then we analyzed the resistive wall wakefields which eventually led to the distortion of the longitudinal phase space within the bunch.Finally,the theoretical predictions of influence of resistive wall wakefield are compared with experiment results on SXFEL,which shows great agreement.The detailed research leads a direction for subsequent FEL optimization.In addition,in order to accelerate the bunch energy to 8 Ge V in Linac,the length of pipes is nearly 1.5 km.We calculated the structure wakefield of 1.3 GHz superconducting cavity,3.9 GHz superconducting cavity,collimator and the resistive wall wakefield of different pipes with different radius.The wakefield combination ratio of the Linac in SHINE is obtained.It provides a favorable theoretical basis for sixdimensional phase space within bunch in Linac.