Research And Design Of Beam Optical Elements For A Mega-electron-volt Ultrafast Electron Diffraction

In order to meet the HUST-UED resolution requirements for the quality of the electron beam,the beam transmission system is designed with the aim to minimize beam mass emissivity,beam spot size and beam orbit displacement.Because of the microwave electric field force of the electron gun,the emittance of the electron beam will increase after leaving the electron gun.A solenoid must be employed to compensate for the emissivity of the beam.Meanwhile,because of the unavoidable trajectory deviation in beam transmission,the corrector also must be employed to adjust beam orbit.Resultantly,the magnetic field parameters of the solenoid and correctors are crucial to improve the UED performance.The main work completed in this paper includes the following:(1)The hard-edge model of the solenoid is derived by solving the equations of motion of charged particles,then the particle tracking program ASTRA is used to simulate the effect of the soft edge solenoid and the hard-edge solenoid in the beam transmission.The fringe field effect on the beam quality is compared and analyzed.The results show that the fringe field effect is alleviated to a certain extent.(2)Using the ASTRA program,the influence of harmonic magnetic field on particle motion is analyzed.A saddle-shaped correction coil that can compensate the multi-pole magnetic field is designed,and the correction effect of the correction coil on aberrations is studied based on the single-event model.The influence of changing the parameters of the correction coil on beam quality is analyzed.Finally,the design of the built-in correction coil is realized.(3)The magnetic field of the finished solenoid is measured,and its integral field uniformity and higher-order harmonic component are compared with the ideal values by data processing.The magnetic field error tolerance is analyzed by the solenoid measurement under the side-axis condition.(4)Based on the requirements of beam track correction,the composite magnets are deliberately designed to correct horizontally or vertically.POISSON and OPERA-3D software are used to correct the two-dimensional and three-dimensional simulation of magnets.The vertical magnetic field is optimized by cushioning and cutting the inclined magnetic pole head,while the horizontal magnetic field is mainly optimized by changing the distribution of excitation current.The relative error of the good field region of the final vertical magnetic field and the horizontal magnetic field is less than±1×10^-3.Based on the complete design of the solenoid correction coil and combined correction magnet,the beam optics of the above two magnet elements is verified based on the Me V UED whole process simulation method.The results show that the beam quality optimization and track correction can be achieved.