| Ultrafast Electron Diffraction(UED)has proven to be a powerful tool to probe the ultrafast dynamic process on the atomic level.The first-generation DC-gun-based ke V UED is strongly limited by the space charge effect and only achieves temporal resolution on the sub-picosecond(ps)level with a charge less than 1 f C.The second-generation RF-gun-based Me V UED accelerates electron beam to near the speed of light,thereby suppressing the space charge effect and improving the temporal resolution to about 100 fs with charge normally above 10 f C,which enables the Me V UED to probe simple structures like metals with a single shot.In order to probe more complex structures such as biological molecules with <100 fs temporal resolution and single shot,the next-generation UED needs to improve the electron beam properties on brightness,bunch length,and jitter suppression.This thesis has carried out theoretical,simulative,and experimental research on the key technologies towards this goal.The main contents are summarized as follows.In order to increase the bunch brightness in Me V UED and realize the single-shot measurement for the irreversible process,a new 1.4-cell RF gun is developed.Compared to the current 1.6-cell RF gun,it enhances the extraction field by a factor of 4.6 at 3 Me V beam energy.Simulation studies indicate that a higher extraction field significantly suppresses both the space charge effect and mirror charge effect during the extraction stage,lowering the bunch length and longitudinal emittance,which enables the electron bunch to maintain high brightness at p C-level charges.Based on the result above,the new1.4-cell RF gun was designed,fabricated,and tested.The testing results are in good agreement with the design.This is the first reported 1.4-cell RF gun.The Me V UED beamline was designed based on the 1.4-cell RF gun.The performance and error tolerance are estimated by particle-tracking simulation.In order to improve the temporal resolution,both Powell and genetic algorithms are carried out to optimize the bunch length multi-dimensionally.The optimized 1 p C electron beam achieves 57 fs bunch length at the sample,which enables the UED to probe complex structures such as biological molecules with 100 fs-level temporal resolution and a single shot.In order to achieve <100 fs temporal resolution,a THz buncher is proposed to realize the bunch compression and time of flight jitter suppression.The analytical model is built based on the electromagnetic property of the THz buncher,and the transfer matrix of the THz buncher is derived.The particle-tracking simulation is performed,which indicates the THz buncher can lower the bunch length and time of flight jitter to 13.97±5.43 fs and 13.9fs,respectively.This corresponds to a temporal resolution of 27.2 fs.In order to measure the bunch length and time of flight jitter at the 10-fs level,a streaking measurement based on the split-ring resonator(SRR)is proposed.By combining the resonance theory of SRR and the beam dynamics,the analytical model of the streaking process is constructed and the formulas of the key parameters such as streak velocity and resolution is provided.The analytical model is verified via particle-tracking simulation.The error is less than 4% under various non-ideal situations.The simulation result shows that this method can potentially achieve a resolution of 1.6 fs,which meets the measurement requirements of bunch length and time of flight jitter in UED. |
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