Beam Dynamics Analysis Of Megaelectron Volt Ultrafast Electron Diffraction

Ultrafast electron diffraction(UED),a new pump-probe technique,has become an effective tool for investigating ultrafast structural dynamics in physics,chemistry,material and biological science.The traditional ke V-UED has the problem of low temporal resolution.Megaelectronvolt ultrafast electron diffraction(MeV-UED)uses a photocathode radio-frequency(RF)electron gun to replace the traditional DC high-voltage electron gun,which significantly improves the quality of the electron bunch and shows excellent potential in terms of high temporal resolution.Besides,MeV-UED has the capability of single-shot imaging owing to the electron bunch with high charge.In this paper,theoretical research and conceptual design of MeV-UED are presented,and reciprocal space resolution and temporal resolution are analyzed and optimized.Based on electron diffraction and the particle tracking software ASTRA,the paper establishes a start-to-end simulation method for MeV-UED.MeV-UED commonly uses an S-band 1.6-cell photocathode RF electron gun,which has a relatively low accelerating gradient on the cathode surface due to the small optimal accelerating phase of the RF field.In this paper,a 1.4-cell photocathode RF electron gun is designed,which increases the optimal accelerating phase to 65 degrees and significantly increases the accelerating gradient on the cathode surface.Compared to the 1.6-cell RF electron gun,the 1.4-cell RF electron gun can achieve better results in both transverse and longitudinal beam dynamics of the electron bunch.The paper designs a solenoid that investigates the influence of parameters of the solenoid on spherical aberration and analyzes the emittance growth caused by spherical aberration.FLUKA is used to confirm whether collimators with different septal thicknesses can narrow the electron bunch as excepted.The influence of collimator diameters on the electron bunch is also discussed.The paper introduces the definitions of reciprocal space resolution and temporal resolution of MeV-UED.Based on the start-to-end simulation method,the paper discusses the dependencies between reciprocal space-temporal resolution and the beam parameters of MeV-UED.The influences of the initial distribution of electron bunch,the amount of charge,the transverse spot size of the laser,and the system jitter on reciprocal space resolution are analyzed.The dependence between temporal resolution and the accelerating phase is given.Furthermore,the influences of the system jitter and space charge effect on temporal resolution are presented.