Experimental Research Based On The Prototype Of MeV Ultrafast Electron Diffraction

As a 4D spatio-temporal probe technique,ultrafast electron diffraction(UED),together with X-FEL diffraction,have become a powerful tool with atomic-scale spatial resolution and hundred-femtosecond-scale temporal resolution in ultrafast dynamics.Recently,the concept of accelerator-based UED and UEM(Microscope)drives the development in universities and institutes.Also,sponsered by National Major Scientific Research Facility Project,we are commissioning the MeV Ultrafast Electron Diffraction and Microscope System,with 40 fs and 0.1 ? spatio-temporal resolution in diffraction mode,and with single-shot 10 ps and 10 nm spatio-temporal resolution in imaging mode.In order to achieve the spatio-temporal resolution,a MeV UED test facility was designed in SJTU.This thesis will give a thorough treatment of construction and commission of this test facility and the following exploratory experiments.In diffraction mode,the laser-induced melting process with single crystal Au film was studied by pump-probe technique.Through laser-induced plasma shadow imaging and simulation optimization,about 1 ps and 0.1 ? temporal-spatial resolution was achieved.Also,further study found the rf phase jitter(measured,1.37°,rms)dominates the system temporal resolution.This result will drive the new project to reach 40 fs temporal resolution.In imaging mode,we improve the electron source in energy spread and study the single-shot MeV electron imaging in the test facility.The spatial resolution of MeV UEM depends on the spherical abberation and chromatic aberration.For MeV electrons,spherical aberration could be ignored,and the compensation of nonlinear energy chirp is the key factor to reduce chromatic aberration and improve the spatial resolution.Traditionally,the energy spread is cancelled through a harmonic cavity.Due to the cost of a new set of rf power and the phase synchronization problem of two different rf system,we first demonstrated the nonlinear dispersion compensation by 2D parallel flat corrugated structure in low energy and low charge regime,which show that the nonlinear chrip of the core electron beam can cancelled,and the projection energy spread can be decreased to 1/3.In addition,we design and conduct the single-shot MeV ultrafast electron imaging experiment with 1-stage objective lens.Under constraint of hardwares and techniques,300 femtosecond and 3 micron spatio-temporal resolution was achieved,under 40 times magnification.This result will drive the cascaded ultrafast electron microscope.