Generation And Measurement Of Sub-femtosecond Electron Pulses Based On Photocathode RF Gun

Ultra-short pulsed electron beam has a wide range of applications in acceleratorbased X-ray sources,wakefield acceleration,ultrafast electron microscopy,etc.The de Broglie wavelength of the electron beam can reach the order of subpicometres,which can probe the microstructure of the matter at the atomic scale.Using pump-probe technology,MeV ultrafast electron diffraction(MeV-UED)can reveal the ultrafast dynamic processes of matter changes at the atomic scale,providing a new research tool for the development of frontier science.Currently,MeV-UED has achieved a temporal resolution of 50 femtosecond(fs).Further improving its temporal resolution to few-fs or even sub-fs will open up new opportunities for frontier scientific research.With the development of ultra-short laser technology,the pulse width of the pump laser can be compressed to few-fs or even sub-fs.Therefore,to improve the temporal resolution of MeV-UED,it is necessary to further reduce the pulse width and arrival time jitter of the probe electron beam.Aiming at this scientific problem,this thesis mainly focuses on exploring new principles for the generation and measurement of ultra-short electron beams based on photocathode RF gun.In this thesis,two schemes are proposed for generating sub-fs electron beams based on photocathode RF gun.The first scheme involves modulating the electron beam close to the cathode surface using a radially polarized laser pulse.Since the cathode as a boundary breaks the symmetrical distribution of the modulated laser field,a train of attosecond bunches with tunable bunch spacing can be generated.When the driving laser pulse is about 50 fs,an isolated attosecond electron pulse can be obtained.Furthermore,to improve the number of electrons in the pulse and reduce the arrival time jitter of the electron beam,we proposed a scheme to modulate the electron beam using two radially polarized laser pulses of varying frequencies.Due to the diffraction effect of the laser pulse,several asymmetric acceleration and deceleration cycles are experienced by the electron beam near the focused laser focus,resulting in energy modulation.By using realistic laser parameters corresponding to the current gigawatt-power-level laser systems,an electron beam with a pulse width of 3.3 fs(rms)and an arrival time jitter of 1.35 fs(rms)can be obtained.In addition,we carried out experimental work on the generation and detection of radially polarized laser pulses,focal spot measurement,reflectivity and damage threshold measurement of the single-crystal copper cathode,which lays out the foundation for the development of principle demonstration experiments.Accurately characterizing the time information of sub-fs electron beams is crucial for the development of related fields.We carried out research on sub-fs electron beam measurement schemes and related laser experiments.Based on the terahertz(THz)streaking with a slit,theoretical simulations are performed to measure the time information of sub-fs electron beams.Simulation results show that the electron beam can achieve a maximum streaking speed of 24 μrad/fs,and the temporal resolution in the beam temporal profile measurement is about 0.8 fs(rms),which verifies the feasibility of the scheme.In order to obtain the time information of the electron beam more visually,this thesis also proposed a method to measure sub-fs electron beams based on the interaction of electron beams with the ponderomotive potential of an optical traveling wave generated by a vortex laser and a THz field.Analysis and simulation results show that the spatial distribution of the electron beam after interacting with the laser pulse can visually reflect its time information.The temporal resolution of this method is related to the period of the traveling wave and angle measurement error,and the time distribution and arrival time jitter of the electron beam can be obtained with extremely high temporal resolution.In addition,we carried out experimental work on the generation and detection of vortex laser pulses,which lays out the foundation for the experimental research on measuring the time information of sub-fs electron beams.A prototype of photocathode RF gun is developed in the Hefei Advanced Light Facility R&D project.To obtain an electron beam with high qualities,we carried out design and experimental research on the shaping and transport system for photocathode RF gun.Birefringent crystals and an aperture are used to shape the laser pulse in time and space.An optical image transport system is designed to achieve high stability of the laser beam position on the photocathode.Experimental results show that a quasi uniform distribution in the three-dimensional space of laser pulse is obtained at the photocathode position,and the lateral position jitter of the laser beam is less than 4μm.The performance of the laser pulse meets the experiment requirements.The beam commissioning results of photocathode RF gun show that all parameters of the electron beam are better than the design values.The successful development of the prototype lays out the platform foundation for the experimental research on sub-fs electron beam generation and measurement based on photocathode RF gun.