Prebunched Terahertz Coherent Radiation Source Based On Photocathode RF Gun

In recent years,terahertz radiation has shown great potential for development in the national economy and basic science research,and there is an increasing demand for high-quality terahertz radiation sources.Terahertz sources based on the principle of coherent radiation from pre-bunched electron beams are favored by researchers for their ability to achieve compactness,high power,and high pulse energy.This thesis is mainly focused on the prebunched terahertz coherent radiation source based on photocathode RF Gun.The longitudinal shaping of the photocathode driven laser,the experimental study on the commissioning of photocathode RF gun,and the improvement of coherent terahertz radiation performance by special structure of pre-bunched electron beam are explored and studied.Firstly,this thesis investigates the generation scheme of laser micropulse train based on chirped pulse beating technique.Starting form the time-frequency domain complex electric field of ultrafast laser,the propagation process of gaussian pulse in linear dispersive element is derived,and the one-dimensional complex field expression of linearly chirped pulse is given.With the utilization of Michelson interferometer,the intensity modulation results of the linearly chirped pulse beating are experimentally demonstrated.The linear relationship between the modulation frequency with the dispersion of Group Delay Dispersion(GDD)as well as the interference time delay is given by theoretical calculation and verified experimentally.In order to improve the uniformity of the intensity of the laser micropulses formed by the frequency beating of the linearly chirped pusles,we propose a scheme for generating uniform laser micro-pulse train combining the polarizated laser beam stacking technique and the chirped pulse beating technique.On the basis of this,a scheme for the generation of two chirped beating pulse trains with independently tuned modulation frequencies is proposed to extend the multi-frequency characteristics of single terahertz pulse radiation.Combining with the linear relationship between the theoretically derived beating frequency and the group delay dispersion of the laser,we present a laser micro-pulse train generation scheme with the time-frequency chirp characteristics of the micro-pulse modulation frequency,and complete the demonstration verification experiment by building an experimental setup.The scheme realizes the chirp control of laser micro-pulse train intensity modulation by introducing additional group delay dispersion difference between beating beams.These prebunched laser micropulse trains with different intensity modulation characteristics can be used to generate prebunched electron beams with special structures by means of photocathode RF gun and laser electron beam interaction.Based on the prebunched coherent terahertz radiation source device being developed in our laboratory,this paper carries out relevant research on improving the performance of prebunched coherent terahertz radiation by using the chirped pulse beat method to modulate laser pulse train.The generation of high charge prebunched electron beam and its power enhancement to prebunched coherent terahertz radiation by laser shaping method with double stacked chirped beat pulse train are analyzed.A twopulse chirped beat pulse train is used to generate the prebunched electron beam with double modulation frequency,and a two-pulse terahertz radiation source with independent frequency tuning and continuous pulse interval adjustment is studied.Based on the experimental results of laser micropulse train with time-frequency chirp,the generation of electron beam with time-frequency chirp and the radiation scheme of ultra-short terahertz pulse generated by this electron beam are studied.Based on the relativistic plasma oscillation effect,the scheme of increasing peak current intensity by pre-cluster electron beam with high charge density is analyzed,the optimal design of device layout is given,and the motion process of electron beam is analyzed.Finally,With the support of Hefei Advanced Light Source R&D project,we developed a prototype of photocathode RF gun.In this paper,the parameters of pulse longitudinal shaping,transverse transmission and laser stability are systematically measured and the measurement results are given.The high power baking process of the electron gun is carried out and the analysis of the field-building process of RF cavity is given based on the power measurement results.The beam commissioning process is introduced,and the optimization and measurement results of electron beam quality are discussed in detail.The performance of electron beam,such as beam energy and energy dispersion,charge quantity and its stability,beam emittance and other indicators all reach the design value,and the development of the prototype is successfully completed.It lays a foundation for the development of the pre-bunched coherent terahertz radiation source based on the photocathode microwave electron gun.