High -brightness Photocathode Injector Characterization And Beam Quality Improvement

High-brightness electron beams are required as driven sources of the short wave-length Free Electron Lasers, Inverse Compton Scattering based X-ray source, ultra-short bunch based radiation probe techniques, etc. The injector is the key system of any ac-celerator who provides the high-brightness beams. The photocathode injector (photoin-jector) can generate an electron beam with a low emittance and a short bunch length, which has been a hot topic of research during the last two decades. The main challenge in designing the photoinjectors is to develop electron beams at the lower emittance, shorter bunch length, higher bunch charge, higher brightness as well as to transcend some technical limitations. Thus it is necessary to investigate the photoinjector char-acterization and beam quality improvement. In this dissertation, we will discuss the methods of the thermal emittance measurement, the dark current characterization and mitigation strategy, the beam dynamics in RF cavity with off-axis beam emission and emittance compensation, the optimization schemes for velocity bunching, and also the idea to enhance the quantum efficiency (QE) of the metal cathodes.At first, we will review the present development of the electron sources, and intro-duce the characteristics of the photoinjectors especially for an S-band photo-gun and a VHF photo-gun. A novel method to measure the thermal emittance which is based on a drift following a RF gun is introduced and compared with other emittance measure-ments.One of the major limitations in gun operation is the field emission from the cavity wall or the cathode material, which will cause dark current downstream of the injector. The situation would be much worse in the case of the continuous wave modes. To un-derstand and solve the issue, we gave a systematic study of the dark current emission from the VHF gun, including characterizaitons of the dark current measurement and the field emission sources. The measurements allowed us to define an effective multifron-t strategy for significantly reducing the dark current, including a passive collimation scheme which has been proved to remove 90% of the dark current.To prolong the QE lifetime of the cathode, off-axis illuminatin is used in an RF photoinjector. The challenge in using the off-axis emission at the cathode is the po-tential emittance growth that such configurations can induce. We will investigate this effect analytically and through simulations, and also define a correction procedure that can largely compensate for the induced emittance growth. A Multi-objective Genetic Algorithm (MOGA) is introduced to optimize the final emittance. And the two codes ASTRA and IMPACT-T are used jointly to overcome the simulation challenges. At the end, we have evaluated the residual emittance growth of the optimized off-axis beam and estimated the wake field effects during the off-axis transportation.Velocity bunching technique is a tool for compressing the electron beam in the modern high brightness photoinjectors, as a result, a high peak current and a short bunch length can be achieved in the injector. This thesis will present two steps of improvemen-t in velocity bunching. Firstly, we have used a travelling wave structure with a lower gradient compared with the common methods. This proposal can realize a beam with a symmetrical current profile and a more tolerance of the phase jitter. Secondly, as a fur-ther progress, a scheme named "brake-applied" velocity bunching (BAVB) is proposed to accomplish the extreme compression regime with a complete compensation of the transverse emittance. The beam is injected into the compressor at a deceleration phase, and a special layout of photoinjector is designed to compensate the beam emittance. The MOGA is adopted to optimize the bunch length and the transverse emittance simultane-ously. As to our simulation results of the BAVB scheme, the compression factor could be over 19 and the emittance would be fully compensated.The polarized laser with oblique incidence could achieve a higher QE of metal cathodes than that with the normal incidence, which however requires the spatial and temporal laser shaping. To simplify the system and maintain the high QE, we have proposed an idea of electron gun design based on the radial polarized laser with the normal incidence on a cone cathode. Analytical estimations and primary simulations are explored.Finally, some problems to be solved and the corresponding improvements in the future works are suggested.