Optimization of an SRF Gun for High Bunch Charge Applications at ELBE

As a cutting-edge technology for photoinjectors, SRF guns are expected to provide CW electron beams with high bunch charge and low emittance, which is critical to the development of future FELs, ERLs and 4th/5th generation light sources. However, existing research has not explored the full potential of SRF guns as predicted by theory. Currently, the research activities at ELBE focus on solving technological challenges of a 3.5 cell SRF gun as well as applying it to high-bunch-charge experiments. This thesis aims to optimize the ELBE SRF gun and the relevant beam transport for future high-bunch-charge applications at pELBE, nELBE, TELBE and CBS experimental stations. Chapter 1 describes the demands of these applications on the SRF gun in detail. Chapter 2 outlines the development of a simulation tool based on ASTRA and Elegant, followed by the optimized gun parameters and the beam transport for the four experimental stations. Chapter 3 introduces beam diagnostic methods and data processing applied in this thesis. Chapter 4 presents results of experiments, including the pulse length measurement of the UV laser for generating electrons from the photcathode, the commissioning of ELBE SRF Gun II, a verification experiment on the LSC effect conducted at PITZ and a beam transport experiment with the bunch charge of 200 pC. Simulation results have determined the effect of each SRF gun parameter on the beam quality and have provided optimized settings according to the requirements in Chapter 1. Experimentally, the LSC effect was confirmed at PITZ, in agreement with simulations which indicated that LSC significantly influences beam quality. The performance of ELBE SRF Gun II was improved and a beam with a bunch charge of 200 pC and an emittance of 7.7 μm from ELBE SRF Gun II has been transported through ELBE without visible beam loss. The development of the simulation tool and beam diagnostics will serve further research at ELBE. Results of both simulations and experiments enrich the understanding of the existing SRF gun as well as the ELBE beamline and will guide continuing improvements. Already, ELBE SRF Gun II can deliver twice the bunch charge and lower emittance compared to the thermionic injector routinely used for ELBE. Ongoing modifications and development of the gun-cavity and photocathodes are expected to provide still further improvements. Progress on high-bunch-charge experiments at ELBE can be expected by applying the SRF gun