Analysis of fringe field effects on measurement errors for Q-scanning-based emittance diagnosis system

As a widely recognized emittance measurement technique, the Q-scanning method typically plays a crucial role in the commissioning process of accelerator-based beam injectors. The Q-magnet, being a pivotal component, significantly influences the measurement capability, thus warranting detailed examination in this study. By reassessing the measurement outcomes of the Q-magnet, the impact of fringe fields on emittance measurement precision is thoroughly scrutinized within this framework. Subsequently, corrections to the transfer matrices of the Q-scanning system under different excitation currents are implemented by revisiting fundamental beam optic principles. Moreover, virtual measurements via both theoretical calculations and beam dynamics simulations are conducted to validate these corrections, utilizing the setup of an existing beam injector established at Huazhong University of Science and Technology. Both the theoretical and experimental results indicate that the measurement accuracy of the Q-scanning technique can be improved by correcting the fringe field effects.