Practical realization of a self-adaptive feed-forward control system for RF gun beam-loading compensation

Beijing Free Electron Laser Facility (BFEL) features the adoption of a thermionic cathode microwave electron gun which is a compact RF linac-based FEL injector with good performance for IR spectral region. For long macro-pulse operation, however, the back-bombardment of the electrons on the gun cathode causes current increasing and hence energy decreasing of the output electron beam due to beam-loading effect. This effect degrades FEL performance because the energy spread of the accelerated beam will be seriously enhanced. In this paper, the experimental evidence of the beam-loading effect is first presented. Then, the analytical procedures for the calculation of the control signals will be briefly discussed that form the basis of the computer programming. The system has the capability to control both the amplitude and the phase of the RF output from the klystron so as to deal with the coupling between the circuit elements for amplitude and phase control. Some difficulties in implementation are also discussed and some practical realization measures are presented accordingly. The control system described above is operational and some preliminary experimental results have been obtained, including the control of amplitude and phase fluctuations of the RF output of the klystron and the microwave field in the gun cavity. These preliminary outcomes suggest that the system developed is effective to improve the beam quality and hence the performance of BFEL.