Transient study of beam instability due to beam loading in standing-wave low-energy electron linear accelerators

This work studies the transient beam instability caused by beam loading within the macropulse in standing-wave (SW) low-energy electron linear accelerators (linacs). A general model is presented to predict the transient outcomes of SW linacs with arbitrary input conditions, such as input power and voltage of the electron gun. In this approach, main outcomes including the current waveform and the energy spectrum can be calculated by combing the power conversion relationship and beam dynamics simulation results under steady conditions. A beam test was performed with an X-band (9.3 GHz) 6 MeV SW electron linac, which indicated that the calculation agreed well with the measurement results. In addition, the proposed method demonstrates that the observed beam instability was caused by beam loading in the test. This work could provide guidance on the design, commissioning, and operation in industrial and medical low-energy electron linac applications.