Suppression of beam break up instability on an S-band travelling-wave linac

This work presents a novel structure design for suppressing beam break up (BBU) effect on a travelling wave (TW) linear accelerator (linac). The higher-order modes (HOMs) trapped in bunching section were demonstrated to be critical to such beam instability. Eliminating the trapped HOMs would significantly improve the peak beam current. In this paper, an optimized accelerator structure with lower passband of HOMs in the bunching section was proposed to relieve BBU instability. In a high-power experiment, 40-mA threshold growth of BBU peak beam current was achieved without wake-field damping or absorbers. Based on detailed time-domain and frequency-domain analysis, the trapped HOMs were significantly weakened. This study provides an effective detuned approach for enhancing output beam power of linac with higher peak beam current.