Study of Wakefield-Induced Beam Breakup Instability on an X-Band Accelerating Structure

In this study, the instability of beam breakup (BBU) was explored both theoretically and experimentally for an X -band linear accelerating structure developed at Tsinghua University. According to Eigenmodes and mode passband results, there were several dipole modes trapped in the bunching section. The beam dynamics calculation was performed considering the wakefield effect. The higher-order modes (HOMs) trapped in bunching cavities were demonstrated as a major contributor to beam instability. Then, a radio frequency (RF) low-power test and high-power experiment were performed on HOMs to investigate BBU instability, with the discontinuity of passband detected in the RF cold test. During the high-power experiment, an accelerating gradient of 18.7 MeV/m was achieved, and the kinetic energy was measured to be 6.1 MeV. The HOM signal was extracted and analyzed. The growth of HOMs was obtained, and the main dipole modes were evaluated. As confirmed by both simulation and experimental results, the BBU instability in the accelerating structure was caused by a combined impact of the HOMs trapped in bunching cavities and the dipole modes of the accelerating section.