Analysis of beam break up instability in an S-band irradiation travelling-wave linac

Studies on beam break up instability are carried out in a 10-MeV travelling-wave linear accelerator test stand for irradiation in Tsinghua University. The high-order modes and the radio frequency parameters are calculated and analysedin the structure with a high Q value. The main frequency components of the wakefield in different accelerating cavity sections are simulated and compared. Results show several trapped modes in the bunching section. A tracking code is built in MATLAB to simulate the dynamics. Then, many investigative experiments are carried out in the test stand with a wake-field measurement system for high-order mode sampling analysis. The micro growth process of the high-order modes is obtained, and the main deflecting modes are analysed. The simulation and the experiments reveal the detailed mechanism of beam break up and prove that instability is mainly caused by the trapped modes in the bunching section.