Arcing-based protection system for the klystron output window of the BEPCII Linac

The Beijing Electron–Positron Collider II (BEPCII) Linac is powered by 20 klystrons, and the output windows of each klystron need to be protected from being broken, which are currently protected by a system based on reflected radiofrequency (RF) power and vacuum levels. The primary weak point of reflection-based protection is the difficulty of accurately measuring the value caused by an uneven waveform. Moreover, vacuum protection is restricted by the long response time, which results in delayed protection. Both of the weak points may damage the output window of the klystron. An arcing-based protection system is then introduced into the BEPCII Linac, considering the constant presence of light during arcing and the experiences of other laboratories and colleagues in the RF group. The system development mainly consists of the design and test of the E-bend waveguide observation window, selection of the arcing detection module, offline joint test, system assembling and online operation. The system was assembled with a new klystron at the K6 position on the summer of 2018. During the following annual operation year, the light signal was captured, and the interlock was successfully realized. The arcing-based protection system assembled online was found to work well. Moreover, the light signal was successfully captured, and the interlock function worked fine. The system is planned to be applied in other klystron locations in the future.