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During the operation of the China ADS Front End Superconducting Demo Linac ofthe China initiative Accelerator Driven System (CiADS), it was found that therewas flashover happened on the surface of vacuum ceramic window in the pick-upcoupler of superconducting cavity, which is uncommon under low temperatureconditions. That seriously affects on the stable operation of thesuperconducting linear accelerator. By comparing the flashover trajectory onthe electrical aging windows from the accelerator, the simulation results ofthe field emission electrons trajectory in the superconducting cavity, as wellas online experiments, we found that the flashover on the window surface wascaused by the bombardment and accumulation of field emission electrons from theSRF cavity. Our results suggest that, unlike most vacuum window flashovers, ofwhich characteristic and causation can be explained by the mechanism of thesecondary electron emission avalanche (SEEA) model. Some phenomena of theflashover in the linac have been initially explained under the guidance of themechanism discussed in the PER flashover model. According to the conclusions, anew design modification to the pick-up coupler was made and applied to thelinac. The Pt (the power coming out the transmitted power coupler) signalflicker problem has not been monitored after the online application of ournewly designed antenna. The research in this article solved the flashoverproblem and improved the performance of stable operation of the accelerator.

Supression of Flashover on Radio-frequency Window of Linear Accelerator at The Temperature of 4.2K