  During the operation of the China ADS Front End Superconducting Demo Linac of
the China initiative Accelerator Driven System (CiADS), it was found that there
was flashover happened on the surface of vacuum ceramic window in the pick-up
coupler of superconducting cavity, which is uncommon under low temperature
conditions. That seriously affects on the stable operation of the
superconducting linear accelerator. By comparing the flashover trajectory on
the electrical aging windows from the accelerator, the simulation results of
the field emission electrons trajectory in the superconducting cavity, as well
as online experiments, we found that the flashover on the window surface was
caused by the bombardment and accumulation of field emission electrons from the
SRF cavity. Our results suggest that, unlike most vacuum window flashovers, of
which characteristic and causation can be explained by the mechanism of the
secondary electron emission avalanche (SEEA) model. Some phenomena of the
flashover in the linac have been initially explained under the guidance of the
mechanism discussed in the PER flashover model. According to the conclusions, a
new design modification to the pick-up coupler was made and applied to the
linac. The Pt (the power coming out the transmitted power coupler) signal
flicker problem has not been monitored after the online application of our
newly designed antenna. The research in this article solved the flashover
problem and improved the performance of stable operation of the accelerator.
