Detection and suppression of the trapped-electrons-transportation-type flashover in a linear accelerator

The polarization energy relaxation around trapped electrons type of flashover happened in the Superconducting Demo Linac of the China Initiative Accelerator Driven Subcritical System project, which greatly limited the acceleration capability of the accelerator. This problem seriously affects the stability of accelerator operation and becoming the most urgent issue to be solved. From the reticulated and radial traces through the inner and outer conductors on the electrical aging windows of the RF couplers, the surface discharge or flashover was inferred from the pick-up couplers. The simulation of field-emission electron trajectory in the superconducting cavity and the online monitoring of the flashover signal showed that the discharge occurred by the bombardment and accumulation of field-emission electrons from the cavity. Therefore, the antenna of the pick-up coupler was redesigned and applied to the Linac. The new design removed the abnormal signal caused by discharges during more than one year operation of the cavity, thereby improving the acceleration capability and stability performance of the accelerator. (paper)