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

TL;DR

This paper investigates flashover issues on a superconducting linear accelerator's RF window at 4.2K, identifies electron bombardment as the cause, and proposes a new design to prevent flashover, enhancing operational stability.

Contribution

It introduces a new understanding of flashover mechanisms at low temperatures and presents a modified pick-up coupler design to mitigate flashover in superconducting accelerators.

Findings

Flashover caused by field emission electron bombardment.

New coupler design prevents flashover and flicker issues.

Enhanced stability of superconducting linac operation.

Abstract

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.