In situ Observation of Dark Current Emission in a High Gradient RF Photocathode Gun

TL;DR

This study uses high-resolution imaging to analyze dark current emission in an RF photocathode gun, revealing dominant emitters and their correlation with surface breakdowns, which impacts gun performance.

Contribution

It provides detailed imaging and analysis of dark current sources and their relation to surface features and breakdowns in high-gradient RF guns.

Findings

Dark current dominated by several strong emitters.

Selected emitters have high field enhancement factors.

Majority of emitters overlap with breakdown spots.

Abstract

Undesirable electron field emission (a.k.a. dark current) in high gradient RF photocathode guns deteriorates the quality of photoemission current and limits the operational gradient. To improve the understanding of dark current emission, a high-resolution (~100 um) dark current imaging experiment has been performed in an L-band photocathode gun operating at ~100 MV/m of surface gradient. Dark current from the cathode has been observed to be dominated by several separated strong emitters. The field enhancement factor, beta, of selected regions on the cathode has been measured. The post scanning electron microscopy (SEM) and white light interferometer (WLI) surface examinations reveal the origins of ~75% strong emitters overlap with the spots where rf breakdown have occurred.