Characteristics of dark current from an S-band rf gun with exchangeable photocathode system

TL;DR

This study investigates dark current emission in an S-band RF gun with exchangeable photocathode, identifying sources and patterns through experiments and simulations to improve gun performance.

Contribution

It provides a detailed analysis of dark current sources in an S-band RF gun using experimental measurements and finite element simulations, aiding in minimization efforts.

Findings

Dark current patterns can be qualitatively reproduced by simulations.

Certain surfaces within the gun significantly contribute to dark current.

Analysis informs improvements in gun and cathode design.

Abstract

Field emission or dark current from RF electron guns is problematic for accelerators for several reasons, and can be more difficult to predict, measure and understand, compared to the main beam. The work presented here is a study of the dark current emitted from the CLARA 3GHz 2.5 cell electron gun with exchangeable photocathode, which is a type of gun used for single pass high quality low emittance beams for uses such as Free Electron Lasers. The main purpose of these studies was to identify sources of dark current emission within the gun, by examining the dark current distribution and patterns on scintillating screens. It is shown that the features of the patterns can be reproduced qualitatively, and to some extent quantitatively, by finite element simulations. Thus, certain surfaces within the gun and cathode apparatus which contribute most significantly to problematic dark current can be identified, and further development and preparation of the guns and their cathodes can be informed to minimise dark current.