Experimental demonstration of dark current mitigation by an over-inserted plug in a normal conducting VHF gun

TL;DR

This paper demonstrates that over-inserting a plug in a VHF gun significantly reduces dark current, improving the performance of high-repetition-rate free-electron lasers through experimental and simulation studies.

Contribution

It presents the first systematic experimental and simulation study showing dark current mitigation via over-inserted plugs in VHF guns.

Findings

Dark current reduced by over two orders of magnitude.

Over-inserted plugs effective across different gun configurations.

Systematic approach combining simulations and measurements.

Abstract

The room temperature continuous wave (CW) very-high-frequency (VHF) gun is one of the candidates for the electron gun of the high-repetition-rate free-electron lasers (FELs). The VHF gun operates with a cathode gradient of ~ 20 MV/m and an accelerating voltage of ~ 750 kV. The gun dark current emission leads to beam loss along the FEL machine, therefore is a critical parameter for the performance of the CW gun. In this paper, we presents a systematic study of the dark current reduction of the VHF gun, including cathode region optimizations, dark current tracking simulations and measurements. Over-inserted cathode plugs were tested in two VHF guns of different acceleration gap sizes, and both demonstrated significant dark current reduction ratios of more than two orders of magnitude.