High gradients at SRF photoinjector cavities with low RRR copper cathode plug screwed to the cavity back wall

TL;DR

This paper demonstrates that copper cathode plugs screwed directly into L-band SRF photoinjector cavities can achieve peak on-axis gradients of around 55 MV/m, marking progress toward high-gradient CW photoinjectors.

Contribution

First experimental confirmation that copper cathodes can reach high gradients in SRF cavities with a simplified design.

Findings

Achieved peak gradients of around 55 MV/m in SRF cavities with copper cathodes.

Validated copper as a suitable cathode material for high-gradient SRF photoinjectors.

Simplified cavity design without RF choke filter or load lock is effective.

Abstract

In recent years we increased the typical maximum peak field on axis gradients obtained in L-band superconducting RF (SRF) photoinjector cavities at vertical tests to around 55 MV/m. This was achieved with niobium cathode plugs directly screwed to the cavity back wall omitting an RF choke filter and a load lock system for cathodes. Copper demonstrated being a suitable cathode material in normal conducting injector cavities used at X-Ray Free Electron Lasers (XFELs) operating with pulsed RF. In this article we present the first experimental confirmation that peak field on axis gradients around 55 MV/m and beyond can be achieved in L-band SRF photoinjector cavities with copper cathode plugs screwed to the cavity back wall. We view this as a major milestone for the development of a high gradient photoinjector operating continuous wave (CW).