Femtosecond-Scale MeV-UED Beamline Using a Stand-Alone Multi-Cell RF Photogun

TL;DR

This paper introduces a multicell RF photogun design capable of producing femtosecond-scale electron bunches for ultrafast electron diffraction, reducing system complexity and enabling high temporal resolution without downstream compression.

Contribution

It proposes a novel multicell RF photogun with tailored phase velocity to generate femtosecond electron bunches directly, eliminating the need for downstream compression.

Findings

Achieves 5-15 fs rms electron bunches directly from the gun.

Demonstrates a travelling wave design with significantly lower power dissipation.

Projects 26 fs rms temporal resolution when paired with high-stability RF sources.

Abstract

The temporal resolution of MeV ultrafast electron diffraction (UED) is fundamentally constrained by the electron bunch length at the sample, motivating the development of new electron sources capable of producing femtosecond scale bunches. In this work, we propose a multicell RF photogun that has a tailored phase velocity profile to generate 5 to 15 fs rms MeV electron bunches directly from the electron gun, eliminating the need for downstream compression. This approach achieves comparable performance to conventional one and a half cell photoguns with downstream compression, while reducing system size, complexity, and power requirements. We examine two implementations: a standing wave (SW) and a travelling wave (TW) design. The TW variant demonstrates over an order of magnitude lower power dissipation than typical SW structures, enabling potential kHz operation. When paired with SwissFEL style C band RF sources, which offer high amplitude and phase stability, the TW photogun is projected to deliver a temporal resolution of 26 fs rms.