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

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.