Temporal resolution of MeV ultrafast electron diffraction based on a photocathode RF gun

Photocathode RF guns serve as electron sources in various ultrafast pump–probe experiments, such as in X-ray free-electron lasers, Thomson scattering X-ray sources, and MeV ultrafast electron diffractions (UED), for studying structural dynamics on the fundamental scales of atomic motions. The temporal resolutions of these experiments are of great concern. In this paper, we study each contributing term to the temporal resolution, including the RF compression effect, the dependence of time-of-flight of the electron pulses on the RF-to-laser timing jitter, and the effect of fluctuation of the RF field amplitude. We employ a simple single-particle model, and demonstrate inherent connections of all these effects to the single-particle dynamics. Particularly, the correlation between the RF compression effect and the fluctuation of time-of-flight induced by the RF-to-laser timing jitter, makes the temporal resolution considerably smaller than simply adding each term quadratically. We apply these results to a MeV UED and evaluate its temporal resolution, showing that a ∼ 100 fs resolution is already achievable with state-of-the-art hardware performances. Since these results are inherent properties of photocathode RF guns, they are also applicable to other facilities.