Development of a 1.4-cell RF photocathode gun for single-shot MeV ultrafast electron diffraction devices with femtosecond resolution

The MeV ultrafast electron diffraction (UED) has proven to be a powerful tool for exploring the ultrafast process at the atomic level with femtosecond resolution. However, MeV UED facilities are struggling to achieve sub-100 fs temporal resolution at pC level charges due to the rapid and severe bunch lengthening near the cathode region. Both analytical work and simulation show that the bunch lengthening near the cathode originates from both the strong space charge effect (SCE) within the sub-relativistic electron beam and the bunch tail trailing due to the mirror charge effect (MCE). In this paper, we report the development of a new 1.4-cell RF gun that significantly increases the extraction field at the cathode to suppress both the SCE and the MCE. Compared to the conventional 1.6-cell RF gun, the new 1.4-cell RF gun is more suitable to produce an electron beam with a shorter bunch length and less distorted longitudinal phase space. The shorter bunch length leads directly to a temporal resolution improvement in UEDs. Meanwhile, the beam longitudinal phase space with better linearity ensures a smaller compressed bunch length when the bunch compression schemes are applied.