Design of ultrabright 270 keV DC photoelectron gun for ultrafast electron diffraction

Compact DC photoelectron guns of high-voltage are highly desired to output ultrabright and ultrashort electron pulses for accessing irreversible processes by using the ultrafast electron diffraction (UED) technique. The high-voltage breakdown, however, is a major technical barrier to providing an intense electric field strength in a condensed space between the photocathode and the anode when the voltage is over 120 kV. In this work, by adopting the concept of voltage division, we propose a novel design of ultrabright near-relativistic DC photoelectron guns for UED. The electric field breakdown mainly caused by micro-particle collision could be avoided such that an optimized three-level acceleration DC gun can work with an electron energy of up to 270 keV and an electric field strength of up to 15 MV/m. N-particle simulations of the electron pulse propagation show that, with such a DC electron gun, it is possible to have ultrabright and ultrashort electron probe pulses with no jitter issue.