Emittance self-compensation in blow-out mode

We report an unusual regime of emittance self-compensation in an electron bunch generated in the blow-out mode by a radio-frequency photocathode gun. This regime is observed for a strong space-charge field on the cathode reaching around 30%-35% of the accelerating field. Simulations clearly show an initial growth and a subsequent self-compensation of projected emittance in a divergent electron bunch originating from the effects of: (a) strong space-charge forces of mirror charges on the cathode, (b) an energy chirp in the bunch and (c) substantial re-shaping of the electron bunch. Furthermore, we show analytically and numerically how a complex interplay between these effects leads to emittance self-compensation at the gun exit-the effect that is normally observed only in the presence of focusing fields.