Emittance self-compensation in blow-out mode

TL;DR

This paper demonstrates a novel emittance self-compensation regime in electron bunches generated by a blow-out mode RF photocathode gun, driven by space-charge effects, energy chirp, and bunch reshaping, occurring in free space.

Contribution

It reveals and analyzes a new self-compensation mechanism for emittance in electron bunches without external focusing fields.

Findings

Simulation confirms emittance growth followed by self-compensation.

Analytical models explain the interplay of effects causing self-compensation.

Self-compensation occurs in free space, not just in focusing magnetic fields.

Abstract

We report an unusual regime of emittance self-compensation in an electron bunch generated in blow-out mode by a radio-frequency photocathode gun. Simulations clearly show an initial growth and a subsequent self-compensation of projected emittance in a divergent electron bunch originating from the effects of: (i) strong space-charge forces of mirror charges on the cathode, (ii) an energy chirp in the bunch and (iii) 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 in free space -- the effect that is normally observed only in a focusing magnetic field.