Improving the beam quality of rf guns by correction of rf and space-charge effects

Abstract

In this paper we describe two possible strategies to attain ultra‐low emittance electron beam generation by laser‐driven RF guns. The first one is based on the exploitation of multi‐mode resonant cavities to neutralize the emittance degradation induced by RF effects. Accelerating cigar‐like (long and thin) electron bunches in multi‐mode operated RF guns the space charge induced emittance is strongly decreased at the same time: high charged bunches, as typically requested by future TeV e−e+ colliders, can be delivered by the gun at a quite low transverse emittance and good behaviour in the longitudinal phase space, so that they can be magnetically compressed to reach higher peak currents. The second strategy consists in using disk‐like electron bunches, produced by very short laser pulses illuminating the photocathode. By means of an analytical study a new regime has been found, where the normalized transverse emittance scales like the inverse of the peak current, provided that the laser pulse intensity distribution is properly shaped in the transverse direction. Preliminary numerical simulations confirm the analytical predictions and show that the minimum emittance achievable is set up, in this new regime, by the wake‐field interaction between the bunch and the cathode metallic wall.