Quality of the self-field dominated beam, with arbitrary aspect ratio, of a high-intensity magnetically focused rf gun

Abstract

In a high-intensity, laser-driven rf gun, such as that designed to feed the high-power rf-FEL of the CEA at Bruyères-le-Châtel, self-field effects are dominant in electron-beam emittance growth. For beams with an aspect ratio of the order of 1 (as are the energetic beams of the forementioned FEL), this emittance growth takes place mainly during extraction or at the beginning of acceleration. It is attended by radial expansion which makes it necessary to focus the beam as soon as possible, i.e. with a magnetic lens located at the exit of the first cavity, before the beam enters further accelerating cavities. We study theoretically the influence of such magnetic focusing on the beam quality, measured by various whole-beam emittances: radial, transverse, longitudinal, 3D, as well as by energy spread. Theoretical modelling treats the self electromagnetic field by Liénard-Wiechert's equations. This provides a rigorous relativistic description of early beam transport and emittance growth for beams of any aspect ratio, in which the velocity spread is enormous at the beginning, so that monokinetic — or space charge — approximations like Kim's cannot be used.