QUALITY OF THE SELF-FIELD DOMINATED BEAM, WITH ARBITRARY ASPECT RATIO, OF A HIGH-INTENSITY MAGNETICALLY FOCUSED RF GUN

In a high-intensity, laser-driven rf gun, such as that designed to fee d the high-power rf-FEL of the CEA al Bruyeres-le-Chatel, self-field e ffects 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 f orementioned FEL), this emittance growth takes place mainly during ext raction or at the beginning of acceleration. It is attended by radial expansion which makes it necessary to focus the beam as soon as possib le, i.e. with a magnetic lens located at the exit of the first cavity, before the beam enters further accelerating cavities. We study theore tically the influence of such magnetic focusing on the beam quality, m easured by various whole-beam emittances: radial, transverse, longitud inal, 3D, as well as by energy spread. Theoretical modelling treats th e self electromagnetic field by Lienard-Wiechert's equations. This pro vides 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 ch arge - approximations like Kim's cannot be used.