Emittance growth due to the wake field driven by an electron beam accelerated in an RF-gun of free electron laser “ELSA”

It appears that the ease of the parameter chosen for “ELSA” photo injector, the influence of the exit aperture, in terms of beam quality, is slight concerning the transverse emittance: (Δε⊥/ε⊥)(z)∼3% at maximum, and negligible concerning the axial emittance. To complete this paper, we recall the results previously obtained concerning the wake field of a closed or open cavity for a beam approaching the anode [6,7]. They had quantitatively specified the expected deep assymetry between the conducting walls regarding their contribution to the total wake field, besides the space–charge contribution. (Given that the radial walls have no time to contribute, these conducting walls are the cathode and the anode.) Thus, concerning the effects on whole-beam emittances, the correction (Δε⊥/ε⊥)(z) entailed by taking the anode contribution into account had been found to be less than 5% at maximum (a maximum reached for t=tg, and for E0=30 MV/m). The field map perturbation due to the aperture in the anode [7] has an effect of the same order of magnitude, i.e. small, on the transverse emittance of a beam reaching this exit aperture. As shown in Fig. 3, this correction is of order of 3% for E0=30 MV/m. It could be of the order of 10% for E0=10 MV/M.