TESLA FEL photo-injector simulations giving high-quality beams

To obtain a high-brightness beam for the TESLA FEL project, operating in the VUV region, a photo-injector providing a high-quality beam is needed, i.e., the rms radial and longitudinal emittances must be, respectively, of the order of 1 mm mrad and 20 mm KeV [1]. With these drastic conditions, we cannot use an analytical model with some approximations or scale laws, a refined study of the beam dynamics with a numerical code is necessary. The quality optimization is all the more difficult since many parameters play a role in the beam dynamics. The beam dynamics is simulated by the numerical code ATRAP (Acceleration and TRAnsport of Particles) [2–5], using the Liénard-Wiechertʹs equations to describe the self-electromagnetic field. To reduce the total time of this optimization, we have chosen the RF peak field on the cathode, between 40 and 60 MV/m, high enough to decrease the space charge effect and not too high to avoid the defocusing effect of the RF field. In this paper, we show the influence of the different parameters on the beam quality.