New simulation results for the S-DALINAC electron source

For the operation of its mid-infrared FEL the superconducting electron accelerator S-DALINAC has to produce bunches with a charge of 6 pC and a duration of 2 ps FWHM. In order to achieve these figures a subharmonic injection scheme was added to the 3 GHz cw accelerator. The electron gun consists of a grid controlled thermionic cathode on a 250 kV terminal. It produces electron pulses with a charge of 20 pC and a width of 800 ps FWHM at a repetition rate of 10 MHz. For a reliable simulation of the subsequent parts of the injector (600 MHz chopper/prebuncher section and a superconducting linac consisting of 2- and 5-cell capture cavities and two 20-cell accelerating structures) the beam parameters at its input have to be known as exactly as possible. Since we expect that due to the pulsed operation of the gun, longitudinal space charge effects might be important, the electron gun and the 250 kV electrostatic acceleration have been studied using the 2 1/2 dimensional PIC code MAFIA-TS2 and the tracking code GPT. This calculation takes into account the time structure of the beam and yields results like bunch shape at the gun exit, correlated and uncorrelated energy spread, as well as the transverse emittance. The results of the simulation as well as a comparison with measurements of the transverse emittance and the electron pulse length at the exit of the electrostatic acceleration will be presented in this paper.