Emittance growth in the FEL RF-gun

A high brightness and low emittance is of crucial importance for the SASE-FEL at the TESLA Test Facility. Therefore a photo-RF-gun has been installed as particle source. Numerical simulations with codes like ASTRA and MAFIA show that the space charge dominated processes inside the RF-gun contribute significantly to the emittance. In this paper we present the results of detailed studies with MAFIA TS2 which clarify the effects resulting in emittance growth for space charge dominated beams. It is shown that the resulting emittance can be minimized by changing the laser parameters like pulse length and spot size on the cathode. Additionally we present the concept of slice emittances which allows a more precise prediction of the real transverse emittance achievable with an emittance compensation scheme.