PROGRESS IN THE DESIGN OF A TWO-FREQUENCY RF CAVITY FOR AN ULTRA-LOW EMITTANCE PRE-ACCELERATED BEAM

Today most of the X-rays Free-Electron Laser projects are based on state of the art RF guns, which aim at a nor- malized electron beam emittance close to 1 mm·mrad. In this paper we report on the progress made at PSI towards a hybrid DC + RF Low Emittance Gun (LEG) capable of producing a beam with an emittance below 0.1 mm·mrad. To reduce the intrinsic thermal emittance at the LEG cath- ode the electrons are extracted from nano-structured field- emitters. A gun test facility is under construction wherein after emission the beam is accelerated up to 500 keV in a diode before being injected and accelerated in a two- frequency 1.5-cell RF cavity. The fast acceleration in the diode configuration allows to minimize the emittance di- lution due to the strong space charge forces. The two- frequency RF structure is optimized to limit the emittance blow-up due to the non-linearity of the RF field.