Beam matching strategies in undulators for SASE FEL operating devices

We discuss the new problems emerging in charged beam transport for SASE FEL dynamics. The optimisation of the magnetic transport system for future devices requires new concepts associated with the slice emittance and the slice phase space distribution. We study the problem of electron beam slice matching and guiding in transport devices for SASE FEL emission by discussing matching criteria and how the associated design of the electron transport line may affect the FEL output performances. We analyse different matching strategies by studying the relevant effect on the FEL output characteristics.