  We discuss the new problems emerging in charged beam transport for SASE FEL
dynamics. The optimization 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 discussing matching criteria
and how the associated design of the electron transport line may affect the FEL
output performances. We analyze different matching strategies by studying the
relevant effect on the FEL output characteristics.
