Strong focusing influence on high gain FEL characteristics

The problem of strong alternating focusing effect on the operation of a linac-driven FEL with a planar undulator is considered. The numerical analysis is made for the single-pass soft X-ray FELs on the basis of TDA code. The effects of the focusing of different types (quadrupole or periodic sextupole respectively) and phase advance per focusing cell are investigated. Three SASE FEL known projects are numerically analyzed.