The universal method for optimization of undulator tapering in FEL amplifiers

Technique of undulator tapering in the post-saturation regime is used at the existing x-ray FELs for increasing the radiation power. There are also discussions on the future of high peak and average power FELs for scientific and industrial applications. Diffraction effects essentially influence on the choice of the tapering strategy. Recent studies resulted in an general law of the undulator tapering for a seeded FEL amplifier. In this paper we extend these results for the case of the Self Amplified Spontaneous Emission (SASE) FEL.