Generation of Longitudinally Coherent Ultra High Power X-ray Fel Pulses by Phase and Amplitude Mixing*

We study an improved SASE (iSASE) scheme to generate narrow bandwidth Xray FEL pulses by introducing repeating delays of the electron beam respect to the radiation field, thus mixing the spikes phase and amplitude, increasing the cooperation length and generating a bandwidth much smaller than in the SASE case. The improved longitudinal coherence is used, in combination with a tapered undulator, to increase the efficiency of energy transfer to the radiation and generate ultra-high peak power. We report results of theoretical and simulations studies for a hard X-ray iSASE FEL at 0.15 nm, using an LCLS-II undulator to generate X-ray pulses with peak power between 0.1 and 1 TW. The analysis is carried out using a time dependent 1-dimensional model and with GENESIS numerical simulation including 3dimensional effects.