Optimization of single-step tapering amplitude and energy detuning for high-gain FELs

We put forward a method to optimize the step-tapering amplitude of undulator strength and initial energy detuning of electron beam with the goal of maximizing the saturation power of high-gain FELs, based on the physics of longitudinal electron beam phase space. Using the FEL simulation code GENESIS, we numerically demonstrate the accuracy of the estimation for parameters corresponding to the linac coherent light source and the Tesla test facility.