  The formation of a double-horn current profile is a challenging issue in the
achievement of electron bunch with high peak current, especially for
high-repetition-rate X-ray free-electron lasers (XFELs) where more nonlinear
and intricate beam dynamics propagation is involved. Here, we propose to
correct the nonlinear beam longitudinal phase space distortion in the
photoinjector section with a dual-mode buncher. In combination with the
evolutionary many-objective beam dynamics optimization, this method is shown to
be effective in manipulating the longitudinal phase space at the injector exit.
Furthermore, the formation of the current spike is avoided after the
multi-stage charge density modulation and electron bunch with a peak current of
1.6 kA is achieved for 100-pC bunch charge.Start-to-end simulations based on
the Shanghai high-repetition-rate XFEL and extreme light facility demonstrate
that the proposed scheme can increase the FEL pulse energy by more than 3 times
in a cascading operation of echo-enabled harmonic generation and high-gain
harmonic generation. Moreover, this method can also be used for longitudinal
phase space shaping of electron beams operating at a high repetition rate to
meet the specific demands of different researches.
