Wang, G.L. Dai, D.X. Wu, G.R. Yang, X.M. Zhang, W.Q. MOC04 laser bunching FEL electron linac 2015 2015-12 The beam energy spread at the entrance of undulator system is of paramount importance for efficient density modulation in high-gain seeded free-electron lasers (FELs). In this paper, the dependencies of high harmonic bunching efficiency in the high-gain harmonic generation (HGHG) schemes on the electron energy spread distribution are studied. Theoretical investigations and multi-dimensional numerical simulations are applied to the cases of uniform and saddle beam energy distributions and compared to a traditional Gaussian distribution. It shows that the uniform and saddle electron energy distributions significantly enhance the performance of HGHG-FELs. A numerical example demonstrates that, with the saddle distribution of sliced beam energy spread controlled by a laser heater, the 30th harmonic radiation can be directly generated by a single-stage seeding scheme for a soft x-ray FEL facility.