Fig. 1. Schematic layout of the harmonic-enhanced HGHG FEL. Here, is a short modulation undulator, where an external seed laser with a wavelength of and a peak power at the megawatt level is used to modulate the electron beam; is a harmonic-enhanced modulation undulator resonant at and is a radiation undulator resonant at ; and are two dispersion chicanes. (a)–(e) Sketches of electron beam distribution in the longitudinal phase space.

Fig. 2. Electron distribution in the longitudinal phase space and the corresponding bunching factor at different harmonics of the seed laser wavelength after (a), (b) , (c), (d) and (e), (f) .

Fig. 3. (a) The evolution of radiation pulse energy along and averaged over 50 GENESIS runs and (b) the FEL spectra for the 50 runs for the case with . The averaged spectrum is plotted as a dark line in (b).

Fig. 4. Bunching factor evolution at the fifth and 20th harmonics of seed laser wavelength along the radiation undulator for the case without .

Fig. 5. Evolution of the radiation spectra (upper) and the longitudinal phase space of the electron beam (lower) along : (a) and (e) at the entrance of , (b) and (f) after the first undulator segment, (c) and (g) after the third segment and (d) and (h) after the fifth segment. The longitudinal coordinates are scaled to . The electron current profiles are plotted as red curves in (e)–(h).

Fig. 6. (a) The evolution of radiation pulse energy along and averaged over 50 GENESIS runs and (b) the FEL spectra for the 50 runs for the case without . The averaged spectrum is plotted as a dark line in (b).

Fig. 7. FEL pulse energy for 200 GENESIS runs with random machine errors.