  We study a self-seeded high-gain harmonic generation (HGHG) free-electron
laser (FEL) scheme to extend the wavelength of a soft X-ray FEL. This scheme
uses a regular self-seeding monochromator to generate a seed laser at the
wavelength of 1.52 nm, followed by a HGHG configuration to produce coherent,
narrow-bandwidth harmonic radiations at the GW level. The 2nd and 3rd harmonic
radiation are investigated with start-to-end simulations. Detailed studies on
the FEL performance and shot-to-shot fluctuations are presented.

Accelerator Physics