Abstract A high-resolution soft X-ray monochromator system is designed for self-seeding the next generation FEL sources. It consists of a single variable-line-spacing (VLS) grating, an exit slit, and pre-and collimating mirrors, and operates in the fixed-focus mode to achieve complete tuning of the seeding energy from 200 to 2000 eV with a nearly constant resolving power of greater than 50000, producing transform-limited seed ranging from 1 ps at 200 eV to 100 fs at 2000 eV. The optical delay is of order 1 ps, matching well with that of an electron chicane of moderate magnetic field strength. The design is based on a coherent Gaussian beam treatment of the FEL beam propagating from the upstream SASE undulator through the entire seeding monochromator system, preserving the transverse beam profile entering the downstream seeding undulator to ensure maximum coupling efficiency with the reentrant electron beam
