The optimal parameter space for an X-ray free-electron laser (FEL) in the self-amplified spontaneous emission (SASE) operation mode is examined. This study focuses on FEL operation with a shorter undulator period and higher undulator strength made available through recent developments in in-vacuum, cryogenic and superconducting undulators. Progress on short-period undulator technologies is surveyed and FEL output characteristics versus undulator parameters are computed. The study is performed on a case of the planned soft-X-ray FEL at the MAX IV Laboratory in Sweden. An extension of the SASE mode into the harmonic lasing self-seeded mode is also analysed.
Keywords: X-ray free-electron lasers; harmonic lasing self-seeding; high brilliance; self-amplified spontaneous emission; undulator technologies.