Two-stage seeded soft-X-ray free-electron laser

We report the first generation of coherent, tunable, variable-polarization, soft X-ray femtosecond pulses, generated by a seeded free-electron laser (FEL) operating in the fresh bunch, two-stage harmonic upshift configuration. Characterization of the radiation proves this FEL configuration can produce single-transverse-mode, narrow-spectral-bandwidth output pulses of several tens of microjoules energy and low pulse-to-pulse wavelength jitter at...