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

A seeded free-electron laserwith a two-stage harmonic upshift configuration provided tunable and coherent soft-X-raypulses. The configuration produced single- transverse-mode, narrow-spectral-bandwidth femtosecond pulses with energies of several tens of microjoules and a low pulse-to-pulse wavelength jitterat wavelengths of 10.8 nm and below.