A Femtosecond X-ray Undulator Source for EUV Pump/X-ray Probe Experiments at FLASH2

The recently implemented APPLE-III device at FLASH2 is an undulator source providing tunable, femtosecond spontaneous X-rays in the range of 2 - 3.5 keV at the 3rd and 5th harmonics. This work demonstrates its applicability for EUV FEL laser pump/probe absorption and diffraction experiments.