Operation of a free-electron laser from the extreme ultraviolet to the water window

We report on the performance of a free-electron laser operating at the wavelength of 13.7 nm where unprecedented peak and average power for a coherent extreme-ultraviolet radiation source have been measured. In the saturation regime, the peak energy approached 170 microJ for individual pulses, and the average energy per pulse reached 70 microJ. The pulse duration was in the region of 10 fs, and peak powers of 10 GW were achieved. A a pulse repetition frequency of 700 pulses per second. The average extreme-ultraviolet power reached 20 mW. The output bean also contained a significant contribution from odd harmonics of approximately 0.6% and 0.03% for the 3rd (4.6 nm) and the 3rd (2.75 nm) harmonics, respectively. At 2.75 nm the 5th harmonic of the radiation reaches deep into the water window, a wavelength range that is crucially important for the investigation of biological samples