Demmler, S.S.DemmlerKlas, R.R.KlasTschernajew, M.M.TschernajewHädrich, S.S.HädrichRothhardt, J.J.RothhardtLimpert, J.J.LimpertTünnermann, A.A.Tünnermann2022-03-132022-03-132016https://publica.fraunhofer.de/handle/publica/40083610.1364/UP.2016.UW2B.6We report on efficient high harmonic generation based on cascaded frequency conversion of a high power femtosecond fiber laser. Using krypton as generation medium, up to 2.8 mW (8·1014 photons/s) of average power have been generated within a single narrowband harmonic at 21.6 eV. Furthermore, a strong window-resonance in argon has been utilized to achieve relative energy bandwidths of DE/E=3·10−3 at 26.6 eV. This narrow harmonic line still contains 1.6 mW of average power. We discuss the scaling towards higher photon flux, smaller bandwidth and potential applications.en620conference paper