We report a passively mode-locked fiber-based oscillator that has no internal dispersion-compensating gratings. This design, which we believe to be the first of its kind, produces 25 nJ pulses at 80 MHz with the pulses compressible to 150 fs. The pulses appear to be self-similar and initial data imply that their energy is further scalable.

Population dynamics of atomic inner-shell vacancy states are analyzed for the possibility of inversion in relation to keV x-ray laser schemes. Transitions between pairs of inner-shell vacancy states are considered in which the states are pumped via electron-impact inner-shell ionization by a femtosecond high-energy electron pulse. For appropriate atomic systems, transient inversion is predicted due to rapid lower state depopulation via Coster-Kronig decay.

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A new physical scheme for femtosecond x-ray lasers, where the upper lasing level (L 23 innershell vacancy level͒ is pumped by x-ray photons and the lower lasing level (M 1 innershell vacancy level͒ is depopulated via a Coster-Kronig radiationless transition, is analyzed for Ca. The transition wavelength is 4.1 nm, which is inside the water window ͑the wavelength range between the K absorption edges of oxygen and carbon͒. The peak spectral brightness of the x-ray laser output at 4.1 nm is predicted to be as large as 5 ϫ10 25 photons/s/͑mm 2 mrad 2 0.1% bandwidth), which is 4 to 5 orders of magnitude brighter than a typical undulator radiation in the similar spectral region. In addition to the high flux, the expected duration of x-ray lasing of ϳ3 fs will be useful for the study of fast dynamics in physical and biological sciences.New regimes of laser matter interaction have opened due to recent advances in the 10 fs range, high-peak-power laser development ͓1-4͔. These lasers are ideal for the generation of femtosecond high-order harmonic radiation in the soft x-ray range ͓5-9͔, and for pumping x-ray lasers based on innershell atomic transitions ͓10-16͔. While the past 15 years have seen significant progress in the x-ray laser research and development, most x-ray lasers operate at lowrepetition rates with pulses in picoseconds or longer. Highrepetition-rate, femtosecond x-ray lasers would be useful for dynamical studies of ultrafast phenomena in nature ͓17͔.Duguay and Rentzepis ͓18͔ first proposed inner-shell x-ray laser schemes in 1967. In their scheme the lower level of the lasing transition was the ground level of the first ion and could not decay. The idea of using an atomic system in which the lower level decays was suggested by Stankevich ͓19͔, elaborated by Arecchi ͓20͔ and Elton ͓21͔, and calculated in detail by Axelrod for K-shell transitions ͓22͔. The technical barrier for the successful realization of these schemes has been the development of a sufficiently fast and energetic x-ray pump source whose time scale is on the order of the lifetime of the keV lasing transitions, i.e., in the 10 fs range ͓10,11,15,22͔. Such pumping sources are now becoming available due to the advent of femtosecond ultrahigh peak-power lasers. Such systems with peak powers of tens to hundreds of terawatt ͑TW͒ have been demonstrated ͓1-4͔ and plans exist for extending peak powers to 1 petawatt at a 20-fs pulse duration. Even with these new lasers, intrinsic problems with the K-shell transition x-ray laser scheme still exist. In particular, electrons produced during photoionization and subsequent Auger decays are energetic enough to collisionally ionize neutral atoms, producing the lower level of the lasing transition and destroying the inversion. If it were possible to create an inner-shell population inversion via atomic processes involving only electrons, the inversion in any photoionization pumped x-ray laser schemes based on the same transitions would be ͑1͒ much less sensitive to secondary electron-collisional filling of th...