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Coherent soft X-ray (SXR) sources that provide a high photon flux in the water window are essential tools for advanced spectroscopy (e.g. of magnetic materials [1] and organic compounds [2] ) or for lens-less bio imaging with nm-scale resolution [3] . To date, such sources are mostly large-scale facilities like synchrotrons or free electron lasers. A promising alternative are laser-driven sources, based on high harmonic generation (HHG) in noble gases. Current state-of-the-art SXR HHG uses frequency converted Ti:Sa lasers (to ~2 µm wavelength to increase the phase matching cutoff) with multi-mJ pulse energies at 1 kHz repetition rate [1] . Most recently, there has been a strong push towards increasing the repetition rate of the driving lasers, and the SXR HHG, to enable faster data acquisition, space-charge-reduced SXR photoelectron spectroscopy or coincidence detection [4] , [5] . In this contribution, we present an approach to SXR HHG that is based on nonlinear pulse self-compression and HHG in the same helium gas-filled antiresonant hollow-core fiber (ARHCF). Because of the intensity enhancement resulting from temporal pulse self-compression, the experiments can be driven by moderate-energy, multi-cycle laser pulses, which facilitates repetition rate scaling. We coupled 100 fs-, 250 µJ-pulses centered around 1.9 µm wavelength at a 98 kHz repetition rate to the ARHCF ( Fig. 1a ). When the fiber length (~1.2 m) and the gas pressure at its output (~3.8 bar) were chosen appropriately, the pulses close to its end ( Fig. 1b ) were self-compressed to 4×10 14 W/cm 2 . At this point, the gas is partially ionized, and the chosen pressure ensures phase matching between the driving laser and the generated SXR light ( Fig. 1b ). It is the first time that this approach is experimentally realized, and we have generated a photon flux >10 6 Ph/s/eV at the carbon K-edge ( Fig. 1c ). To the best of our knowledge, this is the highest photon flux at 300 eV reported to date at a laser repetition rate >1 kHz.

Table-top extreme ultraviolet (XUV) light sources based on high harmonic generation (HHG) have emerged as a user-friendly, complementary technique to large scale facilities like synchrotrons and free electron lasers [1] . The ultrashort pulse duration (femtosecond to attosecond) and coherence of these laser-driven sources are highly advantageous for many applications, e.g. coherent diffractive imaging [2] or XUV spectroscopy of atoms, molecules and ions [3] . In addition to the precise control of temporal and spatial properties of the HHG radiation, investigations of narrow band resonances and detailed spectral features in the XUV require tuning of the discrete harmonic lines. Therefore, there is a strong application-driven demand for powerful, tunable XUV combs. To date, a large variety of tunable XUV sources have been demonstrated e.g. by shifting the driving wavelength using optical parametric amplifiers [4] or soliton-plasma dynamics [5] , among others [6] .

Recent advances in table-top high-harmonic sources allowed for coherent diffractive imaging with a record spatial resolution of 13 nm. Additionally, integration times of only a few seconds will enable 3D tomography, imaging of extended objects and element-specific imaging in the near future.