Life science and biology have advanced with the progress of microscope technology. A strong desire of life scientists is to observe live cells with high spatial resolution. The development of soft x-ray microscopy using a special wavelength called the water window started in the 1970s in order to fulfill this desire. However, the photon fluxes realized so far are still insufficient for this application. Here, we demonstrate the generation of a coherent water window x-ray by extending the plateau region of high-order harmonics under a neutral-medium condition. The observed maximum harmonic photon energy attained are 300 eV and 450 eV in Ne and He, respectively. Our proposed generation scheme, combining a 1.6 mm laser driver and a neutral Ne gas medium, is efficient and scalable in output yields of the water window x-ray. This powerful concept is expected to boost the development of soft x-ray microscopes based on high-order harmonics.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.