We present a laser plasma based x-ray microscope for the water window employing a high-average power laser system for plasma generation. At 90 W laser power a brightness of 7.4 x 10(11) photons/(s x sr x μm(2)) was measured for the nitrogen Lyα line emission at 2.478 nm. Using a multilayer condenser mirror with 0.3 % reflectivity 10(6) photons/(μm(2) x s) were obtained in the object plane. Microscopy performed at a laser power of 60 W resolves 40 nm lines with an exposure time of 60 s. The exposure time can be further reduced to 20 s by the use of new multilayer condenser optics and operating the laser at its full power of 130 W.

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Laboratory water window cryomicroscopy has recently demonstrated similar image quality as synchrotron-based microscopy but still with much longer exposure times, prohibiting the spread to a wider scientific community. Here we demonstrate high-resolution laboratory water window imaging of cryofrozen cells with 10 s range exposure times. The major improvement is the operation of a λ=2.48 nm, 2 kHz liquid nitrogen jet laser plasma source with high spatial and temporal stability at high average brightness >1.5×10(12) ph/(s×sr×μm(2)×line), i.e., close to that of early synchrotrons. Thus, this source enables not only biological x-ray microscopy in the home laboratory but potentially other applications previously only accessible at synchrotron facilities.

Tomographic reconstruction in soft x-ray microscopy is a powerful technique for obtaining high-resolution 3D images of biological samples. However, the depth of focus of such zone-plate-based microscopes is typically shorter than the thickness of many relevant biological objects, challenging the validity of the projection assumption used in conventional reconstruction algorithms. In order to make full use of the soft x-ray microscopes' high resolution, the tomographic reconstruction needs to take the depth of focus into account. Here we present a method to achieve high resolution in the full sample when the depth of focus is short compared to the sample thickness. The method relies on the back-projection of focus-stacked image data from x-ray microscopy. We demonstrate the method on theoretical and experimental data.

Microscopic jets of cryogenic substances such as liquid nitrogen are important regenerative high-density targets for high-repetition rate, high-brightness laser-plasma soft x-ray sources. When operated in vacuum such liquid jets exhibit several non-classical instabilities that negatively influence the x-ray source's spatial and temporal stability, yield, and brightness, parameters that all are important for applications such as water-window microscopy. In the present paper, we investigate liquid-nitrogen jets with a flash-illumination imaging system that allows for a quantitative stability analysis with high spatial and temporal resolution. Direct and indirect consequences of evaporation are identified as the key reasons for the observed instabilities. Operating the jets in an approximately 100 mbar ambient atmosphere counteracts the effects of evaporation and produces highly stable liquid nitrogen jets. For operation in vacuum, which is necessary for the laser plasmas, we improve the stability by introducing an external radiative heating element. The method significantly extends the distance from the nozzle that can be used for liquid-jet laser plasmas, which is of importance for high-average-power applications. Finally, we show that laser-plasma operation with the heating-element-stabilized jet shows improved short-term and long-term temporal stability in its water-window x-ray emission.