# An Easy Technique for Focus Characterization and Optimization of XUV and Soft X-ray Pulses
## 1 Introduction
The photon energy of this fluorescence is typically in the visible spectral region and therefore can be imaged with a microscope objective onto a common CCD or complementary-metal-oxide semiconductor (CMOS) camera. Typically, a resolution on the sub-10 \(\upmu\)m scale is achieved, a single-shot operation with a reduced signal-to-noise ratio is possible, and the fluorescence materials are rather thin (100 \(\upmu\)m to few millimeters). Furthermore, a high conversion efficiency and efficient collection of the secondary photons is needed in order to enable a good signal-to-noise ratio. Wavefront sensing is another common option for the characterization and optimization of XUV and X-ray foci [12]. For this technique, the profile and wavefront of the XUV beam is measured with a 2D pinhole array and a CCD outside of the focal plane, and the focus is reconstructed via back-propagating algorithms. This method enables single-shot measurements, which is important for the focus optimization and the determination of the shot-to-shot stability. Furthermore, it is possible to characterize very small foci because the measurement is performed outside of the focus, where the beam diameter is large [23]. The necessary hardware for the measurement of the XUV wavefront is commercially available, but this technique requires a very sensitive XUV camera, which is typically expensive. The reason for this is the large beam diameter at the camera position, which leads to a low fluence. Furthermore, these kinds of sensors are very sensitive to internal alignment errors and often have to be calibrated via the use of a pinhole as spatial filter that blocks most of the XUV signal [23]. Moreover, this method provides an indirect characterization of the focus and, hence, is very sensitive to measurement errors. If, for example, the wavefront is not measured with high enough resolution or accuracy, this will lead to a false and typically smaller measured focus size. Placing a sensitive XUV or X-ray CCD camera in the focus of the XUV beam would allow for a direct characterization of the XUV focus, but this is typically not a viable option, because these cameras are mounted on a vacuum flange and cannot be placed easily into the focal plane. Furthermore, their pixel size is too large (e.g., 13.5 \(\upmu\)m or 26 \(\upmu\)m for Andor, iKon, and Newton) to enable high enough spatial resolution to characterize XUV foci on the few micrometer scale directly. In this article, we describe the measurement and optimization of the XUV focus of an ellipsoidal mirror by the use of a slightly modified low-budget CMOS camera, which is commonly used for near infrared or visible light measurements. We discuss the resolution, acquisition speed, and signal-to-background capabilities at different wavelength regions from 18-150 eV, as well as the challenges that have to be overcome to apply this technique. With this method, the 2D fluence distribution of an XUV and soft X-ray focus is acquired within a single light pulse of the source. Hence, the characterization of an XUV and soft X-ray focus becomes almost as simple as for common laser beams and much easier than the traditional methods.