# An Easy Technique for Focus Characterization and Optimization of XUV and Soft X-ray Pulses
## 3 Results and Discussion
Due to this short acquisition time, the images from the CMOS camera are ideal as feedback for the alignment of the ellipsoidal mirror. Furthermore, the camera detects the 2D image of the focus without any projection. Hence, cylindrical symmetry does not need to be assumed, and the errors that are caused by this assumption are fully avoided. For example, for the focus image of Figure 4b, the projection along the \(y\)-axis bid the elongation of the focus in the downwards direction, whereas the projection along the x direction indicated a significantly larger focus than it actually was. All following measurements were made with a fully open iris. For the focal measurements presented in this article, the ellipsoidal mirror was aligned in a way that the peak intensity in the images was maximized for the focal plane. The short wavelength and the short focal length made the alignment of the ellipsoidal mirror very sensitive. This is illustrated in Figure 5. Figure 5a,b shows the XUV beam profile at the focal plane with a misalignment of \(\pm\)0.04\({}^{\circ}\) of the azimuth angle around the vertical axis (see Figure 2). As illustrated, this small misalignment led to approximately a factor of 2 reduction in peak intensity. The alignment of the grazing incidence angle was even more sensitive. A misalignment of \(\pm\)0.015\({}^{\circ}\) led to a decrease of more than a factor of 2 in peak intensity at the focal plane. However, it has to be mentioned that the plane with the highest intensity changed for different alignments of the ellipsoidal mirror. This is especially valid for the grazing incidence angle. This entanglement of various alignment parameters means that multiple dimensions have to be scanned repeatedly during the optimization procedure [13]. Therefore, the fast characterization of the XUV focus with the CMOS camera is essential for the alignment. Figure 4: Profiles of the XUV beam in Zr window in and around the focus position with fully open iris. (**a**) XUV beam profile 1 mm in front of the focus. (**b**) XUV beam profile at the focal plane. (**c**) XUV beam profile 1 mm behind the focus. A total of 300 single-shot images were averaged for each figure. The alignment of the ellipsoidal mirror was performed in a way that the peak intensity at the focal plane was maximized. The black lines indicate the horizontal and vertical projections of the corresponding 2D image. The x and y FWHM spot sizes of the projections in the focus were \(7\times 26\)\(\upmu\)m.  
To evaluate the performance of the CMOS camera for different spectral regions within the XUV and soft X-ray, the focus of the ellipsoidal mirror was also observed and optimized for the Pd window and the Al window. It is known that the wavefront of high harmonics can slightly change between the harmonic orders [23]. This was also observed in this experiment, as the optimal alignment of the ellipsoidal mirror was slightly different for Zr and Al windows. The optimized focus for the Pd window is shown in Figure 6a. The shape of this focus was very similar to the one of the Zr window. Furthermore, its peak intensity as well as  
Figure 5: Profiles of the XUV beam at the focus position with different alignment of the ellipsoidal mirror. The rotation of the azimuth angle around the vertical axis was misaligned by **(a)** 0.04\({}^{\circ}\) and **(b)** \(-\)0.04\({}^{\circ}\). **(c)** The ellipsoidal mirror was aligned to achieve maximum peak intensity. The grazing incidence angle was misaligned by **(d)** 0.015\({}^{\circ}\) and **(e)** \(-\)0.015\({}^{\circ}\) with respect to the optimum. A total of 300 single-shot images were averaged for each figure. Figure 6: Measurement of the XUV focus at different photon energy ranges. **(a)** Pd window (\(\approx\)95–150 eV); **(b)** Al window (\(\approx\)18–73 eV).