# An Easy Technique for Focus Characterization and Optimization of XUV and Soft X-ray Pulses
## 2 Experimental Implementation  
For these high-harmonic generation (HHG), characterization, and focus optimization experiments the attosecond beamline of the Relativistic Attosecond Physics Laboratory at Umea University was used (see Figure 1). This beamline applies an energy upscaling approach for HHG in gas medium [24; 25]. With this approach, the sub-5-fs \(>\) 10 TW pulses of the laser, the Light Wave Synthesizer 20 (LWS-20), are utilized to generate intense isolated attosecond pulses with a spectrum up to 150 eV [26]. After generation, the fundamental of the laser was suppressed with thin metal foils. These metal foils act also as filters in order to investigate the behavior of the focus as well as the behavior of the detection method at three different photon energy ranges. The following filter combinations were used: 3\(\times\) 150 nm Zr: \(\approx\)65-150 eV; 2\(\times\) 150 nm Zr + 1\(\times\) 100 nm Pd: \(\approx\)95-150 eV; 2\(\times\) 500 nm Al: \(\approx\)18-73 eV [27]. These energy ranges will further be referred to as Zr window, Pd window, and Al window, respectively. The XUV source provides attosecond pulse energies directly at the source of 40 nJ in the Zr window, 20 nJ in the Pd window, and 30 nJ in the Al window, which was determined with an XUV photodiode [15; 28]. The spectrum of the XUV source for the Zr window and the Pd window is shown in Figure 2. In addition to the spectrum and the pulse energy, the profile of the XUV pulses was measured with an XUV CCD camera (Andor, iKon), giving a full width at half maximum (FWHM) beam diameter of \(\approx\)1.5 mm for all three cases. Figure 1: Experimental setup. The pulses from the LWS-20 laser system are focused with an f = 22 m mirror into a neon gas cell to generate high-order harmonics. Different XUV and soft-X-ray spectral regions are selected by thin metallic filters on a filter wheel. An XUV photodiode measures the XUV pulse energy, and a motorized iris sets the beam diameter. Gold mirrors can send the beam to an XUV CCD camera to determine the beam profile and to an XUV spectrometer. The ellipsoidal mirror focuses the XUV beam, and the focus is observed with a CMOS camera. X, Y, Z: define the coordinate system as it is used in the article; \(\tau\): grazing incidence angle of the ellipsoidal mirror; \(\rho\): azimuth rotation angle of the ellipsoidal mirror. There are two additional flat grazing incidence mirrors in front of the filter wheel that are not included in this illustration. After spectral filtering, the XUV pulses were focused with a gold-coated ellipsoidal mirror (Standa) in grazing incidence geometry. The focal lengths of this mirror were 14,500 mm on the incidence side and 125 mm on the exit side, and the grazing incidence angle was 8\({}^{\circ}\). Considering the transmission of the filter combinations (Zr: 8%, Pd: 3%, Al: 4%) and the reflectivity of all mirrors in the XUV beamline (there were two more grazing incidence mirrors in front of the metallic filters, which are not show in Figure 1), the energies on target were 2 nJ, 1 nJ, and 1 nJ for the Zr, Pd, and Al windows, respectively. The main cause of this low transmission was the metal filters, which have reduced transmission due to a thin oxide layer on their surfaces. The transmission of the filters was experimentally determined. New filters would increase the focused energy [27]. In experiments where high intensity is needed, e.g., ionization studies with the attosecond beamline, the use of only one metal filter is enough, which boosts the transmission by almost a factor of 10. However, for the characterization of the XUV focus, which is described in this article, the suppression of the fundamental laser radiation has to be high, and therefore, the filter combinations according to the Zr window, the Pd window, and the Al window are necessary. For the detection and optimization of the XUV focus, a low-budget CMOS camera with a pixel size of 1.85 um was used (Fir, BFS-U3-12054M-CS Blackfly S USB3, Mono).