## 2 Beamline Description  
The source of the PolyX beamline is a 1.31 T bending magnet from the SOLARIS storage ring. The detailed information about the source is given in Table 1 and the scheme of the beamline is provided in Figure 1. The beamline optics are designed to deliver either a polychromatic white beam or a monochromatized beam to the end station. Two types of interchangeable monochromators are planned: a double crystal monochromator (DCM) and a double multilayer monochromator (DMM). The list of all main components of the beamline with corresponding distances from the bending magnet is shown in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Electron beam energy & 1.5 GeV \\ Maximum current & 500 mA \\ Electron beam sizes (\(\sigma_{x}\), \(\sigma_{z}\)) & 44 \(\upmu\)m, 30 \(\upmu\)m \\ Electron beam emittance (\(\epsilon_{x}\), \(\epsilon_{z}\)) & 8.05 nm-rad, 0.065 nm-rad \\ BM magnetic field & 1.31 T \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Bending magnet (X-ray source) parameters used in simulations. Figure 1: The scheme of PolyX beamline. A—white beam slits, B—white beam filters, C—be window 1, D—monochromator, E—be window 2, F—capillary optics, G—sample. A DCM uses two crystals made of Si(111) that are precisely aligned to diffract X-rays of a specific wavelength with a bandwidth of \(\sim\) 1.4 \(\times\) 10\({}^{-4}\) and a constant vertical offset of 10 mm. The dimensions (width \(\times\) length) of the first and the second crystal are 35 mm \(\times\) 55.5 mm and 35 mm \(\times\) 140 mm, respectively. A DMM uses a pair of multilayer mirrors to diffract and reflect X-rays to achieve monochromatization with a bandwidth of several percent. Each mirror has two different stripes of multilayer structures next to each other. The first stripe was prepared to work with the energy range of 4-8 keV. It has a Ni/B\({}_{4}\)C multilayer coating with 75 periods, a period of 4 nm and a metal layer thickness-to-period ratio of 0.4. The second stripe is dedicated to work in the 7.5-15 keV energy range. It has a Mo/B\({}_{4}\)C multilayer coating with 115 periods, a period of 2.6 nm and a metal layer thickness-to-period ratio of 0.4. The dimensions of both stripes were set to 20 mm \(\times\) 280 for the stripes on the first optics, and to 20 \(\times\) 172 mm for the stripes on the second optics. The vertical offset between the second and the first crystal is again set to the fixed value of 10 mm. In order to achieve the focused beam on the sample, two types of capillary optics will be used at PolyX. Seven different polycapillary lenses (from Helmut-Fisher formerly IfG, Germany and XOS, USA) will be interchangeably used to achieve a focal spot in range from 10 to 100 \(\upmu\)m. A narrower beam will be achieved with an achromatic ellipsoidal single-bounce monocapillary lens (AEXML) with a 30 nm Pt coating from Sigray, USA. The details of the selected optical elements are given in Table 3 (the remaining polycapillaries are described in the supplementary materials). The main measurement modes that will be routinely used at PolyX are elemental microimaging by \(\upmu\)-XRF, elemental speciation by \(\upmu\)-XAS and absorption/phase-contrast imaging by \(\upmu\)-CT. Other techniques that are planned to be available at the beamline are confocal-XRF for 3D elemental imaging, high-resolution plenoptic X-ray imaging and \(\upmu\)-XRD for crystalline phase speciation. Several different X-ray detectors will be installed at the end station. The first group--spectrometry detectors--is composed of two silicon drift detectors (SDD) with 80 mm\({}^{2}\) active area (one with 25 \(\upmu\)m Be window and one with ML3.3 Extreme window) by Hitachi High-Tech. As a digital pulse processor, a DANTE