Figure 1. Beamline layout
The source of synchrotron light at the DEMETER beamline is an undulator (EPU - Eliptically Polarizing Undulator), located in straight section of the storage ring. It provides soft X-rays in a wide energy range of 100-2000 eV. A fully motorized undulator motion control system allows for full polarization control, providing photon beam at any polarization: linear (in any direction), circular (right- and left-handed) and elliptical.
The first optical element just behind undulator is collimating (toroidal) mirror that minimize the astigmatism of a photon beam and enables focus it on a point.
After toroidal mirror, DEMETER beamline has a variable angle plane grating monochromator. A plane mirror and three diffraction gratings are used to cover the full energy range of 100 to 2000 eV. After the grating chamber, there are two toroidal mirrors which focus the beam onto the exit slit of the PEEM and STXM branch. The resolving power is around 8000 at an exit slit of 10 μm and Cff = 2.25.
The measurements using synchrotron radiation can be performed at one time only at one end station, which is selected by appropriate adjustment of focusing mirrors. Each branch of the beamline has exit slits (vertical and horizontal), which allow changing the beam size and energy resolution. The maximum size of the excited area on the sample is (V x H) 100 x 300 μm2 for PEEM and 300 μm diameter for STXM.
The total photon flux at the sample position was measured using an AXUV photodiode (see Figure 2) and it is in the range of 109–1012 photon/s for a current of 400 mA in the storage ring. The flux values were obtained with an exit slit opening at 40 μm and with the optimal Cff coefficient value for the intensity and energy resolution, for the grating of 336 l/mm (black), 1221 l/mm (red) and 1400 l/mm (blue), respectively.
Figure 2. Top row: Experimental intensity curves of the radiation cone with an exit slit of 40 μm for three diffraction gratings: 336 l/mm (black), 1221 l/mm (red) and 1400 l/mm (blue), respectively. Bottom row: Comparison of flux for three different polarization at all measured harmonics for grating 1221 l/mm. The ring operates in electron current decay mode, typically with a current value in the range of 450-200 mA.
The optimized resolving power depends on the opening of the exit slits and the diffraction grating and can reach a value of 8000 for the nitrogen K-edge.
Figure 3. On the left: The X-ray absorption spectrum for the N2 K-edge for grating 1221 l/mm measured with 10 μm exit slits opening and with fitting components. On the right: the resolving power as a function of the exit slit opening for three diffraction gratings, obtained by fitting the measured spectra.