X-ray optics

Two elliptical X-ray mirrors in Kirkpatrick-Baez (KB) configuration are designed to focus the X-ray radiation from the SASE3 undulator down to a sub-micron level. The KB system consists of a pair of 800mm long, actively cooled, and deterministically polished B4C-coated elliptical silicon substrates. Both are equipped with mechanical benders, to compensate different source positions and to achieve focusing in three different focal positions.

It is possible to deliver either the direct, broadband SASE radiation from the undulator, or the monochromatic beam from the exit slit of the SASE3 soft X-ray monochromator to the experiments.

Focus properties

The expected focusing performance of the interim system (purple line) is comparable to the F2 focal position (yellow line) of the final system. Shown are ray-tracing simulations including the contribution from surface polishing errors and diffraction effects. Spot sizes are given in FWHM.

Beamline transmission

The expected overall beamline transmission for both the final and the interim KB systems is very high for almost the entire range of photon energies available at SQS, thus facilitating investigations of nonlinear phenomena. The predicted transmission includes the expected reflectivity of the boron carbide coating of the mirrors, as well as the geometric acceptance given by the finite clear aperture. The latter factor limits the transmission in the low photon energy range; the former cuts the transmission curve at the high photon energy limit.

Soft X-ray monochromator

The monochromator in the SASE3 beamtransport section is located 300 m downstream the undulators. It consists of a pre-mirror that focuses the beam onto the exit slit 100 m further downstream and a plane vls grating. Two pre-mirrors are available, one operating under a fixed incidence angle of 20 mrad, covering the energy range between 250 eV – 1500 eV and the other operating at 9 mrad for 1.5 keV – 3 keV. Currently, a 120 mm long laminar grating is installed as an interim solution. This grating has been manufactured holographically with a central line density of 50 l/cm. A future upgrade with a 500 mm long, mechanically ruled, blazed grating is foreseen.

The figure shows the calculated transmission of the interim grating through a 100 µm wide exit slit (top) and the resolution for both the interim (120 mm) and future (500 mm) gratings.