A XUV and soft X-ray split-and-delay unit for FLASH2

Dreimann M, Roling S, Wahlert F, Eppenhoff S, Kuhlmann M, Toleikis S, Brachmanski M, Treusch R, Plönjes E, Zacharias H

Abstract

A split-and-delay unit for the XUV and soft X-ray spectral range has been installed at beamlines FL23 and FL24 at the FLASH2 Free-Electron Laser at DESY. It enables time-resolved pump-probe experiments covering the whole spectral range of FLASH2 from 30 eV up to 1500 eV. Using wavefront beam splitting and grazing incidence mirrors a sub-fs resolution with a relative pulse delay of -5 ps ≤ Δτ ≤ +18 ps is achieved. Two different mirror coatings are required to cover the complete spectral range and thus, a design that is based on a three dimensional beam path was developed. This allows the choice between different sets of mirrors with either coating for the fixed branch. In the variable branch two coatings are available on the same set of mirrors. A Ni coating allows a total transmission above T > 0.50 for photon energies between hν ≈ 30 eV and 650 eV at a grazing angle of a ϑ = 1.8° in the beam path with variable delay. With a Pt coating a transmission of T > 13 % is possible for photon energies up to hν = 1500 eV. In the fixed beam path at a grazing angle of ϑ = 1.3° a transmission of T > 60 % with a Ni coating and T > 28 % with a Pt coating is possible.