Design of an upgraded XUV and soft X-ray split-and-delay unit for FLASH1

Abstract The split-and-delay unit (SDU) of FLASH1 is upgraded to meet the advances of increasing photon energies of FLASH1 over the years. The SDU enables time-resolved pump/probe experiments at a high temporal resolution in the XUV and soft X-ray regime. With its original design first experiments were performed in 2007 and the SDU was permanently incorporated into the BL2 at FLASH1 in 2010. The upgrade will increase the spectral range of the SDU from former hν = 250 eV to future hν = 750 eV. Two different mirror coatings provide a high transmission in this spectral range. The design that is based on a three-dimensional beam path allows choosing the propagation via two sets of mirrors with these coatings. A carbon coating will allow a total transmission on the order of T > 0.74 for photon energies between hν ≈ 30 eV and hν = 200 eV at a grazing angle of θ = 3.0° in the variable beam path. In the fixed beam path a shallower grazing angle of θ = 2.5° is used, which yields a total transmission of T > 0.79. A Ni coating can be used to cover a range up to hν = 750 eV. This results in a total transmission of T > 0.08 in the variable and T > 0.25 in the fixed beam path. The planned delay range of the new set-up is -1 ps < Δt < +9 ps with a subfemtosecond temporal delay.