A soft x-ray split-and-delay unit for FLASH II

Roling S, Rollnik M, Kuhlmann M, Plönjes E, Wahlert F, Zacharias H

Abstract

For the soft x-ray free-electron laser FLASH II at DESY in Hamburg a new split-and-delay unit (SDU) is built for photon energies in the range of 30 eV < hν < 1500 eV with an option to expand this range to hν = 2500 eV. The SDU is based on wavefront beam splitting at grazing incidence angles. A three dimensional set-up allows for the use of two different beam paths. With grazing angles of θ = 1.3° in the fixed beam paths and θ = 1.8° in the variable beam path a good compromise between a sufficient reflectance (shallow angles) and a large possible maximum delay (steeper angles) has been chosen. The maximum possible delay is -6 ps < Δt < 18 ps. For photon energies in the range of 30 eV < hν < 800 eV the mirrors are coated with Ni providing a total transmission between T = 57 % at hν = 30 eV and still T >; 30 % at hν = 800 eV. For photon energies up to hν = 1800 eV a different beam path with platinum coated mirrors is used enabling a total transmission in the fixed beam path of T > 29 % at hν = 800 eV and T = 24 % at hν = 1800 eV, respectively. In the variable beam path the total transmission in this photon energy range is considerably lower but still sufficient with T = 13 % at hν = 800 eV and T > 6 % at hν = 1800 eV.