The soft X-ray and XUV split-and-delay unit at beamlines FL23/24 at FLASH2

Abstract

A split-and-delay unit for the extreme ultraviolet and soft X-ray spectral regions has been built which enables time-resolved experiments at beamlines FL23 and FL24 at the Free-electron LASer in Hamburg (FLASH). Geometric wavefront splitting at a sharp edge of a beam splitting mirror is applied to split the incoming soft X-ray pulse into two beams. Ni and Pt coatings at grazing incidence angles have been chosen in order to cover the whole spectral range of FLASH2 and beyond, up to hν = 1800 eV. In the variable beam path with a grazing incidence angle of ϑd = 1.8°, the total transmission (T) ranges are of the order of 0.48 < T < 0.84 for hν < 100 eV and T > 0.50 for 100 eV < hν < 650 eV with the Ni coating, and T > 0.06 for hν < 1800 eV for the Pt coating. For a fixed beam path with a grazing incidence angle of ϑf = 1.3°, a transmission of T > 0.61 with the Ni coating and T > 0.23 with a Pt coating is achieved. Soft X-ray pump/soft X-ray probe experiments are possible within a delay range of −5 ps < Δt < +18 ps with a nominal time resolution of t r = 66 as and a measured timing jitter of t j = 121 ± 2 as. First experiments with the split-and-delay unit determined the averaged coherence time of FLASH2 to be τc = 1.75 fs at λ = 8 nm, measured at a purposely reduced coherence of the free-electron laser.

Enabling time-resolved experiments with ultrashort FEL pulses via the split-and-delay unit for FLASH2

“…Optical set-up of the SDU. 19 The Ni coated mirrors Ni1-Ni4 (orange) form the fixed delay path whereas the mirrors BS, DL1, DL2 and RC (gray) form the variable delay path. Another beam path is represented by the Pt coated mirrors Pt1-Pt4 (light blue).…”

“…For the SDU at the FL24 only a short summary is presented here, a more extensive presentation of the SDU is given in ref. 19. The basic principle is shown in figure 1.…”

“…[14][15][16][17][18] These instruments offer the opportunity to split a given pulse into two separated partial pulses with a high temporal accuracy. This paper presents the capability of the SDU at FLASH2 which serves the beamlines FL23 and FL24 19 in terms of time-resolved experiments with ultrashort FEL pulses. The temporal resolution achieved is shown by linear autocorrelation measurements and demonstrates the capability of sub-femtosecond accuracy.…”