We present the temporal characteristics of the split-and-delay unit at FLASH2 via visibility measurements which characterize the temporal resolution of the combined system of the FEL and the split-and-delay unit. The use of the split-and-delay unit at FLASH2 allows the users at the beamlines FL23 and FL24 at DESY to perform such pump-probe experiments. By using wavefront beam splitting, grazing incidence mirrors, and two different coatings the whole spectral region of FLASH2 is covered, and even harmonics up to 1800 eV are transmitted with a transmission of T>0.06. It is concluded that user experiments with a pump-probe scheme from the picosecond regime down into the sub-femtosecond region can be carried out.
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 ≤ ▵τ ≤ +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 ϑvariable = 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 ϑfixed = 1.3° a transmission of T > 60 % with a Ni coating and T > 28 % with a Pt coating is possible.
For the High Energy Density Instrument (HED) at the European XFEL a hard x-ray split-and-delay unit (SDU) is built covering photon energies in the range between 5 keV and 24 keV. This SDU enables time-resolved x-ray pump / x-ray probe experiments as well as sequential diffractive imaging on a femtosecond to picosecond time scale. The set-up is based on wavefront splitting that has successfully been implemented at an autocorrelator at FLASH. The x-ray FEL pulses will be split by a sharp edge of a silicon mirror coated with Mo/B4C and W/B4C multilayers. Both partial beams then pass variable delay lines. For different photon energies the angle of incidence onto the multilayer mirrors is adjusted in order to match the Bragg condition. Hence, maximum delays between +/- 1 ps at 24 keV and up to +/- 23 ps at 5 keV will be possible. Time-dependent wave-optics simulations are performed with Synchrotron Radiation Workshop (SRW) software. The XFEL radiation is simulated using the output of the time-dependent SASE code FAST. For the simulations diffraction on the edge of the beam-splitter as well as height and slope errors of all eight mirror surfaces are taken into account. The impact of these effects on the ability to focus the beam by means of compound refractive lenses (CRL) is analyzed.
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.
For the High Energy Density (HED) experiment [1] at the European XFEL [2] an x-ray split- and delay-unit (SDU) is
built covering photon energies from 5 keV up to 20 keV [3]. This SDU will enable time-resolved x-ray pump / x-ray
probe experiments [4,5] as well as sequential diffractive imaging [6] on a femtosecond to picosecond time scale.
Further, direct measurements of the temporal coherence properties will be possible by making use of a linear
autocorrelation [7,8]. The set-up is based on geometric wavefront beam splitting, which has successfully been
implemented at an autocorrelator at FLASH [9]. The x-ray FEL pulses are split by a sharp edge of a silicon mirror
coated with multilayers. Both partial beams will then pass variable delay lines. For different photon energies the angle
of incidence onto the multilayer mirrors will be adjusted in order to match the Bragg condition. For a photon energy of
hν = 20 keV a grazing angle of θ = 0.57° has to be set, which results in a footprint of the beam (6σ) on the mirror of
l = 98 mm. At this photon energy the reflectance of a Mo/B4C multi layer coating with a multilayer period of d = 3.2 nm
and N = 200 layers amounts to R = 0.92. In order to enhance the maximum transmission for photon energies of hν = 8
keV and below, a Ni/B4C multilayer coating can be applied beside the Mo/B4C coating for this spectral region. Because
of the different incidence angles, the path lengths of the beams will differ as a function of wavelength. Hence, maximum
delays between +/- 2.5 ps at hν = 20 keV and up to +/- 23 ps at hν = 5 keV will be possible.
For the European XFEL [1] an x-ray split- and delay-unit (SDU) is built covering photon energies from 5 keV up to 20 keV [2]. This SDU will enable time-resolved x-ray pump / x-ray probe experiments as well as sequential diffractive imaging [3] on a femtosecond to picosecond time scale. Further, direct measurements of the temporal coherence properties will be possible by making use of a linear autocorrelation. The set-up is based on geometric wavefront beam splitting, which has successfully been implemented at an autocorrelator at FLASH [4]. The x-ray FEL pulses will be split by a sharp edge of a silicon mirror coated with Mo/B4C multi layers. Both partial beams will then pass variable delay lines. For different wavelengths the angle of incidence onto the multilayer mirrors will be adjusted in order to match the Bragg condition. For a photon energy of hν = 20 keV a grazing angle of θ = 0.57° has to be set, which results in a footprint of the beam (6σ) on the mirror of l = 120 mm. At this photon energy the reflectance of a Mo/B4C multi layer coating with a multi layer period of d = 3.2 nm and N = 200 layers amounts to R = 0.92. In order to enhance the maximum transmission for photon energies of hν = 8 keV and below, a Ni/B4C multilayer coating can be applied beside the Mo/B4C coating for this spectral region. Because of the different incidence angles, the path lengths of the beams will differ as a function of wavelength. Hence, maximum delays between +/- 2.5 ps at hν 20 keV and up to +/- 23 ps at hν = 5 keV will be possible.
For the European XFEL [1] an x-ray split- and delay-unit (SDU) is built covering photon energies from 5 keV up to 20 keV. This SDU will enable time-resolved x-ray pump / x-ray probe experiments as well as sequential diffractive imaging [2] on a femtosecond to picosecond time scale. Further, direct measurements of the temporal coherence properties will be possible by making use of a linear autocorrelation. The set-up is based on geometric wavefront beam splitting, which has successfully been implemented at an autocorrelator at FLASH [3]. The x-ray FEL pulses will be split by a sharp edge of a silicon mirror coated with Mo/B4C multi layers. Both partial beams will then pass variable delay lines. For different wavelengths the angle of incidence onto the multilayer mirrors will be adjusted in order to match the Bragg condition. For a photon energy of hν = 20 keV a grazing angle of θ = 0.57° has to be set, which results in a footprint of the beam (6σ) on the mirror of l = 345 mm. At this photon energy the reflectance of a Mo/B4C multi layer coating with a multi layer period of d = 3 nm and N = 200 layers amounts to R = 0.92. For a photon energy of hν = 5 keV a smaller size of the footprint of l = 244 mm is calculated due to the steeper grazing angle of θ = 2.28°. In order to enhance the maximum transmission for photon energies of hν = 8 keV and below, a Ni/B4C multilayer coating can be applied beside the Mo/B4C coating for this spectral region. Because of the different incidence angles, the path lengths of the beams will differ as a function of wavelength. Hence, maximum delays between +/- 3.7 ps at hν = 20 keV and up to +/- 44 ps at hν = 5 keV will be possible.
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