Beamline FL 24

Figure 1

Figure 2

FL24 has an open port for user supplied end stations. FL24 covers the entire wavelength range of FLASH2 (see FLASH parameters). It is equipped with a KB bending mirror system for focussing in spring 2017. A grazing incidence XUV and soft X-ray split-and-delay unit (SDU), described below, has been designed by H. Zacharias et al., Westfälische Wilhelms-Universität Münster and is expected to be operational in the FL24 beamline in spring 2018.

Details for connection of user-supplied endstations:
FL24 KB:
Beam is horizontal
Beam height above ground: ~1704 mm (due to KB system according to
drawing), a 20-30 mm discrepancy from the drawing height should be taken
into account due to the floor uneveness
Beamline end valve: DN63 (for co-linear pump-probe laser beam in-coupling)

FL24 unfocussed:
Beam is horizontal
Beam height above ground: ~1600 mm
Beamline end valve: DN40

The split-and-delay unit (SDU) will serve the beamline FL24 and and the future beamline FL23 at FLASH2. The basic concept relies on wavefront beam splitting and is adapted from the SDU installed at BL2 at FLASH1 [1]. It will allow jitter-free time-resolved experiments in the XUV and soft X-ray-regime.
A sketch of its basic set-up is shown in figure 1. The wavefront of the incoming beam is split by the beam-splitting mirror (BS) in two parts: The upper part of the beam is reflected by the mirror downwards to the variable delay line, whereas the lower part of the beam propagates right below the mirror through the fixed beam path. There it is reflected at a grazing angle of ϴ = 1.3° over a length of l = 5.6 m by mirrors M1-M4. The beam in the lower, variable delay line is reflected at a grazing angle of ϴ = 1.8° to a set of two mirrors (DL1, DL2). These mirrors are moveable along the propagation path. The position of these mirrors set the delay between the two beams. Afterwards, both parts of the beam are recombined at the last mirror (RC) and directed to the experiment.
To cover the whole spectral range of FLASH2, including the harmonics, Ni- and Pt- coated mirrors can be selected by moving the whole optical system horizontally. Figure 2 shows the overall transmission of the SDU from 30 eV to 1500 eV for the respective beam paths (Ni- and Pt-mirror, four reflections, ϴ = 1.3° for the fixed path and ϴ = 1.8° for the delay line). The SDU will allow to choose a delay of -6 ps < Δt < +18 ps. A sub-fs resolution and a sub-fs jitter is expected. The splitting ratio between the pump and probe will be adjustable.
To support higher photon energies with the SDU, e.g. after an upgrade of FLASH, a change of the grazing angle from ϴ = 1.8° to ϴ = 1.3° in the variable delay line has been foreseen. In this set-up, the maximum delay will be reduced to -10 ps < Δt < 2.5 ps.
A filter stage with exchangeable filters will allow two colour pump-probe experiments. Combined with a beam monitor it will be possible to measure the pulse energy of the separated pulses individually. (M Rollnik, S. Roling, F. Wahlert, and H. Zacharias, Westfälische Wilhelms-Universität Münster)

[1] M. Wöstmann et al., The XUV split-and-delay unit at beamline BL2 at FLASH, J. Phys. B: At. Mol. Opt. Phys. 46, 164005 (2013).

Contact: Matthias Rollnik, Universität Münster,