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. 2023 Jul 1;30(Pt 4):662-670.
doi: 10.1107/S1600577523003090. Epub 2023 May 10.

Harmonic radiation contribution and X-ray transmission at the Small Quantum Systems instrument of European XFEL

Thomas M Baumann  1 , Rebecca Boll  1 , Alberto De Fanis  1 , Patrik Grychtol  1 , Markus Ilchen  1 , Ulf Fini Jastrow  2 , Masahiro Kato  3 , Christoph Lechner  1 , Theophilos Maltezopoulos  1 , Tommaso Mazza  1 , Jacobo Montaño  1 , Valerija Music  1 , Yevheniy Ovcharenko  1 , Nils Rennhack  1 , Daniel E Rivas  1 , Norio Saito  3 , Philipp Schmidt  1 , Svitozar Serkez  1 , Andrey Sorokin  2 , Sergey Usenko  1 , Jiawei Yan  1 , Gianluca Geloni  1 , Takahiro Tanaka  3 , Kai Tiedtke  2 , Michael Meyer  1
Affiliations

Harmonic radiation contribution and X-ray transmission at the Small Quantum Systems instrument of European XFEL

Thomas M Baumann et al. J Synchrotron Radiat. .

Abstract

Transmission measurements of the soft X-ray beamline to the Small Quantum Systems (SQS) scientific instrument at the SASE3 undulator of European XFEL are presented. Measurements are reported for a wide range of photon energies (650 eV to 2400 eV), using X-ray gas monitors as well as a bolometric radiometer. The results are in good agreement with simulations for the beam transport and show a transmission of up to 80% over the whole photon energy range. The contribution of second- and third-harmonic radiation of the soft X-ray undulator is determined at selected photon energies by performing transmission measurements using a gas absorber to provide variable attenuation of the incoming photon flux. A comparison of the results with semi-analytic calculations for the generation of free-electron laser pulses in the SASE3 undulator reveals an influence of apertures along the beam transport on the exact harmonic content to be accounted for at the experiment. The second-harmonic content is measured to be in the range of 0.1% to 0.3%, while the third-harmonic contributed a few percent to the SASE3 emission. For experiments at the SQS instrument, these numbers can be reduced through specific selections of the mirror reflection angles.

Keywords: X-ray beam transport; atomic, molecular and optical science; free-electron lasers; harmonic radiation.

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Figures

Figure 1
Figure 1
Schematic view of the X-ray beam transport from the undulator exit to the SQS instrument, seen from the top. The mirrors M1, M2, M3, M4, as well as the SQS KB (VFM and HFM), are depicted in blue. The slit system (SRA), the gas attenuator (GATT) and the intensity diagnostics (XGM) are shown as well. Downstream of the SQS interaction zone (Focus) an additional intensity monitor (XGMD) and a compact room-temperature bolometric radiometer (CBR) were installed for the measurements presented here. Positions are given as distance from the end of the undulator. The figure is not to scale.
Figure 2
Figure 2
Results of the transmission measurements. (a) The data taken by the CBR and the XGMD relative to the upstream XGM, with the FEL confined by the slit system to 1 mm × 1 mm. Data were taken at two beamline configurations with M1 and M2 at 9 mrad and 20 mrad, respectively. The calculated reflectivity of all involved mirrors combined using a 50 nm-thick B4C layer is represented by the dotted lines, while the dashed lines include a 5 nm-thick layer of carbon contamination. (b) The ratio of the data points and the calculation result for 20 mrad and 9 mrad, including contamination. The dashed lines represent the average ratio beased on the XGMD data.
Figure 3
Figure 3
Results of the transmission measurement with an open SRA. (a) The data taken by the XGMD relative to the upstream XGM using a beamline configuration with M1 and M2 at 9 mrad and an intermediate horizontal focus. The dashed line represents a calculated transmission combining mirror reflectivity including carbon contamination and geometric aperture. The dotted line uses the nominal reflectivity without contamination. (b) The ratio of the data points and the calculation result with their average as a dashed line.
Figure 4
Figure 4
Results of the harmonics measurements. (a) The transmission measured with the CBR relative to the upstream XGM as a function of the gas attenuator pressure for M1 and M2 at an incidence angle of 9 mrad. The dashed lines represent the results of a two-parameter fit. (b) Data for an M1/M2 incidence angle of 20 mrad for 730 eV and 1000 eV.
Figure 5
Figure 5
Normalized transverse beam profiles of the fundamental, second- and third-harmonic radiation after the last undulator cell, calculated analytically for three different fundamental energies (730 eV, 1000 eV and 1500 eV).
Figure 6
Figure 6
Calculated dependence of the harmonic contribution on a shift of the aperture center from the FEL axis for three different fundamental photon energies (730 eV, 1000 eV and 1500 eV). The aperture has a size of 1 mm × 1 mm and is located 196 m downstream of the undulator. (a) Second-harmonic contribution, horizontal shift; (b) second-harmonic contribution, vertical shift; (c) third-harmonic contribution, horizontal or vertical shift. The shaded areas represent a 2σ range around the measured values.
Figure 7
Figure 7
Contribution of the second- (a) and third-harmonic (b) radiation in dependence of the fundamental photon energy, calculated from the semi-analytical model for the undulator tapering conditions during the harmonics measurements without including apertures.
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