The soft X-ray monochromator at the SASE3 beamline of the European XFEL: from design to operation

Abstract

The SASE3 soft X-ray beamline at the European XFEL has been designed and built to provide experiments with a pink or monochromatic beam in the photon energy range 250-3000 eV. Here, the focus is monochromatic operation of the SASE3 beamline, and the design and performance of the SASE3 grating monochromator are reported. The unique capability of a free-electron laser source to produce short femtosecond pulses of a high degree of coherence challenges the monochromator design by demanding control of both photon energy and temporal resolution. The aim to transport close to transform-limited pulses poses very high demands on the optics quality, in particular on the grating. The current realization of the SASE3 monochromator is discussed in comparison with optimal design performance. At present, the monochromator operates with two gratings: the low-resolution grating is optimized for time-resolved experiments and allows for moderate resolving power of about 2000-5000 along with pulse stretching of a few to a few tens of femtoseconds RMS, and the high-resolution grating reaches a resolving power of 10 000 at the cost of larger pulse stretching.

Keywords: FEL; beamline; diffraction grating; monochromator; soft X-ray.

Figure 1

Layout of the SASE3 beamline, adapted from the work by Sinn et al. (2012 ▸).

Figure 2

Influence of figure error and VLS miscut on resolution. (a, c) Analytically estimated resolution of the monochromator operating in first diffraction order with designed and installed 50 l mm⁻¹ gratings; slope error on premirror 50 nrad RMS. (b) Instrument response function of the 500 mm-long 50 l mm⁻¹ designed grating operating in first diffraction order at 1.5 keV obtained by wavefront propagation simulations for the cases of different magnitudes of slope error on the grating. (d) Analytically estimated tolerances on the VLS b2 parameter for the 500 mm-long 50 l mm⁻¹ designed grating and for the 120 mm-long 50 l mm⁻¹ grating.

Figure 3

Influence of grating length on resolution and pulse stretching. (a) Dependence of analytically estimated resolution of the 50 l mm⁻¹ grating with 50 nrad RMS slope error, operating in first diffraction order at 500 eV on the grating length. Solid circles – ideal VLS; open circles – VLS b2 = −1.2 × 10⁻⁶ l mm⁻³. (b) Pulse stretching versus open aperture upstream of the 50 l mm⁻¹ grating operating in first diffraction order: solid line – 290 eV; dashed line – 500 eV; dotted line – 800 eV. The stars correspond to 120 mm length of grating illumination.

Figure 4

Time–bandwidth product. (a) Analytically estimated optics transmission in sigma of Gaussian-like beam cross-section; the inserts show the shape of the beam transmitted by the monochromator. (b) Analytically estimated time–bandwidth product in the case of ideal optics with a perfect slope and a perfect VLS law, as well as in the case of the designed grating and installed gratings. Operation in first diffraction order.

Figure 5

Monochromator performance. (a) Analytically estimated resolving power and (b) pulse stretching due to the grating for the 50 l mm⁻¹ 500 mm-long designed grating operating in first diffraction order, for the 50 l mm⁻¹ 120 mm-long installed grating operating in first and second diffraction orders, and for the 150 l mm⁻¹ 120 mm-long installed grating operating in first diffraction order.

Figure 6

Resolution measurements and longitudinal focusing. (a) Transmission spectrum of Ne around the K-edge; circles represent measured transmission spectra; grey lines – modelled transmission; red lines – resulting convolution of modelled transmission with the monochromator IRF. (b) Resolution optimization at the Ne K-edge by aligning the angle of incidence of the LE premirror; operation in first diffraction order.

Figure 7

Transmission. (a) Estimated (lines) and experimentally measured (circles) beamline transmission operating with the 50 l mm⁻¹ 500 mm-long designed grating (20 µm exit slit), with the 50 l mm⁻¹ 120 mm-long installed grating (100 µm exit slit) and with the 150 l mm⁻¹ 120 mm-long installed grating (50 µm exit slit). Solid lines and solid circles – operation with LE premirror; dashed lines and open circles – operation with HE premirror. (b) Instrument response function of respective gratings.