Instrument design

The SPB/SFX instrument is located at an approximate distance of 900 m downstream of the SASE1 undulator of the European XFEL.

The instrument consists of an optics hutch (where the micron KB-focusing mirrors are located) and an experiment hutch equipped with two experiment endstations. Various beam conditioning devices (slits, shutters, attenuators, diagnostics, etc.) are installed in both hutches and some even immediately in front of the optics hutch.

The upstream endstation will be able to utilize the full flux from SASE1, focused by one of the two Kirkpatrick-Baez (KB) mirror systems to either a 1 µm or a 0.1 µm focus into the upstream focal plane. The spent beam of the upstream interaction region could then be utilized to be refocused by compound-refractive lenses (CRLs) to a second focal plane (downstream interaction region). Ultimately, utilizing the same photon beam parameters, two experiments may be run in parallel.

Starting in 2017, only the upstream interaction region will be installed and commissioned. The downstream interaction region will follow approximately a year later.

General layout of the SPB/SFX instrument.

X-ray source

The SASE1 undulator will provide X-ray pulse trains at a repetition rate of 10 Hz. These pulse trains can be filled with up to 2700 X-ray pulses corresponding to a maximum intra-train repetition rate of 4.5 MHz. The fundamental X-ray photon energy will be tunable in the range of 3–16 keV (wavelengths from 4–0.8 Å) with single pulse energies up to 4 mJ, resulting in up to 1012 – 1013 photons/pulse (depending on the photon energy) with the full SASE bandwidth of about 10-3. No monochromator is installed.

Time structure of European XFEL X-ray pulses.

X-ray focusing optics

The SPB/SFX instrument is equipped with two KB mirror systems for focusing the X-ray beam into a common focal plane in the upstream interaction region, creating focal sizes of approximately 1 μm and 100 nm (as a function of photon energy, with larger sizes at lower photon energy). Two different coatings for both mirror systems ensure good transmission over the entire primary photon energy of 3–16 keV.

The micron and nano KB systems for the SPB/SFX instrument.

The micron KB mirror system (MKB) consists of two focusing mirrors and two flat mirrors. The latter are used to keep the optical axis of the focused beam parallel to the primary beam. The MKB system is located in the SPB/SFX optics hutch, approximately 25 m upstream of the first interaction region. The nano KB mirror system (NKB) consists of two focusing mirrors at distances of 3 m (horizontal) and 1.9 m (vertical) upstream of the first interaction region in the upstream sample chamber. A custom differential pumping system between the NKB system and the upstream sample chamber maintains ultra-high vacuum in the NKB mirror system when the pressure in the sample chamber can be some orders of magnitude higher due to sample injection utilizing water and aerosol jets.

An interesting feature of the SPB/SFX instrument is that measurements in two interaction regions can potentially be performed at a same time. To achieve this, in certain use cases, the divergent X-ray beam arriving from the upstream interaction region will (downstream of the upstream detector) be refocused into the downstream interaction region with high efficiency, achieving a focal spot of micrometer scale. The key component for refocusing the X-ray beam is a set of beryllium parabolic compound refractive lenses (CRL), which work over a broad photon energy range (>6 keV) with high efficiency. The design of the CRL assembly is shown in the figure below (courtesy of JJ X-Ray).

Optical lasers

The pump-probe (PP) laser system is located in a laser hutch adjacent to the SASE1 instruments, serving both with 800 nm (15 fs pulse duration) and 1030 nm (500 ps, after compression 850 fs) respectively. The pulse energy strongly depends on its repetition rate, since the saturation level of amplifier system is about 1.2 W and 24 W respectively. The repetition rate can be controlled by externally triggered active laser optics ranging from single shot to 4.5 MHz. The basic parameters of the pump-probe laser are shown in the table below.

Typical Pump-probe (PP) laser operation parameter, as delivered by the Optical Lasers group to the SPB/SFX ILH.

Wavelength (nm)

Intra-burst rep. rate (MHz)

Pulse energy (mJ)

Pulse duration

800

4.5

0.05

15–300 fs

800

1

0.2

15–300 fs

800

0.2

1

15–300 fs

800

0.1

2

15–300 fs

1030

4.5

1

850 fs – 500 ps

1030

1

4

850 fs – 500 ps

1030

0.2

20

850 fs – 500 ps

1030

0.1

40

850 fs – 500 ps

The SPB/SFX instrument will operate an instrument laser hutch, served by the aforementioned PP-laser-system. The variety of samples expected to be investigated at SPB/SFX exhibit very different absorption and reflectivity spectra, and an optical laser needs to generate an appropriate wavelength to excite or align such samples. Those wide wavelength regions, which will be demanded by users of SPB/SFX, are covered (ultimately) by an optical parametric amplifier (OPA). Second harmonic generation (SHG), third harmonic generation (THG) and fourth harmonic generation (FHG) will be provided for experiments that require higher fluence than an OPA system can provide. The fundamental wavelength of 800 nm is tunable from 740–840 nm. It is expected that SHG (370–420 nm) will fit to many sample cases for excitation.

Sample delivery

In order to fully exploit the potential of X-ray FEL sources, diffraction experiments are usually performed at single pulse energies far above damage limits while utilizing the ultrashort duration to obtain still images even from exploding samples. This operation mode requires sample delivery concepts to replenish fresh sample for every shot.

At the SPB/SFX instrument, the upstream and downstream sample chambers are equipped with multiple ports to mount different kinds of gas-focused, liquid jet and/or aerosol injectors for fast sample replenishment of biological samples at high repetition rates. However, for experiments at 10 Hz, a fast, fixed-target scanner can be used which holds samples previously prepared and characterized offline. Such solid state sample systems could also be utilized at non-destructive intensities (i.e., multiple images from one single sample).

In case different, special sample delivery equipment for a particular experiment is to be used users are encouraged to cross check with the instrument scientists about how to best incorporate a specific user setup.

Detectors

The SPB/SFX instrument will (ultimately) feature two AGIPD photon detectors, a 1 Mpx variant in the upstream interaction region and a 4 Mpx variant downstream, with a pixel size of 200 x 200 µm2. The AGIPD provides a dynamic range of >104 photons/px at a photon energy of 12 keV, an electronic noise of >350 e- r.m.s., and is capable to collect data at the full intra-train repetition rate of the European XFEL (4.5 MHz) and store up to 352 frames per train.

The two AGIPD detectors at the SPB/SFX instrument have different mechanical designs. The upstream 1 Mpx detector can be placed between distances of 0.13 m and ~6 m distance to the upstream interaction region while the downstream 4 Mpx detector features sample-detector distances between 0.12 m and 0.52 m. For further details about the mechanical design of both detectors, users are encouraged to contact the instrument scientists directly.

Technical Design Report (SPB)