Instrument design

Information on overall design and individual components—for user conditions see here

  The FXE instrumentation is installed in one single hutch in the experiment hall with a 4-bounce Si crystal monochromator mounted near the end of the upstream tunnel section. Also within this section of the tunnel is a diamond grating mount for delivering three beams: the main beam for the experiment itself, next to two beams (minus and plus first order) that are used for the single shot spectrum analyzer and femtosecond timing tool. Inside the optical section of the FXE experiment hutch are two slits, an attenuator assembly, three beam imaging units, and a CRL focusing device. The experimental section hosts the He sample environment, two independently operated secondary spectrometers, and—in the forward direction—the 1 Mpx Large Pixel Detector (LPD). In addition, an FXE Compound Refractive Lens (CRL) unit is used to collimate the X-ray beam for all energies in the 5–20 keV range. An outline of the entire FXE instrument with its main instrumentation is shown in the figure below.

The X-ray beam energy will be tunable in a 5–20 keV energy range with up to more than 1012 photons/pulse. The beamline optics allows transport of up to about 1000 pulses/train, and with 10 trains/s, more than 1016 photons/s can be expected in the primary beam. The full SASE bandwidth of about 0.1% can be used in the experiments or monochromotized with a 4-bounce Si(111) monochromator near the end of the tunnel system (not shown here), which can be inserted or retracted without changing the X-ray beam path.

Focusing optics are necessary to ensure that as many photons as possible are available for the experiments. This is realized by two CRL units (one in the tunnel, and one inside the FXE hutch). Through use of different lens combinations, the beam size can be varied according to requirements.

A powerful optical pump laser system will be available at FXE and operating with the same time structure as the X-rays. This may be used to tickle, excite, or heat the sample prior to the arrival of the X-ray probe pulse. In addition, a 30 W MHz laser system, also synchronized to the X-ray pulse structure, can be utilized for experiments.

The interaction region, where the beams meet the sample, is situated in air, and can be filled entirely with He atmosphere. A liquid jet injector is available (and an xy-scanner for 10 Hz positioning of solid state samples is under development). Setups to adjust the temperature of the liquid sample are available. A sample stack with one (phi) circle and two cradles (omega and chi) allows adjusting the sample for diffraction experiments on solid state samples in grazing incidence reflection or transmission geometry. Full capability for mapping the diffracted light using a detector mounted on the robot arm is under development. A unit downstream from the sample will feature beam diagnostics (spectrum, intensity and position), which can be connected via the LPD to the sample to allow He atmosphere throughout.

The LPD is a 1M pixel detector with the capability of resolving single pulses inside a train (4.5 MHz). The pixel size is 500 µm and the detector can be located up to 8 m downstream from the sample. It can also be placed as close as 10 cm (or less) to the sample, since the sample environment can be adapted. In forward scattering geometry a central hole in the detector allows the direct beam to pass without any damage to the sensor onto and through the post-sample diagnostic table, in He atmosphere. The beam is finally blocked and fully absorbed by a Cu beam stop behind a B4C beam stop.

In the following, we describe the implemented components for the FXE instrument. Two key components are placed near the experimental hall at the end of the photon transport tunnel, while, closer to the undulator exit (213 m downstream), we operate a compound refractive lens assembly for collimation of the exiting XFEL radiation.

FXE SASE1 tunnel components

Compound refractive lens (CRL) assembly

Up to 10 removable single Be lenses can be inserted into the beam, enabling to collimate the X-ray beam for the remaining 700 m into the experiment hall, for all energies in the 5–20 keV range. Water cooling of the lens units will ensure temperature stability.

Lens stack with 10 units that can be inserted into the beam path. All units are water cooled. FDR, “Beryllium Compound Refractive Lens System for European XFEL Instruments”, JJ X-ray

Si(111) 4-bounce monochromator

A Si(111) mono in 4-bounce geometry and fixed exit allows keeping both the monochromatic and the pink beam (when retracting the mono crystals) paths identical. This is convenient for pump-probe experiments with an external laser beam in the FXE experiment hutch, so that both beams always congruently overlap. The Darwin width of this monochromator is dE/E < 1.5x10-4 for photon energies > 5 keV.

FXE experiment hutch components

The beam height inside the hutch is 1410 +/- 10 mm. The main components are described below.

Dispersive single-shot spectrometer

The spectral intensity is determined from the diffracted X-rays when the beam propagates through a thin diamond single crystal, obtained by CVD. The diffracted X-rays are recorded using a Gotthard detector (specifications see below) on a movable 2Theta arm (in vacuum). The diamond crystal is water cooled to prevent thermal drift of the spectra.

Single-shot spectrometer FDR, “Spectrum analyzer” by JJ X-ray

Focusing lenses (CRL)

This is the main X-ray focusing scheme at FXE. The hardware is identical to the CRL inside the tunnel; however, it is equipped with lenses of higher refractive power and enables focussing the X-rays in front of the sample position. Using the lenses increases the beam divergences to a maximum of 200 µrad. Check back for a listing of focusing options.

Variable attenuator

The attenuator allows inserting solid foils into the X-ray beam, for the purpose of alignment or measurements on sensitive samples.

Von Hamos Spectrometer

The energy-dispersive spectrometer has 16 elements, which can be equipped with suitable crystals tailored to image the entire Kα-Kβ-VTC spectrum of any selected element in the 5–12 keV range. Smart combinations of different crystals permit recording the XES of more elements simultaneously. Check back regularly for an updated crystal selection list.


Crystals for the von Hamos spectrometer


Material (cut)

No. of crystals


Suitable for energy range
(Kα-Kβ-VTC of elements)




?? (Fe, Co, Ni)
Mn Kβ (84°)




Pt Lα (80°)
Fe Kα (75.5°)
Mn Kβ (72.8°)
Zn Kβ (76.3°)
Au Lα (73.2°)
Zr Kα (79.2°)


2 + 3


Cu Kβ (79.9°)
Ir Lα (72.8°)
U Lα (75°)




Co Kβ (83°)
Eu Lα (76.9°)
Zn Kβ (82.5°)
Mn Kα (74.8°)
Au Lα (77.7°)
Cr Kβ (73.2°)
U Lα (77.4°)
Eu Lα (76.9°)
Zr Kα/Kβ (74.2°/75.2°)
Hg Lα (71.8°)
Cu Kα (70.6°)




Fe Kβ (66.7°)
Co Kα (69.3°)

Johann spectrometer

Up to 5 spherically bent crystals can be mounted, with curvatures of R = 1 m, 2 m, 3 m possible. Check back regularly for an updated crystal list.


Crystals for the Johann spectrometer


Material (cut)

No. of crystals available


Suitable for energy range (Kα-Kβ-VTC of elements)

Solid angle covered per crystal




1 m

Fe Kβ (66.7°)
Co Kα (69.3°)




Robot arm

A robot arm can be used to hold and translate samples, as well as light detectors and analyzer crystals. Equipped with 6 degrees of freedom, any trajectory as well as orientation can be followed within the travel limits. The nominal load of the arm is 6 kg with a maximum of 15 kg. Each position can be repetitively set with an accuracy of +/- 35 µm. The arm is mounted on a plate that allows translation in the horizontal (x) direction, perpendicular to the beam propagation.

Robot tower (left) and arm (right) FDR, “Detector Robot and Robot Tower”, JJ X-ray

LPD (Large Pixel Detector)

The LPD has been developed by the Rutherford Appleton Laboratory in the UK for European XFEL. The detector sensitive area is square-shaped with 1024x1024 pixels of 500 µm size and consists of 256 exchangeable rectangular tiles (128x32 pixels). The area is divided into four quadrants that can be moved independently in the detector plane to form an adjustable central hole to let the direct beam through. A 500-µm thick Si sensor enables quantum efficiency over 80% in the 1–13 keV photon energy range and ca. 40% detection efficiency at 20 keV. The LPD has a high dynamic range of up to 105 photons at 12 keV, while providing single photon sensitivity in the lowest amplifier gain setting. The detector electronics supports 3 parallel gain stages (1x, 10x, 100x) and with frame rates of 4.5 MHz up to 512 full frames can be stored into the detector memory, which are transferred every 100 ms to the storage place. It thus operates in the burst mode of the European XFEL pulse train pattern. A fast frame “veto” capability is added to discard poor images based on the online diagnostics, which helps to improve the overall quality of image sets stored within the burst on the detector.

FXE sample environment

Liquid flat sheet (sample delivery)

Currently available are sapphire nozzles for 0.1, 0.2, 0.3, and 0.5 mm jet thicknesses. These operate at speeds up to about 5 m/s and flow rates of up to 500 mL/min. The set of pump and nozzle are suitable for fairly aggressive liquids. Please contact the instrument scientists to enquire about special solvents, other than water, acetonitrile, and certain alcohols. The sample container can be heated or cooled to adjust the temperature of the solvent to better than 0.2 K.

A high-speed thin flat sheet jet (10–70 m/s) is available with adjustable thicknesses in the 1–100 µm thickness range. This jet is presently limited to the delivery of water-based samples under ambient conditions.

Sample stage and environment

The mounting plane (breadboard) is located 230 mm below the x-ray beam. A set of rotation and translation stages, described in more detail below, enables sample alignment and scanning. The beam direction is positive z (y vertical).

Sample mounting stage (SMS) FDR ver. 1.2, “Sample Mounting Stage”, JJ X-ray

Axis No.






+/-20 mm

1 µm/step











-25/+75 mm

0.25 µm/step















+/-50 mm

1 µm/step



+/-50 mm

1 µm/step



+/-10 mm

0.091 µm/step

A robot arm may also be considered for mounting and scanning of samples. Contact the FXE group for possibilities.

The breadboard on the SMS can be used to install protective gas sample environments, sample scanners, and other devices.

Example Data File

Please find an example data file, conatining some "slow data" and LPD test data here.