Free-electron laser FLASH


1-June-2022: Electronic quantum dance in molecules
Scientists watch moving charge density in real-time
An international research team led by DESY scientist Tim Laarmann has for the first time been able to monitor the quantum mechanically evolving electron charge distribution in glycine molecules via direct real-time measurement. The results – obtained at DESY´s brilliant free-electron laser FLASH – are published in the scientific journal Science Advances. Better knowledge of the quantum effects in the motion of electrons at the molecular level can pave the way to controlling, optimising, and engineering ionising radiation to be used for example in radiotherapy for cancer treatment.

In the pump-probe experiment the glycine molecule is first ionised by the high intensity X-ray pulse from DESY's free-electron laser FLASH (left). This induces a correlated motion of the valence electrons and holes, depicted by red and blue lobes. After a variable time delay from 0 to 175 femtoseconds the probe pulse samples the state of the glycine ion and electron motion through further ionisation and measurement of the ionisation products (right). In this example, a time delay of 10 femtoseconds is depicted, which shows two extrema of the oscillatory electron/hole motion, i.e. a half period of the electron coherence. Credit: DESY, David Schwickert

Reference: Electronic quantum coherence in glycine molecules probed with ultrashort x-ray pulses in real time; David Schwickert, Marco Ruberti, Přemysl Kolorenč, Sergey Usenko, Andreas Przystawik, Karolin Baev, Ivan Baev, Markus Braune, Lars Bocklage, Marie Kristin Czwalinna, Sascha Deinert, Stefan Düsterer, Andreas Hans, Gregor Hartmann, Christian Haunhorst, Marion Kuhlmann, Steffen Palutke, Ralf Röhlsberger, Juliane Rönsch-Schulenburg, Philipp Schmidt, Sven Toleikis, Jens Viefhaus, Michael Martins, André Knie, Detlef Kip, Vitali Averbukh, Jon P. Marangos, Tim Laarmann; Science Advances, 8, eabn6848 (2022); DOI: 10.1126/sciadv.abn6848

19-May-2022: Mixing laser- and X-ray-beams
Resonant four-wave mixing spectroscopy at FLASH
A team of researchers from Max Born Institute (MBI) in Berlin, and DESY has now observed a new kind of such wave mixing process involving soft X-rays at FLASH. Overlapping ultrashort pulses of soft X-rays and infrared radiation in a single crystal of lithium fluoride (LiF), they see how energy from two infrared photons is transferred to or from the X-ray photon, changing the X-ray “color” in a so-called third-order nonlinear process. Not only do they observe this particular process with X-rays for the first time, they were also able to map out its efficiency when changing the color of the incoming X-rays. It turns out that the mixing signals are only detectable when the process involves an inner-shell electron from a lithium atom being promoted into a state where this electron is tightly bound to the vacancy it left behind – a state known as exciton. Furthermore, comparison with theory shows that an otherwise “optically forbidden” transition of an inner-shell electron contributes to the wave mixing process.

Two light beams from flashlights will not be influenced by each other when they cross. This is different for very intense laser pulses which meet in a suitable “nonlinear material” – here, beams can be deflected and new beams of different color can be created in a process called wave-mixing. The observation of such wave-mixing phenomena allows researchers to draw conclusions about electronic transitions within the nonlinear material which are otherwise invisible. Researchers from MBI and DESY have now observed how an X-ray beam interacts with a laser beam, paving a route to atom-selective studies of ultrafast processes in the future. (Image credit: Anne Riemann, Forschungsverbund Berlin e.V.)

Reference: Probing electron and hole colocalization by resonant four-wave mixing spectroscopy in the extreme ultraviolet, Horst Rottke, Robin Y. Engel, Daniel Schick, Jan O. Schunck, Piter S. Miedema, Martin C. Borchert, Marion Kuhlmann, Nagitha Ekanayake, Siarhei Dziarzhytski, Günter Brenner, Ulrich Eichmann, Clemens von Korff Schmising, Martin Beye, Stefan Eisebitt, Science Advances (2022). DOI: 10.1126/sciadv.abn5127

22-Mar-2022: Both new accelerating module installed at FLASH
Both upgraded acceleratingIng modules have been installed at position ACC2 and ACC3 in the FLASH injector
Well in schedule, the present FLASH shutdown has seen its first highlights: the successful installtion the two new modules. PXM2.1 and PXM3.1 are refurbished XFEL prototype modules with excellent performace in terms of accelerating gradient.

Accelerating module PXM3.1 is beeing installed into the FLASH injector.

Accelerating module PXM2.1 installed into the FLASH injector.

FLASH is being refurbished and upgraded

16-November-2021 to 14-August-2022

Key items:

  • Energy Upgrade
  • - Exchange two superconducting modules with significantly better performance
    - Upgrade waveguides system to optimize RF power distribution
    - Upgrade of related sub-systems
  • Cryogenics
  • - Modifications for 0.5 bar operation and other refurbishments
  • Injector upgrade
  • - Include a Laser Heater into the first bunch compressor
    - New layout of 2nd bunch compressor to allow optics matching and better performance in terms of beam dynamics
  • FLASH2 Afterburner
  • - Insert an Afterburner undulator to produce circular polarized SASE at short wavelengths
  • FLASHForward
  • - Exchange of plasma chamber, exchange extraction dipole
  • - Exchange ORS-section quadrupoles

Note: The whole facility has not been shutdown, parts of the accelerator, injector and seed lasers, experimental halls and other associated labs are still in operation and require the usual resources and attention.

FLASH Shutdown 2021/2022 - Coordination of FLASH machine related installations


FLASH, the world's first XUV and soft X-ray free-electron laser (FEL), is available to the photon science user community for experiments since 2005. Ultra-short X-ray pulses shorter than 30 femtoseconds are produced using the SASE process. SASE is an abbreviation for Self-Amplified Spontaneous Emission. The SASE or FEL radiation has similar properties than optical laser beams: it is transversely coherent and can be focused to tiny spots with an irradiance exceeding 1016 W/cm2.

The FLASH facility operates two SASE beamlines in parallel: FLASH1 and FLASH2. The electron pulses of FLASH come in bursts of several hundred pulses, 10 bursts per second. The pulses of a burst are shared between the two beamlines providing FEL radiation for two experiments at the same time - with the full repetition rate of 10 Hz. A third beamline houses FLASHForward,
a pioneering beam-driven plasma-wakefield experiment.

The aerial view shows the FLASH facility with its two experimental halls "Kai Siegbahn" and "Albert Einstein" (August 2015).

The SASE process is driven by high brightness electron beams. At FLASH1, the wavelength of the X-rays is tuned by choosing the right electron beam energy. The FLASH accelerator provides a range of electron beam energies between 350 MeV and 1.25 GeV covering a wavelength range between 52 and 4 nanometers (nm). See the table below for details.

FLASH2 has new modern undulators with the possibility to change the gap between the magnetic poles of the undulator magnets. This changes the magnetic field and thus the wavelength of the radiation. With a given beam energy, the experiment at FLASH2 is able to change the wavelength in a wide range without influencing FLASH1.

FLASH - Schematic layout of the facility.
FLASH - Schematic layout of the facility.

Schematic layout of FLASH. Not to scale. The two main SASE beamlines FLASH1 and FLASH2 are in operation for user experiments. A third beamline runs the FLASHForward plasma acceleration experiment.
Image: DESY/Siegfried Schreiber

image/png FLASH layout.png (234KB)

FLASH superconducting accelerating modules. Seven modules are installed, each module has a length of 12 m.

FLASH: from the extreme ultra-violet to the water window

The FLASH accelerator is equipped with seven 12 m long TESLA-type superconducting accelerating modules. Each module contains eight 1 meter long superconducting accelerating cavities operated a radio frequency of 1.3 GHz. The cavities are made of solid niobium and are cooled by liquid helium down to 2 K. At this temperature - just 2 degrees Celsius above the absolute zero -, niobium is superconducting so that the acceleration field can be applied with very small losses. This makes a superconducting accelerator very efficient.

Right after the installation of the seventh module in 2010, the FLASH team accelerated an electron beam to an energy of 1.25 GeV. With this energy, soft X-rays with a fundamental wavelength of 4.1 nm could be produced. For the first time FLASH has generated soft X-ray FEL radiation in the so-called water window. So far this was only possible at FLASH with the third and fifth harmonic of the fundamental wavelength, which are by a factor of thousand fainter.

The water window is a wavelength region between 2.3 and 4.4 nanometers. In the water window, water is transparent to light, i.e. it does not absorb FEL light. This opens up the possibility to investigate samples in an aqueous solution. This plays an important role especially for biological samples, because carbon atoms in these samples are highly opaque to the X-ray radiation, while the surrounding water is transparent and therefore not disturbing.

A selection of FLASH electron beam and photon pulse parameters.
The unit for brilliance is B=photons/s/mrad2/mm2/0.1%bw. Note, that exact values depend on various conditions. Not all combinations within the parameter ranges given are possible.




Electron beam energy

0.35 - 1.25 GeV

0.4 - 1.25 GeV

Normalised emittance at 1 nC (rms)

1.4 mm mrad

1.4 mm mrad

Energy spread

200 keV

500 keV

Electron bunch charge

0.1 - 1.2 nC

0.02 - 1 nC

Peak current

1 - 2.5 kA

1 - 2.5 kA

Electron bunches per second (typ./max)

300 / 5000

300 / 5000

Photon energy (fundamental)

24 - 295 eV

14 - 310 eV

Photon wavelength (fundamental)

51 - 4.2 nm

90 - 4 nm

Photon pulse duration (FWHM)

<30 - 200 fs

<10 - 200 fs

Peak Power (from av.)

1 - 5 GW

1 - 5 GW

Single photon pulse energy (average)

1 - 500 µJ

1 - 1000 µJ

Spectral Width (FWHM)

0.7 - 2 %

0.5 - 2 %

Photons per Pulse

1011 - 1014

1011 - 1014

Peak Brilliance

1028 - 1031 B

1028 - 1031 B

FLASH produces also THz radiation

FLASH1 is equipped with an electro-magnetic undulator able to produce Terahertz (THz) radiation pulses. The THz pulses are produced by the same electron bunch than the SASE radiation. Therefore, both pulses, the soft X-ray and THz are perfectly synchronized (better than 5 femtoseconds).

Parameters of the THz radiation at FLASH. Note, that the exact values depend on pulse length and wavelength chosen.




10 - 230 µm

Photon energy

5 - 125 meV

Photon frequency

1.3 - 30 THz

Photon pulse energy (average)

10 - 100 µJ

First external seeding at 38 nm

The FLASH1 beamline has a section equipped with variable gap undulators for seeding experiments (sFLASH). In April 2012, first seeding at 38 nm has been obtained. The seed source was radiation from high harmonic generation (HHG). An external femtosecond short laser pulse is focused into a gas cell producing higher harmonics of the laser wavelength. This radiation is overlapped with electron bunch and seeds the amplification process in the undulators. The advantage compared to SASE is the narrow bandwidth of the radiation pulses. This is advantageous for certain classes of experiments, where a small bandwidth is required. Today, the sFLASH experiment concentrates under the new name Xseed on the preparation of external seeding in the frame work of the FLASH2020+ project.

The FLASH Accelerator

FLASH is a high-gain free-electron laser (FEL) which achieves laser amplification and saturation within a single pass of the electron
bunches through a long undulator section. The lasing process is initiated by the spontaneous undulator radiation. The FEL works in the so-called Self-Amplified Spontaneous Emission (SASE) mode without needing an external input signal.

The electron bunches are produced in a laser-driven photoinjector and accelerated by a superconducting linear accelerator. The RF-gun based photoinjector allows the generation of electron bunches with a tiny emittance - mandatory for an efficient SASE process.The superconducting technique allows to accelerate thousands of bunches per second, which is not easily possible with other technologies. At intermediate energies of 150 and 450 MeV the electron bunches are longitudinally compressed, thereby increasing the peak current from initially 50 to 80 A to 1 to 2 kA or more - as required for the SASE process to develop.

The FLASH1 undulators.

FLASH1 has a 27 m long undulator made of permanent NdFeB magnets with a fixed gap of 12 mm, a period length of 27.3 mm and peak magnetic field of 0.47 Tesla (K=1.23). The electrons interact with the undulator field in such a way, that so called micro bunches are developed. These micro bunches radiate coherently and produce intense X-ray pulses. Finally, a dipole magnet deflects the electron beam safely into a dump, while the FEL radiation propagates to the experimental hall.

application/pdf FLASH (54KB)
schematic layout (pdf)
Image: DESY / Siegfried Schreiber
image/jpeg FLASH (683KB)
schematic layout (jpg)
Image: DESY/Siegfried Schreiber
An electron gains an energy of 1 electron volt (1 eV) moving across an electric potential difference of one volt (1 V). An every days battery has a voltage of 1.5 V.
One megaelecton volts (MeV) is a million volts; one gigaelecton volts (GeV) is a thousand million volts.

Visible light is in the wavelength range between 380 and 760 nm. 1 nm is a millionth of 1 mm. The size of a molecule is about 1 nm.

The wavelength range of XUV radiation (also called EUV) is in the range of 120 to 10 nm, soft X-ray radiation from 10 to 0.1 nm. Hard X-rays are shorter than 0.1 nm. FLASH produces radiation from the XUV to the soft X-rays.

An outstanding feature of FLASH is to produce ultra-short radiation pulses in the XUV and soft X-ray wavelength range.
Many thousands per second. The radiation pulses have a duration of less than 100 fs. In specific arrangements less than 10 fs.

One femtosecond (= 1 fs) is equal to 10-15 seconds. In one femtosecond, the light travels only 300 nm. To the moon, the light needs 1 second.

You may imaging that you need these extraordinary conditions to study dynamics in molecules or even atoms which are of the size of nanometers.