Free-electron laser FLASH


Quantum imaging with incoherently scattered light from a free-electron laser
Scientists demonstrate a new technique of imaging nanostructures

Physicists at the Friedrich Alexander University Erlangen-Nürnberg (FAU), the University of Hamburg and the research facility DESY have succeeded for the first time in revealing tiny structures using an imaging method that relies on the usual diffraction of light, but which does not require the scattered light to be coherent. With conventional imaging methods using the diffraction of coherent light, scientists have to go to considerable lengths to ensure the coherence of the radiation, i.e. the electromagnetic waves must remain in phase during the scattering process. The new method uses incoherent light instead. The process, which has now been demonstrated for the first time using short-wave ultraviolet radiation, stands to revolutionise the methods of diffractive optics and has been published in the journal Nature Physics.

Diffraction image of a grating consisting of four slits, created using partially coherent light from the Free-Electron Laser Hamburg (FLASH), as used in the experiment. The coherent component of the FLASH radiation produces the horizontal diffraction pattern with a small number of pronounced maxima in its intensity; the incoherent component generates the so-called speckle pattern that can be seen above and below the coherent diffraction pattern. This is precisely the component of the light that was used by the new imaging method. Credit: Raimund Schneider et al.

FLASH - Schematic layout of the facility.
FLASH - Schematic layout of the facility.
application/pdf FLASH layout.pdf (png file: bottom of this page) (70KB)
Schematic layout of FLASH. Not to scale. The third beamline, FLASH3, is being constructed with the FLASHForward experiment. As of the second half of 2017, FLASHForward will use the FLASH electron beam to generate a plasma in order to further accelerate electron bunches.

FLASH is DESY's soft X-ray free-electron laser

FLASH, the world's first 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 the newly installed FLASH2 beamline. Electron pulses of one burst are shared between the two beamlines providing a fully parallel beam for two experiments with the full repetition rate of 10 Hz at the same time.

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 energy. The FLASH accelerator provides a range of electron energies between 350 MeV and 1.25 GeV covering the 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 beam energy given by the required wavelength of FLASH1, the experiment at FLASH2 is able to change the wavelength in a wide range for their experiment without influencing FLASH1.

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

FLASH reaches the water window

The FLASH accelerator is equipped with seven TESLA-type 1.3 GHz superconducting accelerating modules. Each 12 m long module contains eight cavities to accelerate the electron beam. The 1 m long cavities are made of solid niobium and cooled by liquid helium 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.

In September 2010, the FLASH team operated the accelerator with an electron beam energy of 1.25 GeV producing X-rays with a wavelength of 4.12 nm. For the first time FLASH has generated laser light in the so-called water window with the fundamental SASE wavelength. So far this was only possible at FLASH with the by a factor of thousand fainter third and fifth harmonic of the fundamental.

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.

FEL SASE Parameters 2016.
The unit for brilliance is B=photons/s/mrad2/mm2/0.1%bw.




Wavelength Range (fundamental)

4.2 - 51 nm

4 - 90 nm

Average Single Pulse Energy

1 - 500 µJ

1 - 1000 µJ

Pulse Duration (FWHM)

<30 - 200 fs

<10 - 200 fs

Pulses per second

10 - 5000

10 - 5000

Peak Power (from av.)

1 - 5 GW

1 - 5 GW

Spectral Width (FWHM)

0.7 - 2 %

0.5 - 2 %

Photons per Pulse

1011 - 1014

1011 - 1014

Peak Brilliance

1028 - 1031 B

1028 - 1031 B

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 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. 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.


Picture gallery of the FLASH linac (Status 2008)

Schematic layout of FLASH. Not to scale.

An electron gains an energy of 1 electron volt (1 eV) moving across an electric potential difference of one volt (1 V). A normal 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 has a wavelength between 380 and 760 nm. 1 nm is a millionth of 1 mm. The size of molecules is around 1 nm.