Free-electron laser FLASH

 

Albert and Kai

Hamburg’s mayor Olaf Scholz and the Swedish state secretary Anders Lönn name the experimental halls.


At a symbolic ceremony, Olaf Scholz, First Mayor of Hamburg, and Anders Lönn, State Secretary to Sweden’s Minister for Higher Education and Research, named the two FLASH experimental halls after the pioneer physicists and Nobel Prize winners Albert Einstein and Kai Siegbahn.

Kai Siegbahn was awarded the 1981 Nobel Prize in physics for “his contribution to the development of high-resolution electron spectroscopy”.
Albert Einstein received the 1921 Nobel Prize in physics for “his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect”.

FLASH shutdown from 4-May-2015 to 25-May-2015

FLASH starts into a Mini-Shutdown of 3 weeks to complete the FLASH2 radiation shield and to start the installation of FLASHForward into the FLASH3 beamline.

Another 1000 tons of sand will be put on top of the triangle formed by the FLASH1 and FLASH2 beamline tunnels.

FLASHForward is a plasma acceleration experiment to be set-up in the FLASH3 beamline, parallel to FLASH2. During this shutdown, support structures for the extraction beamline and major parts of the laser beamline will be installed. The installtion will continue in 2016.

 

Schematic layout of FLASH. Not to scale. The second beamline, FLASH2, is being commissioned and has seen first lasing at 40 nm on Aug, 20 2014 while FLASH1 was at the same time providing 250 pulses long bunch trains for experiments.


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 50 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 SASE process is driven by a high brightness electron beam. 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.

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 Radiation Parameters 2015

Parameter

Value

Wavelength Range (fundamental)

4.2 - 52 nm

Average Single Pulse Energy

10 - 500 µJ

Pulse Duration (FWHM)

<50 - 200 fs

Pulses per second

10 - 5000

Peak Power (from av.)

1 - 3 GW

Average Power

up to 600 mW

Spectral Width (FWHM)

0.3 - 2 %

Photons per Pulse

1011 - 1013

Average Brilliance

1017 - 1021 photons/s/mrad2/mm2/0.1%bw

Peak Brilliance

1029 - 1031 photons/s/mrad2/mm2/0.1%bw


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 emittances - 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 lasing process in the undulator.

The FLASH undulators.

The 27 m long undulator consists 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.

 
FLASH schematic layout of the facility.
FLASH schematic layout of the facility.
application/pdf FLASH layout.pdf (38KB)
 
Status as of 19-Aug-2014
 

Picture gallery of the FLASH linac (Status 2008)

Glossary
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.
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Glossary
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.
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