Free-electron laser FLASH

 

DESY mourns Helen Edwards
On 21 June, Helen T. Edwards passed away at the age of 80 at her home in Illinois, USA. Helen Edwards was the chief scientist in charge of building and operating the Tevatron at the US Fermi National Accelerator Laboratory (Fermilab), and from the early 1990s onwards she played a key role in developing the TESLA superconducting accelerator technology. She maintained close ties with DESY for over three decades and was together with her husband Don an essential driving force behind years of fruitful collaboration between Fermilab and DESY.

In the course of the TESLA collaboration, she was behind various crucial contributions made by Fermilab towards the design of the linear collider as well as the design and construction of the TESLA Test Facility, from which FLASH later emerged. Numerous colleagues at DESY remember and value Helen Edwards from many years of collaboration, and were extremely fond of her.

With her own brand of curiosity and her desire to get to the bottom of things, she worked with accelerator physicists at DESY until shortly before her death to analyse beam effects at FLASH. She will be remembered at DESY as a competent, dedicated and open-minded scientist who was always open for discussions, and her fond memory will always be cherished. Our thoughts go out to her husband and family.

Helen Edwards

FLASH shutdowns in 2016
FLASH finished its Mini-Shutdown in June and successfully continued the installation of FLASHForward at the FLASH3 beamline.

FLASHForward is a plasma acceleration experiment being set-up in the FLASH3 beamline, parallel to FLASH2. During this shutdown, support structures of the beamline and major components have been installed. The installtion will continue in a longer shutdown end of this year.

Tentative shutdown dates:
FLASH2 shutdown: 22.11.2016 - 3.1.2017 (FL2 Tunnel)
FLASH1 shutdown: 5.12.2016 - 3.1.2017 (FL1 Tunnel + Extraction)

 

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 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 Radiation Parameters 2016.
FLASH2 shows expected values. The unit for brilliance is B=photons/s/mrad2/mm2/0.1%bw.

Parameter

FLASH1

FLASH2

Wavelength Range (fundamental)

4.2 - 52 nm

4 - 90 nm

Average Single Pulse Energy

1 - 500 µJ

1 - 500 µJ

Pulse Duration (FWHM)

<30 - 200 fs

<10 - 200 fs

Pulses per second

10 - 6000

10 - 6000

Peak Power (from av.)

1 - 5 GW

1 - 5 GW

Average Power

up to 600 mW

up to 600 mW

Spectral Width (FWHM)

0.5 - 2 %

0.5 - 2 %

Photons per Pulse

1011 - 1013

1011 - 1013

Average Brilliance

1017 - 1021B

1017 - 1021 B

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.

 
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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