From DESY: “Free-electron laser FLASH”


10 years of SASE at FLASH

Back in 2005, early in the morning of January 14th, first SASE has been observed at DESY’s newly installed VUV free-electron laser. The electron beam has been accelerated to 445 MeV corresponding to a wavelength of 32 nm. In summer 2005, the VUV-FEL turned into a user facility named FLASH.

Spectrum of the first SASE signal measured in the early morning of January 14th, 2005.

FLASH2 generates first laser light

The FLASH II project – the extension of the free-electron laser FLASH – has reached an important milestone: On 20 August 2014, at 20:37 h, the accelerator team of the late shift was able to detect the first laser light at the new undulator line named FLASH2. Simultaneously, FLASH´s first undulator line FLASH1, which is provided with electron bunches from the same accelerator, could continue to operate without restrictions. The undulators are periodic structures of magnets with alternating North-South polarity in which the X-ray laser light is generated. “This makes FLASH the world’s first free-electron laser that serves two laser lines simultaneously and independently from each other,” says project leader Bart Faatz

SASE FEL radiation observed on a Ce:YAG screen of the FLASH2 photon beamline.

Fifth user period started 24-Feb-2014 with its first beam time block

Coming out of a long shutdown to finish up the construction of the new beamline FLASH2, the fifth user period for beamline FLASH1 has started end of February with its first user block. Until April 2015, more than 5000 hours of user experiments are scheduled. Beam time will also be available for accelerator and photon beam line studies as well as for FLASH2 commissioning. FLASH2 saw first beam in March 2014 pushing the beam to the dump for the first time May, 23. Since then, FLASH2 is operated in parallel to FLASH1 whenever possible to finish up the commissioning of beam diagnostics and to refine beam optics. First SASE radiation at 40 nm has been seen on Aug 20, 2014. The next goal is to characterize the SASE radiation, to measure gain length for example for as many other wavelength.

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 a 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 as short as 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 0.37 and 1.25 GeV covering the wavelength range between 45 and 4 nanometers (nm). See the table below for details.

An electron gains an energy of 1 electron volt (1 eV) moving across an electric potential difference of one volt (1 V). 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

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 accelerator modules. Each 12 m long module contains eight cavities. The 1 m long cavities are made of solid niobium and cooled by liquid helium at 2 K. At this temperature just 2 dgC 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 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 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 for 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 2012

Parameter Value

WavelengthRange 4.2 – 45 nm

Average Single Pulse Energy 10 – 500 µJ

Pulse Duration (FWHM) <50 – 200 fs

Peak Power (from av.) 1 – 3 GW

Average Power (example for 5000 pulses/sec) up to 600 mW

Spectral Width (FWHM) 0.7 – 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

FLASH is a science driver

Many scientific disciplines ranging from physics, chemistry and biology to material sciences, geophysics and medical diagnostics use the powerful soft X-ray source FLASH. The ultra-short X-ray pulses in the femtosecond range allow experiments which are not possible otherwise. For example, time-resolved observation of chemical reactions with atomic resolution, single shot diffraction imaging, and many others.

More than 200 publications on photon science have been published already, many in high ranked journals.

First external seeding at 38 nm

In April 2012, sFLASH, the seeding experiment at FLASH, has obtained first seeding at 38 nm. An external seed source of the same wavelength overlaps with the electron beam to seed the SASE process in a series of undulators installed between the accelerator and the FLASH undulators.

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 techniques 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 50-80 A to 1-2 kA – as required for the lasing process in the undulator.

A special superconducting 3.9-GHz module built at Fermilab has been installed in 2010 to improve the quality of the accelerated electron beam. The four cavities in this module operate at the third harmonic of the acceleration field frequency. They shape the electron bunches in a way that the intensity of the laser light is higher than ever before.

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 T. 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.


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