FLASH opens the view through the water window
 
The free-electron laser FLASH (Free-Electron Laser in Hamburg) has set a new record: this weekend, the FLASH accelerator team operated the FEL with an electron energy of 1.25 Giga electronvolts, thus reaching a wavelength of 4.12 nanometres. For the first time FLASH has generated laser light in the so-called water window using the fundamental wavelength – so far this was only reached with laser harmonics. The flashes with the short wavelengths had an average energy of 70 and a peak energy of 130 micro joules.

“Congratulations to the FLASH team for their outstanding work,” said accelerator director Reinhard Brinkmann. “By optimisation of the accelerator, they increased the electron energy by another 50 MeV and – reaching the water window – created brilliant conditions for FLASH scientists.”

The water window is a wavelength region between 2.3 and 4.4 nanometres. In this wavelength region, 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.

Additional measurements with extremely short wavelengths are scheduled for November. From the beginning of April 2011, the FEL radiation will give users the opportunity to look through the water window.

FLASH, the world’s first X-ray free-electron laser is available to the photon science user community for experiments since 2005. Last winter, the facility underwent a major upgrade.

The accelerator is now equipped with a seventh superconducting accelerator module to increase the maximum electron energy to 1.2 Giga electron volts (GeV). Moreover, a special 3.9-GHz module built at Fermilab is installed to improve the quality of the accelerated electron bunches. It shows excellent results: the 3.9-GHz module shapes the electron bunches in a way that the intensity of the laser light is higher than ever before.

„It is absolutely impressing, how fast and promising FLASH is operating after such a substantial upgrade. My compliments to the FLASH accelerator team,” congratulates Reinhard Brinkmann.

With the now obtainable laser wavelength, experiments with carbon in organic molecules are possible, and magneto-dynamics experiments with the third-harmonic wavelength benefit from substantially increased intensities.

This success is also an important milestone for the European XFEL. The accelerator module recently built-in at FLASH is a prototype for the XFEL accelerator, and the properties of the 3.9-GHz module too are decisive for operating the XFEL injector.

Many other improvements of FLASH have been completed. As an example, the RF-stations and waveguide distribution have been substantially upgraded. The injector has now a new RF gun, an upgraded laser system, and a new booster module.

sFLASH, the seeding experiment is being commissioned now. An external laser overlaps with the electron beam to seed the SASE process in a series of new undulators installed between the accelerator and the FLASH undulators.

FLASH, DESY's free-electron laser is a world-wide unique facility delivering intense ultra-short femtosecond coherent radiation in the wavelength range between 44 nm and 4.1 nm.


Since 2005, FLASH is a user facility serving a large variety of experiments.
Typical user operation parameters during the 2nd user period from Nov 26, 2007 to Aug 16, 2009:

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 100 publications on photon science have been published already, many in high ranked journals.

FLASH is a high-gain free-electron laser (FEL) which achieves laser amplification and saturation within a single pass of the electron
bunch through an undulator and does not require a set of mirrors which is needed in conventional lasers. The lasing process is initiated by the spontaneous undulator radiation, and the FEL works then 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 possible with other technologies. At intermediate energies of 130 and 470 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. The 30 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.

Picture gallery of the FLASH linac
updated in May 2008