FLASH
Wavelength World Record at FLASH: 6.5 Nanometers! - Design value for laser flashes reached
Two weeks after the maximum beam energy of 1 gigaelectronvolt was reached, the control room announced another milestone: “On the evening of October 4, we observed lasing at a wavelength of 7 nanometers (nm) at FLASH for the first time.” Only 24 hours later, the FLASH team achieved the facility’s design value of 6.5 nm. In FLASH, the electrons are accelerated to an energy of 986 megaelectronvolts in six superconducting modules. On their flight through the undulator, the electrons now demonstrated the desired behavior also at this high energy: the spontaneous radiation they emit amplified itself to form the desired free-electron laser radiation pulses (SASE-FEL).
FLASH stands for "F"reie-Elektronen-"LAS"er in "H"amburg or the equivalence in other languages.
Many scientific disciplines ranging from physics, chemistry and biology to material sciences, geophysics and medical diagnostics need a powerful X-ray source with pulse lengths in the femtosecond range. This would allow, for example, time-resolved observation of chemical reactions with atomic resolution. Such radiation of extreme intensity, and tunable over a wide range of wavelengths, can be accomplished using high-gain free-electron lasers (FEL).
During the last years a high-gain FEL in the VUV and soft X-ray wavelength regime was set up at DESY, Hamburg. It 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 and in the low-gain FEL. 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.
 "Zoom (20KB)")
The electron bunches are produced in a laser-driven photoinjector and accelerated to by a superconducting linear accelerator. At intermediate energies of 125 and 450 MeV the 1 nC electron bunches are longitudinally compressed, thereby increasing the peak current from initially 50 -80 A to approximately 1-2 kA as required for the FEL operation. The 30 m long undulator consists of NdFeB permanent magnets with a fixed gap of 12 mm, a period length of 27.3 mm and peak magnetic field of 0.47 T. Finally, a dipole magnet deflects the electron beam into a dump, while the FEL radiation propagates to the experimental hall.
A side view of FLASH can be seen here
updated in May 2008
