PHOTON SCIENCE

Both the FLASH facility itself and the user experiments have made tremendous progress during the first two years. Many experiments took promising data and demonstrated the feasibility of new concepts.

Since the first experiments in summer 2005, the operation of the FLASH free-electron laser has already become routine. The stability and reliability of the FEL as well as the average radiation pulse energy have significantly increased, and, according to computer simulations, FEL pulses as short as 10 femtoseconds have been produced. FLASH can now be tuned to any wavelength between 50 and 13 nanometers within a few hours by varying the energy of the accelerator. The demonstration of tunability in spring 2006 was one of the most important milestones for flexible user operation.

The First Experiments

In 2006 and spring 2007, there were 16 active projects involving approximately 200 scientists from 11 countries. Most of the researchers are working in collaborations consisting of several teams, and many of these teams have built new instrumentation dedicated to their experiments at FLASH.

The projects can be grouped into four broad categories:

• Femtosecond time-resolved experiments dealing both with technical developments and the first pump-and-probe experiments.
• Studies of the interaction between extreme-ultraviolet radiation and matter, including experiments dealing with multiphoton excitation of atoms, molecules and clusters. The first diffraction experiments on artificial nano-objects have successfully demonstrated that images can indeed be taken with a single FEL pulse even though the objects explode after the radiation impact. This result is particularly important for research groups aiming to determine the atomic structure of large biomolecules from non-crystalline samples.
• Investigations of very dilute samples. These projects deal with photodissociation of molecular ions, spectroscopy of highly charged ions, and mass-selected clusters.
• Research on surfaces and solids including experiments on laser desorption, surface dynamics, lumiscence, Raman and photoelectron spectroscopy of surfaces and solids with nanometer spatial resolution.

22 weeks of beamtime were scheduled for user experiments in 2006. Typically, beamtime is organised in blocks of four weeks, preceded by three weeks of FEL studies for improving the overall performance of the FEL and the user facility. The remaining time was used for maintenance and machine studies.

The FLASH Upgrade in 2007

From April to June 2007, additional cryomodules will be installed in the accelerator enabling the linac to reach its nominal energy of 1 GeV. Lasing in the soft X-ray regime at 6.5 nm is expected for the next round of experiments beginning in fall 2007.

The exciting research possibilities at FLASH attracted many researchers. 45 new proposals were submitted for experiments starting after the upgrade, they cover six different areas: studies of gas phase samples including atoms, molecules and ions; clusters; imaging and diffraction; high-energy density and warm dense matter; surfaces and solids; and technical developments such as commissioning of new instrumentation.

The projects were reviewed by a project review panel with eleven members from six countries, and due to the very high quality only a few projects were rejected. The procedure selected 32 projects sharing two eight-month beamtime periods, each period including 150 twelve-hour shifts to be distributed among the science projects.

Performance of the FEL Radiation 2005 – 2007