Flash Explores New States of Matter

Usually, it shimmers in a dull silver – simply typically aluminium. Yet given extreme circumstances, the light metal can become translucent, not for normal light, but for soft x-rays. This feature was discovered by an international team of physicists at the Research Centre DESY in Hamburg by way of the light source FLASH. Amongst other things, the result is of relevance for astrophysics and fusion research.

FLASH is a world-wide unique laser and produces ultra short, highly intensive flashes of soft x-ray light. The 300 metre long facility is based on a superconducting accelerator, which accelerates electrons nearly to the speed of light. The particles fly through “undulators” – structures made out of many hundred pairs of magnets forcing the electrons to take a slalom course. In doing so, the particles emit short, strong laser flashes with wavelengths ranging from extreme ultraviolet to x-ray radiation. Experts call the facility a “Free Electron Laser” (FEL). The team around DESY physicist Dr. Sven Toleikis managed to bundle the FLASH flashes into a tiny speck of less than a micrometre in diameter by way of a special mirror. As a result, the flash was so concentrated as to produce a remarkable effect when the researchers hit a tiny piece of aluminium with it: The normally non-transparent metal suddenly became transparent in the area of the soft x-ray radiation.

The reason: The energy rich, intensive x-ray light of the FLASH flash kicks electrons out of one of the inner hulls of the aluminium atoms. This causes a drastic change in the metal’s absorption behaviour: Suddenly, it lets the x-rays pass, the aluminium becomes transparent. As a result, a most peculiar condition of matter forms: While the atomic rumps of the aluminium continue to form a firm grid, the electrons moving almost freely within the grid possess an extreme amount of energy – considerably more than is usual in a solid body. “It is virtually a combination of crystal and plasma”, explains Toleikis. “One can look at it as an initial condition for another exotic form of matter, we call it warm dense matter. This warm dense matter is of relevance for a very different phenomenon. Experts think that it exists in giant planets such as Jupiter. “By examining this state of matter in more detail in future with facilities like FLASH, one can draw conclusions as to what it might look like inside Jupiter”, says Toleikis.

The warm dense matter could also be interesting for the future production of energy, that is, in the artificial nuclear fusion with laser beams. Here, researchers try to shoot at tiny spheres of frozen hydrogen with laser beams in such a manner, that they implode and the hydrogen can melt to form helium. If this succeeds, considerable amounts of energy are releases, which could be converted into electricity in power stations. The warm dense matter is an important transitional state shortly before the actual implosion here. The researchers want to understand it in as much detail as possible, for only then can they optimally activate the hydrogen spheres.

The aluminium experiment is only one of many projects at FLASH. For instance, scientists look at single nanoparticles, research plankton organisms or study highly charged iron ions such as occur in the atmosphere of the sun. Since 2005, FLASH operates routinely. Each year, researchers from all over the world come to experiment at the Hamburg facility. But the laser is totally overbooked: By far not everyone who wants to work with strong x-ray flashes can take their turn. Therefore, DESY now plans a next step called FLASH II in cooperation with the Helmholtz Zentrum Berlin (HZB). The concept: A second undulator is to be built next to the existing one, fed by the same, already existing linear accelerator. A kind of special point will then distribute the fast electrons to both undulators with lightning speed.

The Clou: “The new undulator is adjustable”, says DESY physicist Dr. Bart Faatz. “As a result, it can provide different x-ray wavelengths than the existing undulator.” Furthermore, the laser characteristics of x-ray and UV flashes are to be much more pronounced than is the case in FLASH. Certain experiments thus can be carried out in a much better manner, for instance, the “filming” of chemical reactions. Behind the second undulator, a new experimental hall is to be built. It allows space for up to six measuring stations – practically doubling the FLASH capacity.