Europe’s XFEL creates exotic matter

Experiments at the European XFEL generate states of matter that are close to those found in the interior of planets or in the imploding capsule of an inertial fusion reactor. At the same time, they open the way to the measurement of ultrashort phenomena.

Investigating the extreme conditions reached in the interior of planets, including Earth, or during a fusion reaction is a major challenge. By focusing the extremely powerful X-ray laser of the European XFEL on a copper foil, the scientists created and probed a state of matter very far from equilibrium, coined warm dense matter (WDM), which resembles such exotic environments.

Their discoveries have made remarkable advances in the understanding and characterization of this elusive state of matter that is essential to the progress of inertial fusion, a process that holds the promise of clean and abundant energy. The research is published in a journal Natural physics.

Heat can drastically change the state of matter. Depending on the temperature, substances are solid, liquid or gas. In a certain temperature range, matter also occupies a state known as warm dense matter (WDM): it is too hot to be described by condensed matter physics, but at the same time too dense for weakly bound plasma physics.

The boundary between warm dense matter and other states of matter is not precisely defined. A temperature range of 5,000 Kelvin to 100,000 Kelvin at pressures of several hundred thousand bars, with one bar corresponding to the air pressure at the Earth’s surface, is often reported.

WDM is not stable in our everyday environment and is very difficult to manufacture or even research in the laboratory. Scientists typically compress samples in diamond anvils to achieve high pressures or use powerful optical lasers to convert solids into WDMs in a tiny fraction of a second.

The intense X-ray pulses of the European XFEL have now proven to be a very useful tool for the generation and analysis of warm dense matter. The researchers used copper as a sample material. “The high intensity of the pulses can excite the electrons in the copper foil to such an extent that they go into a state of warm dense matter,” explains Laurent Mercadier, the SCS instrument scientist who led the experiment. “This can be seen in the change in its light transmission.

A metal that is irradiated with an intense X-ray pulse can become transparent if the electrons in the metal absorb the X-ray energy so quickly that there are no electrons left to get excited. The remaining end of the pulse can then penetrate the material undisturbed. This is known as saturable absorption (SA).

Conversely, the metal can become increasingly opaque if the front of the pulse creates excited states that have a higher absorption coefficient than the cold metal. The end of the pulse is then absorbed more strongly, an effect known as reverse saturable absorption (RSA). Both processes are commonly used in optics, for example to generate a specific pulse length using lasers.

Scientists at the European XFEL have now beamed sharply focused 15 femtosecond X-ray pulses onto a 100 nanometer thick copper film. They then analyzed the transmitted signal using a spectrometer.

“The spectrum depends strongly on the intensity of the X-ray pulse,” explains Mercadier. “At low to moderate X-ray intensities, copper becomes increasingly opaque to the X-ray beam and exhibits RSA. However, at higher intensities, the absorption saturates and the foil becomes transparent.”

These drastic changes in opacity happen so quickly that the atomic nuclei in the metal don’t have time to move. “We are dealing with a very exotic state of matter where the lattice is cold and some of the ionized electrons are hot and not in equilibrium with the remaining free electrons of the metal,” explains Mercadier.

“To explain this, we developed a theory that combines the physics of solids and plasmas.” For the researchers, the change in opacity is a sign that they have succeeded in creating and characterizing the warm dense matter in the laboratory.

Understanding material opacity under these extreme conditions is urgently needed for inertial fusion. In the latter case, intense energy is used to compress and heat the fuel target, creating the conditions necessary for fusion. Opacity determines how much radiant energy is absorbed or transmitted by the material, which is necessary to ensure that the energy used for compression does not escape, allowing for efficient fusion reactions.

Short is not short enough

“In fact, these effects happen so fast that we need even shorter X-ray pulses to fully resolve the electron dynamics,” says Andreas Scherz, principal scientist on the SCS instrument. “Recently, the European XFEL demonstrated the ability to generate attosecond pulses, opening the door to so-called attosecond physics.”

With the help of attosecond X-ray pulses, it is possible to precisely “film” the movement of electrons during the formation of warm dense matter or during chemical reactions, significantly improving our understanding of e.g. chemical processes or the functioning of catalysts.

The 2023 Nobel Prizes in Physics awarded to French-Swedish physicist Anne L’Huillier, Hungarian-Austrian physicist Ferenc Krausz and French-American physicist Pierre Agostini show that this is an extremely timely research topic.