Exploring 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 European XFEL on a copper foil, researchers have created and investigated a state of matter very far from equilibrium, coined warm dense matter (WDM), that resembles such exotic environments. Their findings make remarkable strides in understanding and characterizing this elusive state of matter, which is crucial for advancing inertial confinement fusion, a process that holds promise for clean and abundant energy.
Heat can drastically change the state of matter: depending on the temperature, substances are solid, liquid or gaseous. In a certain temperature range, matter also assumes a state known as warm dense matter (WDM): it is too hot to be described by the physics of condensed matter, but at the same time too dense for the physics of weakly coupled plasmas. The boundary between warm dense matter and other states of matter is not precisely defined. Often a temperature range of 5,000 Kelvin to 100,000 Kelvin is specified at pressures of several hundred thousand bar, whereby one bar corresponds to the air pressure on Earth surface. WDM is not stable in our daily environment and is very difficult to produce or even examine in the laboratory. Typically, scientists compress samples in diamond anvil cells to reach high pressures, or use powerful optical lasers to turn solids into WDM for a tiny fraction of a second.
The intense X-ray pulses of European XFEL have now proved to be a very useful tool for generating and analysing 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 it switches to the state of warm dense matter,” explains Laurent Mercadier, a scientist at the SCS[1] instrument who led the experiment: “This can be seen in a change in its light transmission.”
A metal that is irradiated by an intense X-ray pulse can become transparent if the electrons in the metal absorb X-ray energy so fast that there are no electrons left to excite. The remaining tail of the pulse can then penetrate the material unhindered. This is known as saturable absorption (SA). Conversely, a metal can become increasingly opaque if the front of the pulse creates excited states that have higher absorption coefficient than the cold metal. The tail of the pulse is then absorbed stronger, an effect known as reverse saturable absorption (RSA). Both processes are routinely used in optics, for example to generate a specific pulse length with lasers.
Read more on European XFEL website
Image: Laurent Mercadier checks the setup in the experimental chamber
Credit: European XFEL