Warm Dense Matter

 


New experiments at the giant Euro laser called the European XFEL generate states of matter that are within close range of what occurs in the interior of planets. The experiments are also showing the imploding capsule of an inertial fusion reactor and unwrap a new procedure to measure ultra-short phenomenon. Fusion reactions, or the extreme conditions obtained at the interior or planets (like Earth) are very difficult to research. 

Recently, scientists focused the X-ray laser at the European XFEL on copper foil. They have constructed a state of matter that is very far from equilibrium. This new state is termed “warm dense matter.” (WDM). WDM is similar to the foreign environments of the interior of planets. 

Inertial confinement fusion has great promise for abundant and clean energy. The findings make huge strides in research and development of this difficult to track state of matter. The information was recently published in the journal Nature Physics.

It is widely known that heat can significantly change states of matter. Substances can be solid, liquid or gas depending on the temperature. Warm dense matter is too hot to be described by condensed matter physics, and too dense for weakly coupled plasma physics. 

In the past, the boundaries hadn’t been defined between warm dense matter and other states of matter. WDM is not stable and that makes it difficult to create or even look at in a laboratory. Scientists compress materials to reach high pressures, or use strong optical lasers that turn solids into WDM for a tiny fraction of a second. 

The X-ray pulses of the European XFEL are the perfect tool for measuring and creating warm dense matter. The scientists used copper as an illustration substance.

"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," said Laurent Mercadier. He is a scientist who led the experiments and reports, "This can be seen in a change in it's light transmission."

If the electrons in a sample metal absorb x-ray energy so quickly that there are no electrons left to excite, a metal becomes irradiated. The leftover tail of a pulse can impale the sample unchecked. This penetration is called saturable absorption (SA). 

A metal can also become more opaque. This happens if the front of a pulse makes excited states that are more absorbing than the cold metal. The pulse tail is then absorbed vigorously. This effect is called reverse saturable adsorption (RSA). These processes are used in optoelectronics. For example, it creates a specific pulse length of lasers. 

XFEL researchers are using 15 femtosecond long X-ray pulses directly onto a 100 nanometer thick copper film square. The results are then analyzed by a spectrometer.  "The spectrum heavily depends on the intensity of the X-ray pulse...at low to moderate X-ray intensity, copper becomes more and more opaque to the X-ray beam. However,  at higher intensities, absorption saturates and the foil becomes transparent," reports Mercadier. 

The extreme changes of opacity occur so quickly that the atomic nuclei of the metal don't 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 are not in equilibrium with the remaining free electrons of the metal.  To account for this, we developed a theory that combines solid state and plasma physics," reports Mercadier. 

Material opacity is needed to understand inertial confinement fusion. Powerful energy is used to compress and heat a sample target, creating the right environment for fusion. 

Opacity influences the amount of radiation energy that is absorbed or transmitted. This is important to ensure that compression energy does not escape, allowing for the efficiency of fusion reactions. 

Andreas Scherz is head scientist of the SCS instrument. He is reported as saying, "Actually, these effects happen so fast that we need even shorter X-ray pulses to fully resolve the electron dynamics. Recently, the European XFEL has demonstrated the capability to generate attosecond pulses, thus opening a door to the so-called attosecond physics." 

The movement of electrons during the creation of warm dense matter can be filmed using attosecond pulses. It can also film chemical reactions that are occurring. This filming can help researchers greatly improve our knowledge of chemical processing in general and the functioning of catalysts. 

The 2023 Nobel Prize in Physics was awarded to scientists directly involved with warm dense matter exploration. 



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