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How does radiation travel through dense plasma? First-time experimental evidence challenges conventional theories of how plasmas emit or absorb radiation.

Most people are familiar with the three states of matter: solids, liquids and gases. However, a fourth state of matter, called plasma, is the most common form of matter in the universe, found throughout our solar system in the sun and other planetary bodies. Because dense plasma—a hot soup of atoms with freely moving electrons and ions—typically only forms under extreme pressure and temperature, scientists are still working to understand the fundamentals of this state of matter. Understanding how atoms react under conditions of extreme pressure – a field known as high energy density physics (HEDP) – gives scientists valuable insight into the fields of planetary science, astrophysics and fusion energy.

An important question in HEDP is how plasmas emit or absorb radiation. Current models representing radiative transfer in dense plasmas are heavily based on theory rather than experimental evidence.

no new work published in nature communication, researchers at the University of Rochester’s Laboratory of Laser Energy (LLE) used the LLE’s OMEGA laser to study how radiation travels through dense plasma. The research was led by Suxing Hu, a distinguished scientist and group leader of LLE’s High-Energy-Density Physics Theory Group and Associate Professor of Mechanical Engineering, and Philip Nilson, a senior scientist in LLE’s Laser-Plasma Interaction Group, delivers for the first time experimental data on the behavior of atoms under extreme conditions. The data will be used to improve plasma models, allowing scientists to better understand stellar evolution and potentially aiding in the realization of controlled nuclear fusion as an alternative energy source.

“Experiments with laser-driven implosions on OMEGA have produced extreme matter with pressures billions of times higher than atmospheric pressure at the Earth’s surface, so we can study how atoms and molecules behave under such extreme conditions,” says Hu. “These conditions correspond to the conditions within the so-called envelope of white dwarf stars as well as inertial fusion targets.”

Using X-ray spectroscopy

The researchers used X-ray spectroscopy to measure how radiation is transported through plasmas. In X-ray spectroscopy, a beam of X-rays is directed at a plasma of atoms – in this case, copper atoms – under extreme pressure and heat. The researchers used the OMEGA laser both to generate the plasma and to generate the X-rays aimed at the plasma.

When the plasma is bombarded with X-rays, the electrons in the atoms “jump” from one energy level to another, either emitting or absorbing photons of light. A detector measures these changes and visualizes the physical processes taking place in the plasma, similar to the X-ray diagnosis of a broken bone.

A break with conventional theory

The researchers’ experimental measurements indicate that as radiation travels through a dense plasma, the changes in atomic energy levels do not follow the conventional theories currently used in plasma physics models — so-called “continuum depression” models. Instead, the researchers found that the measurements they observed in their experiments can only be explained using a self-consistent approach based on density functional theory (DFT). DFT offers a quantum mechanical description of the bonds between atoms and molecules in complex systems. The DFT method was first described in the 1960s and was the subject of the 1998 Nobel Prize in Chemistry.

“This work shows fundamental steps to rewrite current textbook descriptions of how radiation generation and transport occur in dense plasmas,” says Hu. “According to our experiments, using a self-consistent DFT approach more accurately describes radiative transfer in a dense plasma.” Nilson says: “Our approach could provide a reliable way to simulate radiative generation and radiative transfer in dense plasmas found in stars and inertial fusion targets.” . The experimental scheme described here, based on laser-driven implosion, can be easily extended to a wide range of materials, paving the way for wide-ranging studies of extreme atomic physics at enormous pressures.”

Researchers from Prism Computational Sciences and Sandia National Laboratories, as well as other LLE researchers, including physics PhD students David Bishel and Alex Chin, also contributed to this project.

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