How does radiation travel through dense plasma? First-of-its-kind experimental evidence challenges conventional theories about how plasmas emit or absorb radiation. –ScienceDaily

Most people are familiar with solids, liquids, and gases as three states of matter. However, a fourth state of matter, called plasma, is the most abundant 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 free-moving electrons and ions, typically forms only under extreme pressures and temperatures, scientists are still working to understand the fundamentals of this state of matter. Understanding how atoms react under extreme pressure – a field known as high-energy density physics (HEDP) – offers scientists valuable insights into the fields of planetary science, astrophysics and fusion energy.

An important question in the field of HEDP is how plasmas emit or absorb radiation. Current models describing radiation transport in dense plasmas are heavily based on theory rather than experimental evidence.

na new article published in Nature communications, researchers at the University of Rochester’s Laboratory for Laser Energetics (LLE) used LLE’s OMEGA laser to study how radiation travels through dense plasma. The research, led by Suxing Hu, a distinguished scientist and group leader of the high-energy-density physics theory group at the LLE and an associate professor of mechanical engineering, and Philip Nilson, a senior scientist in the laser-plasma interaction group at the ‘LLE , provides unique experimental data on the behavior of atoms under extreme conditions. The data will be used to improve plasma models, which allow scientists to better understand the evolution of stars and can aid in the implementation of controlled nuclear fusion as an alternative energy source.

“Experiments using laser-guided implosions on OMEGA have created extreme matter at pressures several billion times atmospheric pressure at the Earth’s surface to allow us to probe how atoms and molecules behave under such extreme conditions,” says Hu. “These conditions match the conditions inside the so-called envelope of white dwarf stars and inertial merger targets.”

Using X-ray spectroscopy

The researchers used X-ray spectroscopy to measure how radiation is transported through the plasma. X-ray spectroscopy involves aiming a beam of radiation in the form of X-rays at a plasma made of atoms – in this case copper atoms – subjected to extreme pressure and heat. The researchers used the OMEGA laser to both create the plasma and to create the direct plasma X-rays.

When plasma is bombarded with X-rays, electrons in atoms “jump” from one energy level to another by emitting or absorbing photons of light. A detector measures these changes, revealing the physical processes occurring within the plasma, similar to X-ray diagnostics of a broken bone.

A break with conventional theory

The researchers’ experimental measurements indicate that, when 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 drawdown’ models. Instead, the researchers found that the measurements 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 Nobel Prize in Chemistry in 1998.

“This work reveals critical steps to rewrite current textbook descriptions of how radiation generation and transport occurs in dense plasmas,” says Hu. “Using a self-consistent DFT approach more accurately describes radiation transport in a dense plasma, according to our experiments.” Says Nilson: ‘Our approach could provide a reliable way to simulate radiation generation and transport in the dense plasmas encountered in stars and inertial fusion targets. The experimental scheme reported here, based on a laser-guided implosion, can be easily extended to a wide range of materials, paving the way for far-reaching investigations of extreme atomic physics under tremendous pressures”.

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

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Materials provided by University of Rochester. Note: Content can be edited for style and length.

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