Defying Conventional Theories: How Does Radiation Travel Through Dense Plasma?

First-of-its-kind experimental evidence contradicts traditional theories about the emission and absorption of radiation by plasmas.

While solids, liquids, and gases are familiar states of matter, there is a fourth state called plasmas which is the most common form of matter in the universe. It can be found in the sun and other celestial bodies in our solar system. However, dense plasma, which is a hot mixture of atoms with free-moving electrons and ions, only occurs under extremely high pressure and temperature conditions, making it challenging for scientists to fully understand this state of matter.

Research in high-energy-density physics (HEDP), which investigates how atoms behave under extreme pressure conditions, can provide valuable insights into fields such as planetary science, astrophysics, and fusion energy.

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

In a new paper published in the journal Nature Communications, researchers at the University of Rochester 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 LLE’s Laser-Plasma Interaction group, provides first-of-its-kind experimental data about the behavior of atoms at extreme conditions. The data will be used to improve plasma models, which allow scientists to better understand the evolution of stars and may aid in the realization of controlled nuclear fusion as an alternative energy source.

“Experiments using laser-driven implosions on OMEGA have created extreme matter at pressures several billion times the atmospheric pressure at Earth’s surface for us to probe how atoms and molecules behave at such extreme conditions,” Hu says. “These conditions correspond to the conditions inside 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. 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—under extreme pressure and heat. The researchers used the OMEGA laser both to create the plasma and to create 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 by either emitting or absorbing photons of light. A detector measures these changes, revealing the physical processes that are occurring inside the plasma, similar to taking an x-ray diagnostic of a broken bone.

A Break From Conventional Theory

The researchers’ experimental measurements indicate that, when radiation travels through a dense plasma, the changes in atomic energy levels do not follow conventional quantum mechanics theories often used in plasma physics models—so-called “continuum-lowering” models. The researchers instead found that the measurements they observed in their experiments can be best 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 reveals fundamental steps for rewriting current textbook descriptions of how radiation generation and transport occurs in dense plasmas,” Hu says.

“According to our experiments, using a self-consistent DFT approach more accurately describes the transport of radiation in a dense plasma,” says Nilson. “Our approach could provide a reliable way for simulating radiation generation and transport in dense plasmas encountered in stars and inertial fusion targets. The experimental scheme reported here, based on a laser-driven implosion, can be readily extended to a wide range of materials, opening the way for far-reaching investigations of extreme atomic physics at tremendous pressures.”

Reference: “Probing atomic physics at ultrahigh pressure using laser-driven implosions” by S. X. Hu, David T. Bishel, David A. Chin, Philip M. Nilson, Valentin V. Karasiev, Igor E. Golovkin, Ming Gu, Stephanie B. Hansen, Deyan I. Mihaylov, Nathaniel R. Shaffer, Shuai Zhang and Timothy Walton, 16 November 2022, Nature Communications.
DOI: 10.1038/s41467-022-34618-6

The study was funded by the National Nuclear Security Administration, the New York State Energy Research and Development Authority, the National Science Foundation, Sandia National Laboratories, and the University of Rochester. 

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