Researchers determine new method for measuring high energy density plasmas and facilitating inertial confinement fusion

An international team of scientists has discovered a new way to advance the development of fusion energy through a better understanding of the properties of warm dense matter, an extreme state of matter similar to that found at the heart of giant planets like Jupiter.

The results, led by Sophia Malko of the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), detail a new technique for measuring the “stopping power” of nuclear particles in plasma using ultra-intense, high-repetition-rate lasers. Understanding the proton stopping force is particularly important for inertial fusion (ICF).

power sun and stars

This process contrasts with the generation of fusion in PPPL, where plasma is heated to temperatures in the millions of degrees in magnetic confinement facilities. Plasma, the hot, charged state of matter made up of free electrons and atomic nuclei or ions, powers fusion reactions in both types of research aimed at replicating fusion on Earth, using the sun and stars as a safe, clean source drives and virtually unlimited energy to generate the electricity of the world.

“Stopping force” is a force acting on charged particles due to collisions with electrons in matter, resulting in a loss of energy. “If you don’t know the proton stopping power, for example, you can’t calculate the amount of energy deposited in the plasma and therefore design lasers with the right energy level to produce fusion ignition,” said Malko, lead author of a paper outlining the findings in nature communication. “Theoretical descriptions of the stopping force in high-energy-density matter, and particularly warm dense matter, are difficult, and measurements are largely lacking,” she said. “Our work compares experimental data on proton energy loss in warm dense matter with theoretical models of the stopping power.”

That nature communication The research investigated the stopping power of protons in a largely unexplored area, using low-energy ion beams and laser-generated warm, dense plasmas. To create the low-energy ions, the researchers used a special magnet-based device that selects the low-energy fixed energy system from a broad spectrum of protons generated by the interaction of lasers and plasma. The selected beam then passes through laser-driven warm dense matter and its energy loss is measured. A theoretical comparison with experimental data showed that the closest fit with classical models sharply disagreed.

Instead, the closest agreement came from recently developed first-principle simulations based on a quantum mechanical approach involving many bodies, or interactions, Malko said.

Precise stop measurements

Precise stop measurements can also improve understanding of how protons produce something called fast ignition, an advanced inertial fusion scheme. “In proton-driven fast ignition, where protons have to heat compressed fuel from very low temperature states to high temperatures, the stopping power of the proton and the material state are tightly coupled,” said Malko.

“The stopping power depends on the density and temperature of the material state,” she explained, and both are in turn affected by the energy deposited by the proton beam. “Therefore, uncertainties in stopping power translate directly into uncertainties in the total proton energy and laser energy needed for ignition,” she said.

Malko and her team are conducting new experiments at Colorado State University’s DOE LaserNetUS facilities to extend their measurements to the so-called Bragg peak region, where the maximum energy loss occurs and where theoretical predictions are most uncertain.

The co-authors of this paper included 27 researchers from the US, Spain, France, Germany, Canada and Italy.