IT'S A QUANTUM UNIVERSE

Nuclear Fusion Experiment Just Made Something Very Strange Happen

Jess Thomson - Yesterday 

In a recent study, charged atoms, also known as ions, have been found to behave strangely during nuclear fusion reactions, in ways that scientists did not expect.


Stock image of an atom. Nuclear fusion experiments have found that ions in nuclear fusion reactions behave differently to what was expected.© iStock / Getty Images Plus

According to a paper published on November 14 in the journal Nature Physics, researchers at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory discovered that when deuterium and tritium ions, which are isotopes of hydrogen with one and two neutrons, respectively—are heated using lasers during laser-fusion experiments, there are more ions with higher energies than expected when a thermonuclear burn starts.

"The process of inertial confinement fusion (ICF) squeezes a small (1mm radius) capsule filled with a layer of frozen deuterium and tritium (isotopes of hydrogen) surrounding a volume of deuterium and tritium gas down to a radius of about 30 micrometers. In the process, these isotopes of hydrogen ionize and a plasma of electrons, deuterium and tritium nuclei [is the result]," Edward Hartouni, a physicist at NIF and a co-author of the paper, told Newsweek.

"This plasma is so dense that collisions of these charged particles (electrons and ions) happen very frequently," Hartouni said. "At low temperatures, the ions mostly scatter elastically, as if they were billiard balls. But as the temperature of the plasma increases, which it does as it is squeezed, some of these collisions result in the fusion of the ions. The fusion releases tremendous energy."

"Of the three types of fusion that can occur, the fusion of the deuterium and tritium ions occurs more frequently, and releases the largest amount of energy," he continued. "This energy is in the form of the kinetic energy the fusion [produces], which for deuterium and tritium fusion are an alpha particle (the helium ion) and a neutron," Hartouni said.

In essence, the lasers heat the hydrogen fuel to enormous energy levels, leading them to collide and fuse together to form helium atoms—this is the reaction that powers the sun. This reaction also releases huge amounts of energy, which further heats the hydrogen fuel.

This extra energy can eventually power the reaction without the need for the lasers, having become what is known as a "burning plasma." This "ignition" was only achieved for the first time in 2021, also by NIF, in a milestone achievement for the field.

"If the conditions are right, this process 'runs away' and we have thermonuclear burn," Hartouni said. "It is the goal of the research to study the conditions that lead to controlled thermonuclear burn, which could be an energy-producing technology."

"The goal of the National Ignition Facility is to study this process and learn how to create these conditions. NIF is the first facility to routinely achieve burn plasma conditions and enable experiments to compare with our theoretical expectations. We would expect to be surprised as we haven't (previous to NIF) been able to study burning plasmas experimentally," Hartouni said.

The researchers measured the temperature of deuterium and tritium fuel ions by analyzing the distribution of the neutrons that are flung out during these fusion reactions and found that there are more ions with higher energy in reactions where a burning plasma is achieved compared to previous experiments with non-burning plasmas. This suggests, the authors say, that ions behave differently in a burning plasma.

"We don't know the reason for this at the moment. We have looked back at previous shots and see that our most 'successful' shots have a larger departure from our expectation than the 'unsuccessful' shots; the measure of success being how large the shot yield (measured in the number of neutrons produced) compared to the calculated yield. Since the latest data point in the paper, subsequent shots with higher yields, and thus more robust thermonuclear burn, reveal that this departure from Maxwellian behavior is getting larger," Hartouni said.

These results are surprising, and show the importance of funding for research in such a growing field, said Stefano Atzeni, a physicist at the Università di Roma "La Sapienza", in Italy and author of an accompanying Nature Physics News and Views paper.

"This result has only been possible thanks to extremely sophisticated (and large, and expensive!) instrumentation. The main lesson learned from these measurements is that when a new 'regime' is entered, fundamental research is needed. Theoretical expectations help, but must be confirmed," he told Newsweek.

"These results make clear that we cannot take for granted our models, developed for plasmas under different conditions. More generally, the lesson is that we cannot rely on large extrapolations of previous results."

The results will also help to make future experiments in fusion more accurate.

"The design of possible future laser-fusion energy sources will depend on our ability to make simulations, and those simulations rest on the foundation of our basics science understanding of the process," Hartouni said. "The simulations are based on models, and experimental results allow us to confront these models with reality. This study explores the new territory opened up by NIF creating burning plasmas, and the sophisticated set of diagnostics, both the hardware and the analysis which have been developed for this exploration."

"As we extend our observations and analysis of the experiments, and conduct new experiments to address our hypotheses, we will inform our basic science understanding of the laser-fusion process," Hartouni said. "This understanding will allow us to incorporate more realistic models and make better predictions of the laser-fusion process on which to base potential future designs."