Scientists who are working toward the dream of nuclear fusion, a form of power that could potentially provide abundant clean energy in the future, have discovered surprising and unexplained behavior among particles in a government laboratory, reports a new study. The results hint at the mysterious fundamental physics that underlie nuclear fusion reactions, which fuel the Sun and other stars.
Researchers at the National Ignition Facility (NIF), a device at the U.S. Department of Energy’s Lawrence Livermore National Laboratory (LLNL), recently celebrated the milestone of creating what’s known as a “burning plasma,” which is an energized state of matter that is mostly sustained by “alpha particles” created by fusion reactions. The NIF has also reached the threshold of producing “ignition,” meaning fusion reactions that are self-sustaining, which is a major breakthrough, though it will likely still take decades to develop a fusion reactor—assuming it is possible at all.
Now, a team led by Ed Hartouni, a physicist at LLNL, has revealed that particles inside burning plasmas have unexpectedly high energies that could open new windows into the exotic physics of fusion reactors, which “could be important for achieving robust and reproducible ignition,” according to a study published on Monday in Nature Physics.
“This is a new regime of plasma; NIF diagnostics have made it possible to study these things in ways we couldn't do before,” said Hartouni in a call with Motherboard that also included study authors Alastair Moore, a physicist at LLNL, and Aidan Crilly, a research associate in plasma physics at Imperial College London. “We're able to see things at a level that we hadn't been able to see before, and there are surprises with these plasmas in an actual laboratory.”
“It's a really exciting time for us to finally have an almost-igniting facility and experiments to understand this physics that we haven't really been able to understand before and begin to get to the point where we can think about what a future fusion facility might look like,” added Moore.
The team discovered the strange behavior of the ions while examining observations from several experiments that have occurred at NIF in recent years. These tests involve fusion between particles called ions, which are atoms that don’t have the same number of positive and negative components (protons and electrons), leaving them with an electric charge.
Using dozens of lasers to heat deuterium and tritium ions, which are both heavier versions of hydrogen, NIF researchers generate fusion reactions between the ions. In a burning plasma, the reactions between the ions produce new entities, called alpha particles, that drive up higher temperatures that, in turn, spark even more reactions as part of a thermonuclear burn.
Hartouni and his colleagues have now shown that NIF experiments that produce alpha particles consistently show ions with higher energies than predicted by models, though the source of these energy boosts “is an open experimental question,” according to the study. The team presented four possible explanations for the observation, including so-called “kinetic effects” that have been speculated about in previous theories, but it will take more experiments and meticulous research to understand the underlying mechanisms at work in the plasma.
“It's a mystery, but there's multiple hypotheses,” said Crilly. “Whether it's one on its own, like this kinetic effect, or it's a combination of them and they all add their little bit to that gap.”
“It's worth thinking about how extreme these conditions are and why it's hard,” he added, noting that the NIF fusion reactions occur at temperatures around 180 million degrees Fahrenheit and in conditions that are 30 times more dense than the Sun. Within this otherworldly environment “we need to understand exactly how an alpha particle bumps into all these other particles and distributes its energy and how they all collide,” Crilly noted.
To that end, the team plans to continue searching for clues about the weird ion behavior in both models and experiments. Given that NIF has produced this unprecedented glimpse into the weird world of fusion reactions, the facility is bound to find strange new insights no matter what direction their research takes them.
“We've never been able to study this before,” Moore said. “This is the first burning plasma that we've ever created on the planet, so it's pretty amazing.”