Nuclear

A flash of ‘net energy gain’ as scientists cheer a nuclear fusion breakthrough

Nuclear fusion energy is expected to take decades to become economically viable, but researchers said the laboratory achievement was an important step nonetheless.
A flash of ‘net energy gain’ as scientists cheer a nuclear fusion breakthrough
Dr. Arati Prabhakar, Director of the Office of Science and Technology Policy (OSTP), called the nuclear fusion breakthrough an "engineering marvel."

Researchers at the Lawrence Livermore National Laboratory (LLNL) in California for the first time produced more energy in a nuclear fusion reaction than was used to ignite it, a long-sought accomplishment known as net energy gain.

The extremely brief fusion reaction, which used 192 lasers and temperatures measured at multiple times hotter than the center of the sun, was achieved Dec. 5.

[Researchers] never lost sight of this goal,” said Dr. Arati Prabhakar, Director of the Office of Science and Technology Policy (OSTP). She said those researchers “shot a bunch of lasers at a pellet of fuel and more energy was released from that fusion ignition than the energy of the lasers going in.”

She called the breakthrough for nuclear fusion energy an “engineering marvel beyond belief.”

The experiments were conducted at LLNL’s National Ignition Facility (NIF), a facility large enough to accommodate three football fields. NIF is regarded as one of the world’s most precise and reproducible laser systems. It guides, amplifies, reflects, and focuses its array of laser beams into a target about the size of a pencil eraser in a few billionths of a second, in the process delivering more than 2 million joules of ultraviolet energy and 500 trillion watts of peak power.

With temperatures in the target exceeding 180 million degrees Fahrenheit and with pressures of more than 100 billion Earth atmospheres, hydrogen atoms in the target fuse and release energy in a controlled thermonuclear reaction.

NIF is a part of the National Nuclear Security Administration’s Stockpile Stewardship Program which is charged with maintaining the reliability, security, and safety of the U.S. nuclear weapon deterrent without full-scale testing. The High Energy Density science program studies material behavior under extreme pressure, enabling researchers to conduct weapon physics experiments in a controlled laboratory environment once possible only with underground testing.

The target chamber of LLNL’s National Ignition Facility, where 192 laser beams delivered more than 2 million joules of ultraviolet energy to a tiny fuel pellet to create fusion ignition on Dec. 5, 2022. Credit: LLNL

Proponents of fusion, the energy that powers the sun and stars, hope that it could one day also produce nearly limitless, carbon-free energy, helping accelerate the planet away from fossil fuels. Commercial nuclear fusion energy is expected to take decades to become economically viable. It will need to produce significantly more power and for longer period of time than was achieved in the laboratory setting. Backers called the LLNL milestone an important step, nonetheless.

“This is a landmark achievement for the researchers and staff at the National Ignition Facility who have dedicated their careers to seeing fusion ignition become a reality, and this milestone will undoubtedly spark even more discovery,” said Energy Secretary Jennifer Granholm at an announcement event Dec. 13.

Achieving net energy gain has been challenging because fusion happens at such high temperatures and pressures that it is incredibly difficult to control.

Fusion works by pressing hydrogen atoms into each other with such force that they combine into helium, releasing enormous amounts of energy and heat. Unlike other nuclear reactions, fusion does not create radioactive waste, a potential benefit.

In the case of fusion that takes place in the sun, its intense heat and the pressure exerted by its gravity allow atoms that would otherwise repel each other to fuse.

Decades of effort

Scientists have long understood how nuclear fusion has worked; they have tried to duplicate the process on Earth as far back as the 1930s.

Current efforts have focused on fusing a pair of hydrogen isotopes — specifically, deuterium and tritium — a particular combination that DOE said releases “much more energy than most fusion reactions” and requires less heat to do so.

While experimentation has taken place many times in recent years, LLNL scientists on Dec. 5 designed an experiment in which the fusion fuel stayed hot enough, dense enough and round enough for long enough to achieve ignition.

To create fusion ignition, the National Ignition Facility’s laser energy is converted into X-rays inside the hohlraum, which then compress a fuel capsule until it implodes, creating a high temperature, high pressure plasma. Credit: LLNL

In short, the experiment “produced more energies than the lasers had deposited: About two megajoules (MJs) in, about three megajoules out,” said Dr. Marvin Adams, who serves as Deputy Administrator for Defense Programs. “The energy production took less time than it takes light to travel one inch.”

Ions behave differently

LLNL researchers in mid-November reported that ions undergoing fusion have more energy than previously expected. They said the observation provided insight for the future design of a laser­–fusion energy source.

According to the researchers, experimentation specifically showed that the average neutron energy produced is higher than expected for a deuterium-tritium (D-T) plasma that is in thermal equilibrium.

The findings are featured in a new paper in the Nov. 14 issue of Nature Physics.

While researchers said they lacked a clear understanding of what drove that observation, they said it is one of the most direct measurements of the ions undergoing fusion.

Researchers said this allowed them to diagnose important imbalances in the drive and capsule symmetry that can lead to poor implosion performance. It also gave researchers a window into how hot the plasma is. For a hotter plasma, the ions are on average all moving faster in every direction, so the deuterium and tritium ions collide at higher velocity and that extra energy is shared among the neutron and alpha particle released by the reaction.