What is nuclear fusion, and why is everyone so excited about it?

In this 2012 image, technicians inspect optics inside a preamplifier support structure at Lawrence Livermore National Laboratory in Livermore, California.Damien Jamieson/AP

U.S. Department of Energy officials will announce a major milestone for nuclear fusion, the process that powers stars and could one day provide Earth with a long-term, carbon-free energy supply.

At a news conference scheduled for Tuesday, researchers at the National Ignition Facility at Lawrence Livermore National Laboratory in California are expected to announce that they have achieved a net energy gain with long-term experiments.

That threshold meant that fusion reactions released more energy during the experiment than was needed to trigger them. If confirmed, such a feat would set a new benchmark for the field and add further impetus to the many companies trying to develop commercial fusion energy pathways.

The media has been buzzing about the expected announcement since it was first reported by the Financial Times on Sunday. But those who follow the field say the news is not a surprise.

In August, 202 scientists at US labs reported that they had achieved a net energy gain of 70% and did not foresee any obstacles to achieving their goal.

The facility was built in the late 1990s and the first tests were carried out in 2009. It uses the world’s most powerful laser to blow up small BB-sized targets filled with hydrogen isotopes. The force of the laser blast compresses the isotopes to a state where hydrogen can be converted to helium, releasing energy instantaneously in the process.

This approach, known as inertial confinement, is one of two approaches to fusion that have long been used in government-funded mega-megaprojects.

The other, called magnetic confinement, involves trapping hot plasma in a powerful magnetic field until fusion reactions occur. That’s the strategy behind ITER, a gigantic demonstration reactor nearing completion in France and expected to start operating in 2025.

Neither facility is intended to use its energy output to provide electricity. Experts say it will be years, if not decades, before the engineering challenges are overcome on a meaningful scale.

At the same time, many companies have been on the lookout for new technologies that may offer shortcuts to commercial convergence.

These include Commonwealth Fusion Systems in Massachusetts, which is currently building its first demonstration reactor, a more compact form of magnetic confinement, also expected to be operational by 2025.

Vancouver-based General Fusion has a different reactor design that combines elements of magnetic and inertial confinement. Its demonstration facility will be ready for testing in 2027.

High public excitement has become a regular feature of fusion announcements in recent years. It boils down to the promise of an unlimited source of energy that can run continuously and be independent of fossil fuels.

Fusion is also considered less risky than conventional nuclear power, which is based on a different type of nuclear process called fission. Fusion also doesn’t produce nuclear waste in the form of spent fuel, a big downside of fission energy.

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