On Wednesday, U.S. government scientists announced that they had taken an important step toward making nuclear fusion – the process that powers stars – a viable energy source for humanity.

Using the world’s largest laser, the researchers coaxed fusion fuel to heat itself beyond the heat they zapped into it for the first time, achieving a phenomenon known as a burning plasma that marked a step toward self-sustaining fusion energy.

The amount of energy produced was small – roughly the equivalent of nine nine-volt batteries used to power smoke detectors and other small devices. However, the experiments at the Lawrence Livermore National Laboratory in California represented a watershed moment in the decades-long quest to harness fusion energy, even though the researchers cautioned that years of additional work are required.

Nuclear fusion, which is the combining of atomic nuclei to release energy, was used in the experiments to produce self-heating matter in a plasma state. Plasma is one of the four states of matter, the others being solid, liquid, and gas.

“If you want to make a camp fire, you want to get the fire hot enough that the wood can keep itself burning,” said Alex Zylstra, an experimental physicist at Lawrence Livermore National Laboratory, part of the United States Energy Department, and lead author of the study published in Nature.

“This is a good analogy for a burning plasma, where the fusion is now beginning to self-sustain,” Zylstra said.

192 laser beams were directed at a small target containing a capsule less than a tenth of an inch (about 2 mm) in diameter filled with fusion fuel consisting of a plasma of deuterium and tritium – two hydrogen isotopes, or forms.

When the nuclei of deuterium and tritium fuse at very high temperatures, a neutron and a positively charged particle called a “alpha particle” – consisting of two protons and two neutrons – emerge, and energy is released.

“Fusion necessitates heating the fuel to extremely high temperatures in order for it to burn – similar to a regular fire, but for fusion we need around a hundred million degrees (Fahrenheit). We’ve been able to induce fusion reactions in experiments for decades by putting a lot of heat into the fuel, but this isn’t good enough to produce net energy from fusion “According to Zylstra.

“For the first time, fusion reactions in the fuel provided the majority of the heating, indicating that fusion is beginning to take precedence over the heating we did. This is a brand-new regime known as a burning plasma”, according to Zylstra. Unlike burning fossil fuels or the fission process used in existing nuclear power plants, fusion promises abundant energy with no pollution, radioactive waste, or greenhouse gas emissions. Nuclear fission energy is created by splitting atoms. Fusion energy is created by fusing atoms together, just like inside stars like our sun.

Several private-sector ventures, including dozens of companies and institutions, are also pursuing a fusion energy future, with some oil companies even investing.

“Fusion energy is the holy grail of clean limitless energy,” said Annie Kritcher of Lawrence Livermore National Laboratory, who led the experiments at the National Ignition Facility in 2020 and 2021 and was the first author of a companion paper published in the journal Nature Physics.

In these experiments, fusion produced roughly ten times the energy used to heat the fuel, but less than ten percent of the total amount of laser energy because the process is still inefficient, according to Zylstra. In each experiment, the laser was only used for about 10 billionths of a second, with fusion production lasting 100 trillionths of a second, according to Kritcher.

The progress, according to Zylstra, is encouraging.

“Making fusion a reality is an enormously complex technological challenge that will necessitate significant investment and innovation to make it practical and cost-effective,” Zylstra said. “I see fusion as a decadal-scale challenge to become a viable energy source.”