The largest fusion reactor in the world fired up in Japan. Here's how the $600 million device compares to the US's revolutionary fusion machine.

• Japan's experimental nuclear fusion reactor is up and running.
• JT-60SA is currently the most powerful experimental fusion reactor in operation.

Nuclear fusion has the potential to revolutionize the world by creating abundant, emissions-free energy.

It's estimated that fusion could produce nearly 4 million times as much energy as fossil fuels like coal or oil.

We just have to figure out how to harness all that power.

It's easier said than done. Scientists have been trying to figure it out for decades.

Now, they have a new weapon to wield in their fight: JT-60SA, the most powerful nuclear reactor ever built.

Japan's $600 million nuclear reactor

Japan's new 50-foot-tall, 40-foot-wide reactor is a type of donut-shaped device called a tokamak.

Inside the tokamak, gas heats up to hundreds of millions of degrees Fahrenheit, which forms a plasma where hydrogen atoms can bind together and fusion can take place.

JT-60SA can hold roughly 30% to 40% more plasma than the UK's Joint European Torus, which used to be the largest operating tokamak.

Located in Naka, Japan, JT-60SA is a collaboration between Japan and the European Union, and it officially began operations on December 1.

The tokamak cost an estimated 560 million Euro (about $600 million) to build and is an experimental device, meaning it's not designed to provide commercial electricity.

Instead, the science that experts learn from it — like how plasma behaves — will go on to help inform research for an even larger tokamak called the International Thermonuclear Experimental Reactor (ITER).

ITER will be twice the size of JT-60SA and generate a volume of plasma five times larger. It was originally scheduled to fire up its first plasma in 2025 but has since been delayed.

Attaining net energy

"JT-60SA is officially the most powerful tokamak in the world until ITER is operational," Aris Apollonatos, a communications specialist at Fusion for Energy, the organization managing the EU's portion of ITER, told Business Insider via email.

The reason ITER is so important is because it's designed to achieve net energy. In other words, once ITER is operational, the plasma it generates is designed to eventually produce 10 times more energy than the energy required to form the plasma in the first place.

That type of breakthrough could pave the way for a new era of fusion energy that would revolutionize the energy sector.

Why is JT-60SA important?

"JT-60SA offers a big opportunity to the fusion community because it will give scientists the opportunity to learn, get trained, and analyze further plasma parameters," Apollonatos said.

Over the next two years, researchers will make further improvements to JT-60SA, including increasing the heating power to generate fusion reactions and adding new devices to measure the plasma's performance.

Japan is also working on DEMO, a power plant that would demonstrate the capability of harnessing electricity from fusion. They hope to have it running by 2050, and JT-60SA's experiments could prove useful for that facility as well.

Once the updates to JT-60SA are complete, scientists there will "develop plasmas that are similar to those expected in ITER and in DEMO so that we can make experiments to help them prepare," Apollonatos said.

"Are there gotchas there? Can you keep [the plasma] stable for that long?" Vincent Tang, principal deputy director for the National Ignition Facility & Photon Science Directorate at Lawrence Livermore National Laboratory, told Business Insider. "All that's important."

Tang is part of a team that recently achieved one of the most important breakthroughs in nuclear fusion in decades.

How NIF is revolutionizing nuclear fusion

In December 2022, for the first time in history, NIF researchers generated more energy from a fusion reaction than was used to power the reaction.

It was a major breakthrough that scientists at NIF then surpassed the next year. But unlike JT-60SA, which uses a tokamak to produce fusion, NIF uses lasers to implode a capsule of fuel, a process known as inertial confinement fusion.

Tang said NIF is the only facility able to study burning plasma until ITER comes online.

"We are in a completely burning plasma-ignited regime where that fusion reaction is completely bootstrapping itself, heating fuel further," Tang said.

"NIF was never designed to be energy efficient," Tang said. And its main purpose is for weapons research. Yet its breakthroughs have prompted more interest in inertial confinement fusion as an energy source.

The US Department of Energy, for example, recently announced it's investing $42 million in NIF-style fusion for energy purposes.

Both tokamaks and inertial confinement devices have science and engineering challenges to overcome before fusion power plants become a reality.

"I think it's very important to have multiple paths toward fusion energy," Tang said. "The need is so large, it's not going to be just one solution."