Nuclear fusion is back in the news. This week, the US Department of Energy announced what it called a “major scientific breakthrough” in fusion power research: for the first time, a fusion experiment had produced more energy than the energy used to kick off the reaction. It’s not the first time we’ve heard about fusion progress. There have been decades of headlines touting breakthroughs large and small, usually implying that we’re closer than ever to generating all the clean energy we’ll ever need from nuclear fusion.

It’s a lot to take in, so The Verge put together this guide to fusion power with the help of some experts. Below, we’ve summarized scientists’ dreams for fusion, as well as the harsh realities the technology faces to bring the power of fusion from scientific ambition to commercial reality.

What is nuclear fusion?

Nuclear fusion has been an elusive energy dream for the better part of a century. In theory, it sounds sort of simple. Stars, including our Sun, create their own energy through a process called fusion, which is when atoms get fused together at high temperatures and pressures to create a heavier atom. Typically, this involves hydrogen atoms combining to form helium. The reaction releases a ton of energy, which is why scientists on Earth want to replicate it in a controlled way. (They’ve managed to do it in an uncontrolled way before. It’s called a hydrogen bomb.)

How is nuclear fusion different from nuclear fission?

The nuclear power plants we have today generate electricity through fission, which is sort of the opposite of fusion. Fission releases energy by splitting atoms apart rather than fusing them together.

What are the advantages of nuclear fusion?

In theory, once humans figure out how to make nuclear fusion happen in a controlled way, the possibilities are endless. Hydrogen is the simplest and most abundant element in the universe. You can get it from seawater, for example. And if you do, a single gallon of seawater can generate as much energy as 300 gallons of gasoline, according to the Department of Energy. 

Today’s nuclear reactors have a big mess to clean up, thanks to fission. By splitting heavy atoms, fission leaves behind radioactive waste. What to do with that nuclear waste for millions of years to come is an environmental nightmare that the US still hasn’t figured out.

Fusion doesn’t have these problems. With fusion, you’re building new atoms — usually helium, as in the stuff that’s in balloons. It doesn’t generate greenhouse gas emissions. What’s more, this is a potentially limitless energy source that doesn’t rely on the weather, which is still a challenge with renewables like solar and wind power.

Why haven’t we been able to make ignition happen?

Well, turns out, it’s really hard to recreate a star in a lab. To trigger fusion, you need tremendous amounts of pressure and heat. The environment in the heart of the Sun naturally provides the extreme pressure needed for fusion to take place. Here on Earth, scientists don’t have that kind of pressure just lying around and need to hit temperatures even hotter than the Sun to get the same reaction. Historically, that’s taken more energy than scientists have actually been able to generate through fusion in a lab.

This also takes extraordinary amounts of money and highly specialized technology. With all that in mind, it’s amazing that we’ve managed to make any scientific progress at all. Actually commercializing it? That’s got another mountain of issues that we’ll talk about in just a little bit.

What’s this new “nuclear fusion breakthrough” everyone’s talking about?

On Monday, December 5th, at 1:03AM, researchers at the Lawrence Livermore National Lab achieved “fusion ignition” for the first time on Earth.

Simply put, “They 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,” White House Office of Science and Technology Policy Director Arati Prabhakar said at a press conference announcing the achievement on December 13th.

Specifically, the experiment yielded 3.15 megajoules of energy compared to the 2.05 megajoules the lasers used to trigger the fusion reaction. That’s about a 1.5 gain in energy. It’s modest, but achieving a net energy gain was an important first for fusion research nevertheless.

How did they do that?

Researchers used the world’s largest and highest-energy laser system, called the National Ignition Facility (NIF). NIF is as big as three football fields, capable of firing 192 powerful laser beams at a single target. To reach fusion ignition, energy from those 192 laser beams squeeze fuel within a diamond capsule roughly the size of a peppercorn and 100 times smoother than a mirror. The capsule holds hydrogen isotopes, some of which “fused” together to generate energy. All in all, about 4 percent of that fuel was converted to energy.

Lasers are neat. Tell me more about the diamonds, too.

“The fuel capsule is a BB point sized shell made of diamond that needs to be as perfect as possible,” Michael Stadermann, Target Fabrication Program manager at Lawrence Livermore National Laboratory, said during the December 13th press conference. “As you can imagine, perfection is really hard, and so we’ve yet to get there — we still have tiny flaws on our shells, smaller than bacteria.”

Symmetry plays a huge role in achieving ignition when it comes to both the target and its implosion. The lasers need to be aligned properly, and when it comes to the target, you need to maintain near-perfect symmetry while blasting your target with intense pressure and heat. It’s like compressing a basketball down to the size of a pea, experts say, all while maintaining a perfect spherical shape. If you deviate from that shape, you waste too much kinetic energy and won’t get ignition.

Does this mean we’re going to have nuclear fusion power now? 

Not by a long shot. While the lab achieved “ignition,” they based their achievement on a limited definition of a “net energy gain” focused only on the output of the laser. While the lasers shot 2.05 megajoules of energy at their target, doing so ate up a whopping 300 megajoules from the grid. Taking that into account, there was still a whole lot of energy lost in this experiment. 

To eventually have a fusion power plant, you need a way, way bigger win than a 1.5 net energy gain. You’ll need a gain of 50 to 100 instead.

So, where do we go from here?

There’s a lot of work to do. Researchers are constantly trying to craft even more precise targets, aiming for that perfectly symmetrical sphere. This is incredibly labor-intensive. So much so that a single pellet target might cost about $100,000 today, according to University of Chicago theoretical physicist Robert Rosner. Rosner has previously served on NIF’s External Advisory Committee. That cost per pellet needs to drop down to a few pennies if nuclear fusion is to go commercial, Rosner says, because a fusion reactor might need a million pellets a day.

And if you want to reach ignition again using lasers, you’ll need a setup that’s more efficient, and that can work much faster. The NIF, as powerful as it is, is based on 1980s laser technology. There are more advanced lasers today, but the National Ignition Facility is a behemoth — its construction started in 1997, and it wasn’t operational until 2009. Today, the NIF can shoot its laser once every four to eight hours. A future fusion power plant would have to shoot 10 times a second, according to Lawrence Livermore National Laboratory plasma physicist Tammy Ma.

“This is one igniting capsule, one time. To realize commercial fusion energy, you have to do many things; you have to be able to produce many, many fusion ignition events per minute,” Kim Budil, Lawrence Livermore National Laboratory director, said at the press conference. “There are very significant hurdles, not just in the science but in technology.”

Are there other ways to fuse atoms together?

Yep, lasers certainly aren’t the only strategy used to trigger ignition. The other major strategy is to use magnetic fields to confine plasma fuel using a device called a tokamak. A tokamak can be much cheaper to construct than the NIF. Even private companies have built tokamaks, so there’s been more widespread research in this realm. 

A tokamak has yet to reach ignition. But the magnets it uses have the potential to sustain a fusion reaction for a longer period of time. (At NIF, fusion reactions occur within a fraction of a nanosecond.) Ultimately, breakthroughs in either branch of research can help bring fusion power within closer reach.

What does reaching “ignition” actually accomplish, then?

“We got to the top of the hill,” Gianluca Sarri, a physics professor at Queen’s University Belfast, tells The Verge. He says accomplishing ignition was essentially the “hardest step” in fusion power research, and it is essentially “downhill” from here even if there’s still a long way to go. 

That said, reaching ignition is more of a scientific breakthrough than one with practical application for our energy system — at least not for many more years.

When it comes to nuclear defense and non-proliferation, however, reaching ignition might have a more immediate impact. 

Wait, what’s this about nuclear weapons?

NIF was initially developed to conduct experiments that would help the US maintain its stockpile of nuclear weapons without actually having to blow any of them up. The 1996 Comprehensive Nuclear-Test-Ban Treaty barred all nuclear explosions on Earth, putting an end to underground test explosions. NIF broke ground the following year. The nuclear ignition it was finally able to achieve in its December 5th experiment essentially mimics the uncontrolled fusion that takes place when a nuclear bomb detonates. The hope is that reaching ignition in a controlled way in a lab will allow researchers to validate the computer models that they’ve developed to replace live test explosions.

Cut to the chase. When are we going to have nuclear fusion power plants?

The most optimistic experts The Verge spoke to hope that we might have the first fusion power plant within a decade. But most experts, while still excited about the future of fusion power, think that we’re likely still several decades away. 

Is this going to solve climate change?

No matter how long it takes, we can’t afford to wait a decade or more for fusion power to clean up pollution from our energy system. To keep global warming from reaching a point at which humanity would struggle to adapt, research shows that the world needs to cut greenhouse gas emissions down to net zero by around 2050. By 2030, carbon dioxide emissions from fossil fuels need to be cut roughly in half. That’s much faster real-world progress than fusion research has ever been able to achieve.