Sun in a Bottle

What promise does nuclear fusion hold in the search for greener energy?

We’re all very excited by the fact that renewables are decreasing in cost and increasing in reliability. However, nobody thinks renewables can take us all the way—probably most of the way. To truly decarbonize, we need something to fill the gaps when the wind doesn’t blow and the sun doesn’t shine. It’s actually very hard. People have talked about having batteries that store enough energy, but the U.K. figures I am familiar with, due to my work on U.K. energy policy, show that during the month of December in England, the wind is terribly light and the offshore wind turbines don’t produce very much at all. There is not much sun either, so storing enough energy to power the whole month of December in the U.K. requires a battery that doesn’t exist yet and probably never will. So energy storage is a very difficult thing.

We need what’s called firm energy sources. These are ones you can turn on and turn off. Fossil fuels do that in a beautiful way. The gas turbine is a very cheap and easy technology to turn on when your wind doesn’t blow, right? But we can’t do that, and the options now are very few and far between: there’s nuclear fission, and there’s carbon capture and storage, which we could do for a bit but not for very long because we don’t have enough places to store the carbon. This is where fusion would really fill the gap.

Fusion is an energy source that has enough resources that could power the planet until the planet doesn’t exist anymore, about 4.5 billion years. Here at Princeton in 1994, we were the first people to really set the bar—10 million watts of fusion for a second, which meant that we got the conditions for fusion. You must take the fuel up to a temperature of about 200 million degrees (Celsius) when it’s in the form of what’s called a plasma, an ionized gas. We got the conditions for fusion, and it was a very big achievement; we were the first to demonstrate controlled fusion on Earth despite being a long way from commercial fusion. Since then, people have created fusion in a number of devices, and each one has gotten a little bit better. The consensus is that we can produce fusion but not at a competitive cost yet. I believe, like many people, that once you know you can do something, then innovation will take you all the way to make a product.

Fusion works. It will probably take a sizable percentage of the energy market because it can replace coal-fired power stations, any kind of power station, and make electricity in large amounts in a very flexible way. The world energy market is about $10 trillion a year. If you capture 10% of the market, it is a huge incentive to get fusion to work. About two and a half, three years ago, the U.S. government sort of turned around and said, “Well, we’ve been kind of moseying along with fusion research, really understanding a lot of the science behind it, but actually, now we need to think about the endgame. How do you get to a commercial reactor?”

Simultaneously, many private fusion companies are sharing ideas on how to make a reactor. This is a very exciting time for fusion. As we try to accelerate our progress to try to deliver fusion, I believe we will have some electricity by the end of the 2030s. Some of the private companies are promising electricity by the end of this decade. I don’t think that’s possible, but I want to be surprised.

Why is fusion so much more difficult than nuclear fission?

To get a fusion reaction to happen, you must get two nuclei together and fuse them. Usually, it is very light nuclei like hydrogen. The easiest way to do it is between two kinds of hydrogen: deuterium, which is heavy hydrogen, and tritium, which is super-heavy hydrogen. Both are positively charged.

As you bring them together, they repel at a distance because of those positive charges. But when you get them very close, they attract because of the strong force which binds the nucleus together. To get it to happen, you have to ram them together with sufficient energy that they get over that repulsion to attract, stick together, and fuse. That energy is quite considerable. In order to actually get it to happen, you have to take your deuterium and tritium and heat it up to 200 million degrees and get it to start fusing when it’s there. That’s basically what goes on inside the sun and inside the stars. But stars confine themselves by huge gravitational fields.

We have figured two ways to do this. One is called magnetic fusion, where you hold the fuel in a cage of magnetic field, and that’s what we do at Princeton. The other is inertial fusion, where you make a tiny little pellet, and you heat that pellet and compress that pellet with a laser until it reaches these extreme conditions and then it explodes. You don’t try to contain it, you just let a little explosion go off. Two years ago, the National Ignition Facility [at the Lawrence Livermore National Laboratory in California] made for the first time a net amount of energy out of the pellet by making a little explosion. The amount of energy they’re getting out of the pellet now is several megajoules. A megajoule is the equivalent energy of a hand grenade. This tiny little thing that’s the size of a peppercorn explodes with the energy of three hand grenades. It’s really spectacular. But to make that happen, they had to put two megajoules of energy in laser light into it to compress it, but it was just a phenomenal piece of science.

We’re hoping very soon to be able to show with a magnetic system that we can hold and make it fuse for fractions of an hour and sustain its own fusion reactions in the next generation of machines. But, again, that’s not commercial fusion. That’s just demonstrating we can do it. Commercial fusion will mean that the object must do it 24/7 and it has to do it when you turn it on at the (switch on the) wall. So there’s a lot of technology development. One of the good things that has helped renewables was that each unit is quite small: you can make one windmill, or you can make a solar panel of solar cells and test out efficiencies and optimize the engineering. Every fusion device we’ve ever made has been very expensive. We’re making an international fusion experiment called ITER in Southern France. That experiment is over $20 billion in cost. It’s hard to do innovation in an environment where every step costs multiple billions. But we’re doing it because governments around the world are motivated and this may be the only solution.

How is the Princeton Plasma Physics Lab bringing us closer to the goal of reliable fusion?

We’re about to turn on a device with a novel configuration that we believe will shrink the size and scale of a fusion reactor. Instead of being very big and producing lots of energy, it’s something that’s smaller, cheaper, and faster. It may meet the market need better because we prefer dispersed markets these days. This device is called a spherical tokamak, an idea we started experimenting with 20 years ago. The first experiments were very promising, so we made a bigger one. And now, we’ve made an even bigger one. We have reached a scale where we’re going to see fusion conditions in this device. It’s not that we have had all the innovation we need; we need to drive the innovation for a few more years to get all the ideas in place.

From an environmental standpoint, how does fusion compare with fission?

It produces very little environmental impact. The waste product is helium, which is neither radioactive nor reactive; it’s an inert gas. At the end of the life of your reactor, you let it cool down, and then you have very little waste to dispose of. So it doesn’t have the waste burden of fission. Fusion reactors can be made safe. They don’t have the possibility of runaway fusion reactions or meltdown situations like those you might worry about with fission reactors. I don’t want to dismiss fission reactors, because the enemy right now is climate change. Climate change is going to affect everybody’s lives. The sooner we reduce our carbon emissions, the less impact we’re going to get from the nasty things that climate change can bring. I’m just saying that fusion is more attractive than fission in many ways. The difference is that we can do fission today and we have to wait for fusion.

The question with fusion again is not if but when we are going to do fusion. When we started trying to do fusion 60 or 70 years ago, we knew that the sun did fusion, but back then we wondered if we could do it on Earth. Extraordinary conditions inside of our machines—I mean, 200 million degrees, that’s just incredible. But we’ve actually done that. And in some sense, we’ve put the sun in a bottle. But you know, the end of the road isn’t just putting the sun in a bottle; it’s putting the sun in a cost-effective bottle and selling electricity to the country.