The favorite fusion reaction of star builders is the one between deuterium and tritium, which, compared to other fusion reactions, is easy to do. It’s one hundred times more likely to result in fusion than the deuterium-deuterium reaction, for the same input energy. Deuterium-tritium fusion also releases a lot of energy when successful, which is why NIF, the Culham Centre for Fusion Energy, Tokamak Energy, and First Light Fusion are all planning to use it to deliver net energy gain. There are only a handful of star builders not using this reaction.
However, even though the deuterium-tritium fusion reaction has the best odds of any fusion reaction, the chances of a successful collision are low; it’s still throwing darts blindfolded.
The particle-smashing approach leads to many more misses than hits, and always uses up more energy than is created by successful reactions.14 For the star builders, it isn’t good enough, says Professor Sibylle Günter of the Max Planck Institute for Plasma Physics. “If we just smash individual nuclei into each other, it can be easily demonstrated that the energy required for accelerating the particles would exceed the energy gained from fusion.” Particle smashing is never going to be a route to fusion energy. “The key issue is not to demonstrate the nuclear reactions,” she explained, “but to gain energy in a profitable way.”
For nuclear fission, physicists eventually realized that they could sidestep the problem entirely by chaining reactions together. In fact, it was Rutherford’s “moonshine” comment that prompted another scientist to work out how to do it. With the right nucleus, a neutron can cause a fission reaction that releases neutrons, those neutrons can split more atoms that release neutrons, and so on, with each step in the chain releasing nuclear energy. If there’s more than one neutron released, the chain grows too, increasing the energy released in each generation. Because there are stray neutrons in the environment, there’s not even any need of a particle accelerator; put enough fissile material in one place (a critical mass) and a chain-reaction spontaneously begins. The detritus from so many atoms splitting is what creates radioactive waste.
Fusion doesn’t split atoms, it combines them, so it can’t use the same trick. For fusion reactions to come anywhere close to being able to produce net energy gain, nuclei need to collide in phenomenally huge numbers repeatedly. It’s like buying millions of lottery tickets instead of one. Fortunately, nature has provided the perfect crucible for fusion reactions.
What Every Star Machine Needs
Star builders need a way to crash particles together not just once, but over and over again. To achieve this, they’ve turned to a strange state of matter; one that is rare on Earth, but prevalent in the rest of the universe: plasma. It’s the stuff that stars are made of, and it’s perfect for fusion.
Plasmas are the fourth state of matter, after solids, liquids, and gases. You’ve probably heard of solids, liquids, and gases, but you might not have heard of plasmas. They’re very different from their cousins, in ways that are beguiling, mysterious, and—for the star builders—often frustrating. To us, plasmas are the most rarely encountered of the four states of matter. But when we turn our telescopes to the visible universe, they’re the most common state of matter; they make up 99 percent of it. You can see plasmas on Earth too, whether in the form of lightning, the aurorae at the Earth’s poles (also known as the Northern and Southern Lights), or fluorescent bulbs—you might even be reading this book by light generated from a plasma.
While the other three states of matter are composed of atoms, plasmas are composed of atoms that have separated into their constituent nuclei and electrons. Plasmas are clouds of ripped apart atoms that don’t behave anything like the other three states of matter.
The state of matter of any element varies depending on the conditions it’s in, chiefly the temperature and the pressure. On Earth’s surface, nitrogen (which makes up 78 percent of air) is a gas, water is mostly a liquid, and silver is a solid. However, take nitrogen down to below -210 degrees Celsius (-346 degrees Fahrenheit), and it becomes a solid; water turns into a gas at 100 degrees Celsius (212 degrees Fahrenhiet); and you have to heat silver to 962 degrees Celsius (1,764 degrees Fahrenheit) before it turns into a liquid. Changes in temperature and pressure cause the same atoms to appear in a different form.
Temperature is a measure of average speed at the atomic scale, so more heat means more movement, and more vibration. The molecules in liquid water are bound together by relatively weak forces. Put in a little heat, and the water molecules vibrate more. With enough heat, the energy of the vibrations is more than the energy of the bonds and the molecules break free, turning into a gas. Once free, they zip along until they bounce off another molecule or a wall.
Add more energy to a gas, raise the temperature even higher, and eventually the bonds within the atoms will also sever—this is a plasma. Each atom of hydrogen separates out to become a nucleus and an electron. Because the electromagnetic force acts over long distances, the cloud of electrons and nuclei that exists in a plasma moves in complex synchronized motions; an endless, frenetic dance.
Only when the plasma is hot enough do nuclei collide with each other with enough energy to overcome the electromagnetic repulsion of their positive charges, and for fusion to occur. For nuclear fusion reactions to be successful, particles have to slam together at around 3 million meters (a little under 10 million feet) per second, and do so over and over again. In a successful collision, the nuclei snap together in a fusion reaction. Star builders must put lots of energy into their plasmas, obtaining temperatures of many millions of degrees, to maximize the chances that enough collisions will occur with enough energy to result in fusion. Plasmas are as important for fusion reactors as ice is for making an igloo.
While plasmas are the best bet for getting significant numbers of fusion reactions to happen, that doesn’t mean it’s particularly easy. Understanding and controlling plasmas is one of the biggest challenges facing star builders. The problem is that plasmas are incredibly complex. The frenetic dance of charged particles means that every part of the plasma is pushing or pulling on all of the other parts. Particle physicists, like those working at CERN, try to understand what happens when two particles collide. The long-range electromagnetic forces in a plasma mean that star builders are trying to understand what happens when 100 to 1015 (that’s 10 followed by 15 zeroes) particles collide simultaneously with one another. Plasmas are connected, kind of like a jelly: if you push on one bit, you’ll find everything else moving. The way that a plasma interacts with itself results in weird and unexpected behaviors; for example, plasma often caused a communications blackout during space shuttle reentry, and the wrapper of plasma in the Earth’s atmosphere, called the ionosphere, allows shortwave radio enthusiasts to speak to one another at distances of thousands of miles.
To make progress toward understanding—let alone controlling—plasmas, physicists have had to patch together classical mechanics, quantum mechanics, laser science, nuclear physics, extreme engineering, statistical mechanics, thermodynamics, experimental physics, computer science, and electromagnetism, to name a few. The most recent addition to this mix is quantum electrodynamics, the field of physics that describes how charged particles and light interact. The difficulty of understanding plasmas is why Sibylle Günter’s entire institute is dedicated to the study of them, and why the most prestigious prize in mathematics, the Fields Medal, was awarded for painstaking steps toward a better understanding of them in 2010. “It is only the plasma itself which does not ‘understand’ how beautiful the theories are and absolutely refuses to obey them,” Hannes Alfvén said in his 1970 Nobel Prize lecture.15 Plasma physics is complicated.
This matters. Star builders’ lack of understanding of plasmas is a threat to how quickly they can reach net energy gain. It’s been a problem right from the start. “Of plasma instabilities,” the person who first came up with using lasers for fusion, John Nuckolls, has reflected, “we knew nothing (and had a lot to learn).”16 And it’s still a probl
em today. Mark Herrmann, NIF’s director, confessed that he doesn’t think anyone fully understands the plasma physics involved in his machine. “There are parts of the problem that we understand better and there are parts of the problem that we understand worse.” Mark said that by driving so much energy into the plasma, as they do at NIF, it sets up waves within it, like a big rock chucked into a pond.
“Those [plasma] waves can conspire in lots of different ways; some of them can conspire to reflect light back out.” That’s not good for the success of their fusion machine.
The kind of fusion with which Ian Chapman’s machine is leading the world is also highly dependent on plasma physics. The plasma created at Culham can go from flowing smoothly to chaotically, as if a stream suddenly gushed like a waterfall. One of the Centre’s academic consultants, Professor Howard Wilson from the University of York, said: “I would contest that if it wasn’t for plasma turbulence we would have a fusion reactor working now; we probably would have had a fusion reactor working for some decades!”
At Tokamak Energy, “plasma!” was the response I got when I asked about their biggest scientific challenge. Their competitor, First Light Fusion, said that being able to fully predict what went on in a fusion plasma was “the dream” but “a bit far-fetched.” Although she is scientific director of the Max Planck Institute for Plasma Physics, Sibylle Günter’s response when I asked if she and her colleagues fully understand the plasma physics on their fusion machine was “No, of course not.” I asked Mark Herrmann whether a perfect understanding of plasma physics would allow net energy gain to be achieved. “That understanding would certainly tremendously speed up the rate of progress,” he replied with enthusiasm. Plasmas are key to building a star.
Getting the plasma hot is not the only thing that the star builders need to worry about. They need to keep the plasma hot too; to stop its energy from escaping by putting it into a container. But these temperatures are literally too hot to handle. There is no known material that can do the job. The highest melting point of any metal is that of tungsten, at over three thousand degrees. Fusion plasmas reach over 100 million degrees. If a fusion plasma touches a material, that material is vaporized. Catastrophically for fusion, the plasma also loses its energy and cools down.
Finally, getting more energy from fusion reactions than is put in is all about collisions between nuclei within the plasma. How close together the particles are in the plasma is just as important as the temperature. If you’re in a mostly empty train carriage, you’re unlikely to bump into other passengers despite the shaking of the train as it goes over the tracks. Now imagine it’s rush hour. There’s barely a patch of free space. No matter how small the bump, you’re so close to your neighbors that you’re constantly colliding with them. That’s why social distancing is used to slow the spread of pandemics. And it’s also why packing more nuclei into the same amount of space—or increasing the plasma density—means more collisions, and more chances for fusion.
For fusion to work, the plasma has to be kept hot, as dense as possible, and well confined.
Rutherford died in 1937, just one year before physicists demonstrated that chains of fission reactions could be strung together to scale up indefinitely the energy released. Because of that discovery, we’ve had fission power stations, and fission-based nuclear weapons, for decades. Star builders know full well what they need to show that nuclear fusion as a power source isn’t moonshine either—they just haven’t managed it yet.
The trio of temperature, density, and confinement are the most important properties of the plasma in any star machine. Get them right, and the odds for fusion can tip the scales in favor of net energy gain. Different star machines approach the task differently: some are hotter, some denser, and they vary in how they confine the plasma. But there are just three serious methods for confining hot, dense plasma for fusion reactions—by using gravity, magnets, or inertia. Okay, technically there is one more: creating a universe from scratch.
I. The ratio of the speed of an object to the speed of sound in the surrounding medium, best known in the context of supersonic flight by aircraft.
II. They also found a second reaction that produced an isotope of helium, a neutron, and 3.3 MeV of energy.
III. Three scientists did this in 1941 by bombarding mercury with neutrons. But the gold they created was contaminated with radioactivity, and the experiment would have cost much more than the gold was worth. So not a path to fortune.
CHAPTER 4 HOW THE UNIVERSE BUILDS STARS
“I am aware that many critics consider the conditions in the stars not sufficiently extreme enough to bring about the transmutation—the stars are not hot enough. The critics lay themselves open to an obvious retort; we tell them to go and find a hotter place.”
—Arthur Eddington, 19271
Nature is good at fusion. Really, really good. It’s galling for the star builders. On Earth, they’re trying to build the most sophisticated machines ever devised to trap the stuff of stars, plasma, and initiate fusion reactions in it. When star builders explain what they’re doing, their ideas can sound crazy, like barely plausible science fiction. But, out there in the universe, fusion is happening all of the time, and on scales that make fusion on our planet seem like a whisper in a whirlwind.
The star builders are inspired by the universe’s bounty of fusion-produced energy. It tells them how they might achieve fusion on Earth. More important, what is going on in the cosmos shows them that fusion is possible, likely, and even ubiquitous. A glance at the night sky demonstrates that nuclear fusion is the universe’s most visible, most prevalent energy source.
So how, and where, does nature do fusion so easily? And what can the star builders learn from it? To find out we have to go on a journey through space and time to the very start of the universe.
If you trace everything that has ever happened back far enough, every person, every atom, every packet of light, they all lead back to the Big Bang approximately 14 billion years ago. Currently, the universe is expanding, and like dots on a balloon as it inflates, everything is growing farther apart from everything else.
Run the events backward to right after the Big Bang, and the universe was very hot, and very dense. Initially, in the first millionths of a second, it was too hot for even subatomic particles to form; there was too much energy around. As the universe expanded, it cooled, and the balance shifted so that protons and neutrons could form. The universe’s regular isotope of hydrogen, with one proton, was born and remains to this day the most common element.
With what is astonishing precision, we can say that one hundred seconds after the Big Bang, nuclear fusion reactions began in earnest for the first time. The hot, dense environment hit a Goldilocks zone for fusion. Protons and neutrons fused to form deuterium. Deuterium and neutrons fused to make tritium. Tritium and protons fused to make much of the universe’s helium. Energy from fusion was confined not by a reactor vessel, but by the universe itself. The chain of different fusion reactions proceeded, making more and more massive nuclei as it went.
It wasn’t long, around nine hundred seconds, before the universe had expanded and cooled so much that fusion stopped. Particles had less energy when colliding; lower density meant they were less likely to bump into each other in the first place. In enough time to oven-cook a frozen pizza, the universe’s first fusion factory created the four elements up to beryllium, with trace amounts of more massive nuclei.
The question is: Can those trying to do fusion on Earth learn anything from nature’s first fusion symphony? Big Bang Nucleosynthesis, as this type of fusion is known, is hard to do if you don’t have a fresh universe at hand, and the star builders don’t. We can’t learn much from it about how to do fusion on Earth; all we can say is that it’s another example of how fusion is a natural process that, with the right circumstances, will happen. But there’s another type of fusion reactor that’s ubiquitous across the universe and by which generations of star builders have been inspired: t
he stars.
The First Fusion Reactors
When you look up at the night sky and see those pinpricks of light, you’re seeing the energy released by vast numbers of enormous fusion reactors whose power output greatly exceeds the wildest dreams of Earth-born attempts to control and use fusion.
To find out how the first stars came to be, I’ve come to talk to someone who studies the period when they formed: Dr. Emma Chapman, a Royal Society Dorothy Hodgkin Fellow at Imperial College London. We’re talking in her office on the top floor of the physics department, where all of Imperial’s astrophysicists are located—presumably, to be closer to the skies they study. The room is decorated in a style that might be called “academic—classic,” with books taking on a structural role. Emma is engaging and humble despite her vast knowledge, and frequently distracted by her own excitement. She’s perhaps best known publicly for her battles to improve how women are treated in physics. Unfortunately, she has personal experience with the worst of this; she was sexually harassed by a senior colleague at University College London. She has used her own negative experience to improve, and prevent, the same poor treatment of other scientists by campaigning hard for universities to recognize the problem and to treat victims more fairly. Today, however, I’m here to find out about her research.
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