Death Star
There’s one more way for fusion to happen in space, although it’s not one that any star builder would want to replicate on Earth, even if they could. Even natural fusion reactors can’t go on forever, because they eventually run out of fuel. What happens next depends on the star.
The largest stars, super giants, have radii a thousand times that of the Sun. The hottest have surface temperatures thirty times that of the Sun. Some are so bright that you’d have to orbit them at five times the distance between the Sun and Pluto to get the same amount of light as we do from the Sun on Earth.8
In stars like the Sun, the cycle that turns hydrogen into helium eventually leads to the core being dominated by helium as the hydrogen runs low. However, the fusion reactions that use helium as an input, rather than an output, need much hotter temperatures. So the helium plasma sits around not doing much. In low-mass stars, where the helium gets mixed throughout, this is the end of the road for fusion reactions. These stars gracefully retire as “white dwarfs”—smaller, brighter stars that gradually cool. They do have their surprises though; as Arthur Eddington put it, they’re so dense that a ton of their plasma could fit in a matchbox.9
Medium stars have similar masses to our own Sun, and take a different route to the same end. As the hydrogen fusion reactions in their core sputter out and the temperature falls, they lose their stability. The star’s core collapses in on itself until the particles themselves can get no closer, at which point the core begins to heat up. It eventually gets so hot in the core that helium itself begins to fuse into more massive elements, mainly carbon. The temperature required to fuse helium is twenty-five times what’s required to fuse hydrogen. In the core’s periphery, previously cool hydrogen gets hot enough to fuse too, producing yet more energy. The increase in temperature expands the outer layer of the star substantially—the Sun will eventually do this and be reborn as a red giant, gobbling up the Earth. The Sun has approximately 5 billion years of hydrogen fuel left before this happens, so don’t worry too much. Once the Sun has depleted its red giant–helium burning phase, it too will retire to become a white dwarf, probably in 8–10 billion years.10
Bigger stars, with more than six times the mass of the Sun, undergo even more types of nuclear fusion. They can go from fusing helium into carbon, at 100 million degrees Kelvin, to fusing carbon into neon at 600 million degrees Kelvin, to fusing neon into oxygen at 1.2 billion, to fusing oxygen into silicon at 1.5 billion, to, finally, fusing silicon into iron at a whopping 2.7 billion degrees Kelvin.II Each time the star splutters and stops doing its old fusion reaction, it contracts, heats, and then begins another type of fusion. It’s a system so wonderfully well adapted to producing successively heavier elements that it’s hard not to think of big stars as factories pumping out the building blocks of everything in the universe. This procession of reactions continues until silicon is fused to make iron. This is the limit because it’s the last reaction that produces energy from fusion, resulting in the most stable isotope, Fe-56. Did alchemists ever imagine that the starlight of a moonless sky was generated by vast stellar factories forging heavier and heavier elements? The truth is more incredible than they could have dreamed.
Stars that reach this stage are like a giant red onion, each layer responsible for a different fusion reaction. Right in the center of it all is an inert core of iron that no longer produces fusion energy. Once that core is produced, things get really interesting. All of the four fundamental forces are involved in one way or another. When the core reaches 1.44 times the mass of the Sun, the remaining particle pressure against collapse is overwhelmed by gravity, and within minutes, the core begins to collapse in on itself. It does so with ferocious speed, taking seconds to go from a radius of thousands of kilometers to just ten.
The collapsed core takes up a fraction of the space of the original core, leaving a big hole in the center of the star. The new, even denser core could become a neutron star: a star so dense that all protons and electrons are forced to combine into neutrons. With enough initial mass in the star, it could become a black hole—an object so dense that not even light can escape. Meanwhile, the rest of the star, its middle layers, have nothing supporting them and they begin to collapse inward too. Since the core can’t get any denser, the incoming material bounces off it at a fraction of the speed of light, tearing back out again. A razor-thin 0.02 seconds after the bounce, the expanding shock wave slams into the outermost, and yet to collapse, layers of the star. Energy released from the core in the form of high-energy particles energizes the shock wave as it travels outward at about 10 percent of the speed of light, triggering explosive nuclear fusion reactions in its wake. Even fusion reactions that use energy are possible, and this is our main source of elements bigger than iron.
The expanding envelope of star material reaches far away into space from what was, moments earlier, a giant star. The brightness is 10 billion times the Sun’s: a single exploding star can outshine entire galaxies. This is nuclear fusion on an epic scale and it fully deserves its spectacular name: supernova.III
Such events only happen in Earth’s patch of space once in every half billion years, so your chances of seeing one are small. However, in 2020, some scientists grew excited observing that Betelgeuse—just seven hundred light-years away and previously the eleventh brightest star in the night sky—had entered into an unusual pattern of dimming that might precede its going supernova. In fact, that event is expected to happen soon—but only soon in astronomical time, which could mean at any point in the next one hundred thousand years.11
The death of stars is what has enabled us to exist. Mostly, humans are cleverly arranged bundles of hydrogen, carbon, oxygen, nitrogen, calcium, phosphorous, sodium, potassium, and sulphur. Apart from hydrogen, these elements are mostly forged in the last, dramatic moments of a star’s life. We’re all made of dead stars, and hydrogen. Without fusion reactions, complex life, which is based on a variety of atoms, wouldn’t exist. Our relationship with stars and nuclear fusion goes deeper than even ancient Sun worshiping cultures could have suspected.
I. There are other proton-proton fusion chains active in the Sun, but this is the dominant one.
II. The Kelvin scale is the Celsius scale +273.15. The difference is that Kelvin starts at absolute zero. At these large temperatures, the difference is meaningless.
III. There are different kinds of supernovae. Another involves a white dwarf sucking up mass from another nearby star. The different kinds synthesize different elements in their fusion reactions.
CHAPTER 5 HOW TO BUILD A STAR WITH MAGNETIC FIELDS
“We say that we will put the Sun into a box. The idea is pretty. The problem is, we don’t know how to make the box.”
—attributed to Sébastien Balibar (CERN) and Pierre-Gilles de Gennes (Nobel laureate in physics)1
As soon as nuclear fusion reactions were discovered, scientists realized that smashing together the ingredients for fusion—deuterium and tritium, the special isotopes of hydrogen that combine in the easiest to achieve reaction—was not a viable route to energy. They also knew that stars were really, really good at producing fusion energy. They had a conundrum: How could they make their machines starlike enough for fusion to happen?
The answer is to get the fusion fuel hot. Very hot. And so I’ve come to the hottest place in the solar system: a village in Oxfordshire.
Arriving is surreal. Although the station, Culham, is on the line between London and Oxford, the fast intercity trains don’t stop here. I had to change trains and take a ponderous service that seemed to stop at every house and siding. When I do finally get off at Culham, I find I’m in a very small, plain Oxfordshire village: an unlikely location to ring in the future of global energy production. For a moment, I fear I’m in the wrong place entirely. Somewhat reassuring is the unusually large number of other passengers disembarking, more than you’d expect for an out-of-the-way English village. They all troop, improbably, toward an overgrown count
ry path behind the station. It takes me a moment to realize that what I’m seeing is a stream of scientists and engineers heading for the Culham Science Centre. I follow them through the greenery.
When I get to it, I find that the Culham Science Centre is a myriad of public and private research institutions located on a disused Second World War–era military airfield. The Centre for Fusion Energy, where the magnetic confinement fusion experiment I’m visiting is located, occupies a large portion of the site. Some of the buildings show their age. Many have lackluster squares of glass alternating with dark green panels. In among the aggressive architecture, there are hints of the future. As I walk toward the building that houses Culham’s star machine, several autonomous vehicles pass me, scanning the quiet roads of the facility with mounted radar. I see signs for firms with names like Reaction Engines (reusable space launch vehicles), GeneFirst (molecular diagnostics), and Neuro-Bio (Alzheimer’s treatments).
When I get to the lobby of the Culham Centre for Fusion Energy, I find that it’s also a contradiction in time—it resembles a careworn smoking room last redecorated in the 1970s, and yet vivid printed posters showing the “future of fusion” take up one wall.
I’m here to see the Joint European Torus (JET), the most successful fusion reactor in history. It’s the apex of an approach to fusion that borrows the barely imaginable temperatures of stars—and goes beyond them—to get fusion working. JET was built and funded by European countries and is run by the UK Atomic Energy Authority (UKAEA). Although it’s an EU project run by a UK government research organization, the staff, the support, and the mission are very much global—whatever is discovered is shared worldwide so that it can inform future fusion machines.
My first meeting is with Lorne Horton, who goes by the odd-sounding title of JET exploitation manager. He’s going to show me the machine up close, or as up close as anyone can safely go while it’s in operation. Lorne is a friendly, straight-talking Canadian engineer whose job it is to knit together the program of experiments that European countries want to do on JET with the day-to-day operations of the facility. He’s wearing a suit, unlike many of the other scientists I see around the facility, and he looks back at me with small blue eyes under a crop of receding blond hair.
I want to find out what makes star builders like Lorne tick. Given that scientists and engineers have worked on fusion for decades and could have had more lucrative careers in the oil and gas industry, or even in scientific fields of inquiry with more immediate rewards, you’ve got to wonder what makes them go on day after day working on the Promethean challenge of nuclear fusion. Although Lorne doesn’t seem like he’d be fazed by much, he’s surprised that I’m interested in his motivations.
“I was always interested in energy as a problem,” Lorne says. He grew up in what he describes as Small Town Canada, where his father worked on Canada’s own brand of nuclear fission technology, CANDU reactors. For the Horton family, nuclear energy is the family business. I press him on what he means by energy as a problem. The global economy, the well-being of everyone on the planet—it all comes down to energy, he tells me. He has a crystal-clear memory of being at school when the first oil crisis blew through North America, spiking prices to unheard of levels. He thinks fusion is both the best solution and a great way to take the geopolitics out of energy.
Like so many other star builders who I talk to, Lorne can’t help but be attracted by the gargantuan challenge of taming the stellar fires here on Earth. But he’s also effusive about the other aspects of working on a big, international scientific program. Because JET is supported by so many countries, there’s a global community working on it that he enjoys being part of. Not that there isn’t healthy competition with international colleagues not at JET, he says.
Next, he leads me down to the engine room of star power, what he calls the diagnostic hall. It’s a vast warehouse-like structure. I hear the unceasing whirr, clank, and buzz of machinery: the air pumps, cooling systems, heating systems, experimental and safety diagnostics, and the hundreds of other mechanical arteries that feed this star engine.
The large volume of the hall is subdivided into rooms with thin walls and plastic windows. They fill the space with confusing alleys. Lorne strides through them. At the center of the hall is the thick bunker that houses JET’s reactor chamber, a thirty-meter-sided (approximately one-hundred-foot) cube of two-meter-thick (approximately six-foot) concrete. On the inside, there’s a boron-rich scree to absorb stray fusion-produced neutrons; otherwise they’d hit regular materials and could make them radioactive. The whole reactor-containing cube is depressurized to further isolate any isotopes that they do not want escaping into the atmosphere. It can only be entered through an airlock. Any significant change to the reaction chamber means moving a set of giant concrete slabs, nine hundred tons apiece, that serve as doors and are set into the concrete cube.
As Lorne is explaining all of this to me, we pause while a countdown crackles out over the public address system. This is science at extremes, but it’s so normal for star builders like Lorne that when the countdown is on, he just waits patiently. “Ten,” the voice says, nine, eight, and so on, and suddenly, I’m standing just a few hundred meters away from what is—for a few seconds—the hottest place in the entire solar system.
150 Million Degrees
Inside JET, the temperature just hit 100 million degrees, substantially more than the 15 million in the Sun’s core. Those high temperatures are just what’s necessary for fusion to be possible on JET, Lorne says. On the face of it, there isn’t much difference between a cold object and the same object but hot. They’re both made from exactly the same material, and more often than not, they look the same too. So what changes when objects get hot?
Temperature is a measure of average energy, and that in turn describes how fast particles are moving around within a material. Imagine a schoolyard full of children pretending to be particles. The temperature scale has a minimum, called absolute zero. This is equivalent to the children staying still like statues, improbable though that might be. Now imagine that the children begin to run around until they bump into one another and change direction. This is what particles are doing as you add energy, and the temperature rises. The more energy, the higher the average child’s speed, and the higher the temperature. High speeds and energies increase the chances of two particles—or children—colliding. When two particles (but hopefully not children) collide with enough energy, there’s a chance that nuclear fusion will happen. Below a threshold collision energy, there’s almost no chance. The temperatures on JET are equivalent to an average deuterium speed of more than 1 million meters (approximately 3 million feet) per second—enough to make fusion more likely, but not guaranteed.
Making a big batch of fusion fuel hot at once creates lots of chances for fusion: if one high-energy collision isn’t successful, it doesn’t much matter, because the next one might be, or the one after that. When enough fusion reactions are happening, the energy released can be enough to keep the reactor going. When the reactions become self-sustaining, star builders call it “ignition.”
Before getting to ignition, you have to keep pumping energy in to replace any energy that’s escaping. It’s like having a leaky bathtub that constantly has to be topped up. Lorne describes putting this energy in as applying a blowtorch to the fusion fuel. The ratio of fusion energy coming out to energy put in per second is called “Q” in magnetic fusion, and it’s the primary measure of the success of these reactors. The immediate goal for magnetic confinement star builders is getting to net power gain, a Q greater than one, because it means there’s more energy coming out from fusion per second than is being pumped in. But they want to do far better than that: it’s possible for no external heating at all to be required, for fusion to be completely self-sustaining. In that case, Q becomes infinite: this is ignition.
There are different types of Q: when most star builders talk about Q, they mean the ratio of energy out to energy in per
second for the plasma. But, for practical power generation, the “wall-plug” Q is important too; this refers to the ratio of energy out to energy in per second for the whole machine. For now, JET is aiming to get to a plasma Q of 0.3 to 0.5 for five seconds, which would be a new record for total fusion energy.
After leaving the diagnostic hall, we head to JET’s control room. Before long, Lorne and I are looking at live data coming in from an experiment just like the one I heard the countdown for over the public address system. There are more than a dozen scientists scurrying around the computers. Contrary to popular belief, they don’t wear white coats. Most are wearing casual shirts or blouses, a few T-shirts and jeans. What’s important to them is science, and understanding how the rules of the universe play out. Some of the technicians are chatting, some are doing analysis, some are checking observational equipment. Most are staring at the bank of monitors arrayed in front of us. The most compelling and, it must be said, understandable of these monitors shows a direct video feed of what is going on inside the fusion reactor. Mostly, the image is dark, but we can see a wispy purple-red fog clinging to the bottom and sides of the inside of the vessel. This is 100-million-degree hydrogen radiating away energy as light. Ideally, JET would reach 150 million degrees, because that’s the temperature at which it’s easiest to get to high values of Q.
Such temperatures seem entirely fantastical, but they can be measured; as an earl in Britain’s House of Lords once asked, what kind of thermometer do you need to measure a temperature of 100 million degrees without it melting? To which a viscount quipped, “A rather long one!”2 The true answer is that the calculation is made by firing a laser into the reactor chamber and measuring how much the particles of light in the beam, the photons, are changed following their collisions with electrons in the plasma. The hotter the electrons, the faster, on average, they’ll move in any particular direction, and the more light hitting them gets bounced around. It’s like if you dropped tennis balls from a bridge down onto two opposing lanes of motorway traffic: you’d get a much wider range of tennis ball speeds from fast moving traffic than you would from stationary traffic.I This interaction between light and electrons is called Thomson scattering after J. J. Thomson, Ernest Rutherford’s first boss in the UK.3 Unlike the National Ignition Facility, JET only uses lasers to diagnose what’s going on during experiments. The lasers don’t play a role in creating the conditions for fusion.
The Star Builders Page 9
