The Star Builders

by Arthur Turrell

  It seems unnecessarily complicated, and you might well wonder why it wouldn’t be easier just to have a ball of deuterium and tritium. Star builders at Livermore worked out early on that such a target design would be terribly inefficient. To make fusion happen, the X-rays would have to raise the whole thing to the right temperature and density, which means putting a lot of energy in up front. It would be like making a fire by raising the temperature to the point where every log burst into flames. Using a simple blob of deuterium and tritium like this has a natural energy release limit of twenty times the energy that you put in and so is just not good enough for commercial fusion, which needs gains of thirty to one hundred.

  Instead, the design of the capsules lights a match of fusion reactions right in the center, and these spread outward. It takes a lot less input energy to do this, but it does require the capsule’s layers to be cleverly engineered. The outer layer is a thin sheen of diamond just 30-millionths of a meter thick. It acts as rocket fuel for the collapse of the capsule, helping to raise its density. Next, there’s a 160-millionths-of-a-meter-thick solid ice layer of deuterium and tritium. This layer provides the big, heavy logs of the fusion fire and accounts for most of the mass of the capsule. Sitting at the center, and making up most of the volume of the capsule, is deuterium and tritium gas. It takes really special conditions for this core of gas to coexist with the solid deuterium and tritium layer next to it; most states of matter exist as either solid, liquid, gas, or plasma at a given pressure and temperature. To sustain the unusual multi-state structure, the star builders need to maintain a temperature of just nineteen degrees above absolute zero (or -254 degrees Celsius, -425 degrees Fahrenheit) until the X-rays arrive. This final layer of gas is the kindling for fusion, the place where the initial spark begins.10

  Because of the way reactions start in a hot plasma that’s formed in the core of the collapsed capsule, this potentially high-gain strategy is called hotspot ignition, and it’s by far the most successful form of laser fusion.

  Hotspot ignition is a great idea in principle and a thunderously difficult one to execute in practice. Somehow a match must be held to the fuel right at the center of a dense ball of plasma without disturbing its perfectly spherical geometry.

  The star builders’ great innovation is to use the shape of the laser pulse to ignite the hot spot. NIF’s scientists introduce a series of peaks into the energy delivered by the laser over time to, indirectly, launch a series of shock waves into the capsule. With careful timing, these shock waves both dramatically increase the density of the fuel and create the hot temperatures needed in its dead center.

  Shock waves aren’t that common in everyday life, but once you start looking, they’re everywhere. They’re one of my favorite phenomena in physics. One way to think about them is as a sudden leap, from low density to high, or low temperature to high. It’s no coincidence that we use the same word, “shock,” to describe a sudden change of state in emotion too. In physics, shock waves appear whenever something moves through a medium faster than the waves of the medium themselves can move. Fast-moving ships can create bow shocks ahead of them in water. The space shuttle reentering the Earth’s atmosphere creates one around its nose that’s hot enough to turn air into plasma. Supersonic jets create them as they travel faster in air than sound waves do, and you can actually hear them as a loud BOOM-like thunder. Thunder is itself the shock wave created by lightning turning air into plasma. Explosions can create shock waves too.

  A single shock wave would only push up the density and temperature of the fusion fuel so far; certainly not enough to reach the thirty-thousandfold density increase that is needed for net energy gain. So, to push the fuel all the way, three (or more) shocks are fired off in succession using laser pulse shaping. These shocks have increasingly high energies, so, cleverly, the later ones travel faster. The intended result is that their effects coalesce only once they are within the capsule’s fuel. This mammoth combined shock compresses the cold, icy fuel to ten times the density of the Sun’s core and creates more pressure than an aircraft carrier balanced on a penny. The capsule is shrunk to just a thirtieth of its original radius, and if all goes to plan, the fuel is engulfed by fusion reactions.11

  Seeing Stars

  To make this house of cards stand up requires everything to work perfectly together: the laser, the hohlraum, the capsule, the human operators, and even the computer codes. “Relatively small things can make a big difference,” Mark Herrmann told me, when we talked about the painstaking precision that’s needed. A few percent error in the energy at the start of the laser pulse can degrade the conditions needed for fusion by as much as 50 percent.12

  So far, NIF has not reached net energy gain. The best experiments have reached a fusion energy yield of 3 percent of the 1.8 sticks of dynamite’s worth of energy in the laser beams. Given how fusion energy can scale up suddenly in the right conditions, this is far closer to JET than the difference in percentages alone might indicate, and the scientists here are front-runners in the race to build a star. NIF leads the pack on inertial confinement fusion and, some of the star builders here tell me, has the only machine that’s up and running and could go all the way to net energy gain. This ambition is even reflected in the “ignition” part of the facility’s name.

  To learn more about what’s stopping them from leapfrogging JET, I tack back to NIF’s main building with Bruno. We head for the heart of the reactor, a catacomb of concentric shells of concrete and metal that house the business end of NIF. Inside the gap between the spherical aluminum target chamber and a bubble of thick concrete we can see metal gantries arrayed at all heights. These give access to the target chamber, which is neatly fed by tubes and cables from every direction.

  Bruno points out the equipment that scientists use to see into a star. Every tube, box, and wire that isn’t bringing a laser beam in is part of a diagnostic that tries to pick apart, often indirectly, what happened during each shot. Just as a doctor looks for symptoms to try to diagnose a disease, so scientists at NIF collect data on each implosion to diagnose what went right or wrong.

  There are 250 people solely working on diagnostics here, whether that be on cameras that look into the chamber, fast laser pulses that probe the fusion plasma, neutron detectors, spectrometers, or the design of “diagnostic” targets that are modified to give firsthand information about conditions during an implosion.

  One of those staffers is X-ray imaging diagnostician and group leader in physics Dr. Louisa Pickworth, whom I’d arranged to meet back in the visitors’ center.

  I had a strong memory of Louisa from my time at Imperial College London, where she’d spent hours in the bowels of the physics department working on the subterranean two-storey plasma machine known as MAGPIE. MAGPIE is a z-pinch, a machine that runs a large current through thin metal wires stretched vertically. The current breaks the wires down into a plasma and also creates a strong magnetic field. The combination of current and magnetic field pinches the created plasma into a dense vertical column. The “z” is because physicists often call the vertical direction the z direction.

  MAGPIE can be used for lots of exciting science, for example re-creating in miniature the supersonic jets of plasma fired out of stars as they form.13 Louisa researched new diagnostics sensitive enough to probe these experiments. Turning oil, metal, and electricity into scientific discovery using MAGPIE required both manual dexterity, to fashion targets and repair machinery, as well as mathematical rigor, to understand what was happening. She’s now applying the same skills at NIF, a machine on a dramatically different scale.

  “We haven’t got fusion schemes working yet, and one of the most important things for us to do is to understand why,” Louisa tells me over coffee. “We have many models and many computational codes which inform us how to put an experiment together and what we expect that experiment to give us, but then we do that experiment and nature gives us the real answer. And the job of the diagnostician is to listen to what
nature is telling us.”

  Louisa first got interested in fusion back when she was sixteen, when she spent her summer acquiring work experience on JET. She was a diagnostician right from the start, developing an infrared camera to watch JET’s magnetic confinement fusion plasmas. “My first experience of physics was working as a physicist at a big, expensive fusion facility,” she says, “and to some degree I just fell in love with it.”

  Apart from the machinery, Louisa also enjoyed the international environment (something that both JET and NIF have) and the sheer complexity of coordinating so many people to try to make something amazing happen. But what she fell in love with most was trying to understand what’s happening in extreme conditions.

  “These are pretty challenging environments,” she says, talking about the small size of the capsule during the implosion. “It’s spherical,” she adds, “which is a challenging geometry to image, and it emits a really wide range of radiation.”

  The diagnosticians look at all the types of light radiation that come streaming out of the fusion plasma. This includes optical light, from the capsule’s periphery, and high-energy gamma rays, but Louisa’s specialty is X-rays. From the pattern of X-rays that are emitted, she and her team have to work backward to find out how the implosion proceeded.

  “Part of the diagnostic challenge is to try and take the information that’s coming out and turn it into something useful!” she says.

  One of the many difficulties she and her colleagues face is that the diagnostics on a real fusion shot must be indirect, and not interfere at all with the target. That’s why they rely so much on sensors that work at a distance. That doesn’t mean the tools are sophisticated; one of the workhorses is the pinhole camera, first developed in Ancient Greece. “It’s literally a hole in a piece of material with a detector not dissimilar to film behind it,” Louisa says. “It might tell us how round the imploded plasma is, how small it got, what sort of X-rays were there.”

  Those X-rays appear because electrons in the capsule’s core are constantly being accelerated and decelerated as they encounter other charged particles. A charged particle rapidly changing direction creates an electromagnetic wave, in this case an X-ray, much like a speedboat pulling a tight turn creates a big wave in water. Because it happens as electrons screech around corners, this radiation is named bremsstrahlung, after the German words for braking, bremsen, and radiation, Strahlung. As you’ll know if you’ve ever broken a bone, X-rays can penetrate material that regular light can’t. Even though the plasmas produced at NIF are opaque to visible light, bremsstrahlung X-rays can pass through them. It’s one of the ways for energy to escape from a fusion plasma that John Lawson put in his equation for net energy gain. Scientists like Louisa can use the escaping X-rays as a window on the experiment.

  Part of Louisa’s detective work is tracking down the instabilities that can ruin otherwise perfect implosions. Like magnetic confinement fusion, inertial confinement fusion is bedeviled by plasma instabilities. Those instabilities are a big reason why NIF hasn’t gone further than a 3 percent yield of energy. “Nuckolls thought a kilojoule laser would do it,” NIF’s Jeff Wisoff told me. “When you start stacking in realities, Mother Nature has made this very hard. Mother Nature made actual instabilities much harder.”

  The most ubiquitous and feared instability among inertial confinement fusion star builders is the Rayleigh-Taylor instability. It happens when a less dense material pushes a more dense one, and its effect is to mix up the two materials involved. Like black coffee and milk, once mixed, the materials can’t be unmixed. It may sound innocuous enough, but it’s perhaps the single biggest issue for inertial fusion machines because it stops a hot, central core of fusion fuel from forming and taking hold. At NIF, it rips through the delicately arranged capsule layers.14

  Recently, when I went to visit my old school in the UK’s Peak District, we created our own Rayleigh-Taylor experiment with glasses of water and laminated sheets of paper. We filled the glasses to the brim, placed the laminate on top, and then turned the whole thing over. Try it at home. If you’re really, really careful then you can remove the laminate and have an entirely full glass of water sitting, quite happily, upside down.

  Instabilities need a tiny seed from which their exponential growth can start, just as a pandemic starts with a single infected person. Without any imperfections in their interface, water and air can sit in a delicate, unstable balance. It was at this point, with two dozen glasses of water precariously looming over the classroom, that I told the students to give the water the tiniest of nudges with a finger or a pen tip and—almost instantly, as the instability grew, the water lost to gravity at every desk. We were mopping up the classroom for some time afterward—but the students had learned an important lesson about instabilities.

  It’s also the Rayleigh-Taylor instability that causes the very worst escapes of energy via bremsstrahlung radiation. “A flake of the hohlraum can fall off and get into the capsule,” Mark Herrmann told me when we were discussing the latest challenges. This can create a jet, a finger of Rayleigh-Taylor growth, which pushes all of the capsule’s layers deep into the fuel’s hot core. That includes the outer layer of diamond-like carbon, which is thirty-six times more effective at radiating bremsstrahlung than hydrogen. Once carbon gets into the core, it aggressively cools it. “We can see it creates a meteor of X-rays,” he added. Louisa Pickworth’s team is tasked with tracking bits of the hohlraum, the gold, and the diamond-like carbon, to see how and why they get to where they have no business being. Although the Rayleigh-Taylor instability is the most acute, there are dozens of other ways for the perfect inertial confinement experiment to go wrong.

  Given these truly formidable scientific challenges, you may wonder why inertial confinement fusion star builders have kept going. Just like star builders doing magnetic confinement fusion, they know that Lawson’s equation means that net energy gain is possible in principle. But star builders pursuing inertial confinement fusion have another ace up their sleeve, one that the magnetic confinement fusion star builders don’t: there’s evidence that inertial confinement fusion can produce net energy gain not just in theory, but in practice. There’s just one problem: that evidence remains top secret.

  In the late 1970s and 1980s, a series of classified experiments, known as Halite and Centurion, that used nuclear explosives was conducted underground by the US national laboratories at Livermore and Los Alamos, respectively. We don’t know much about them because they’ve remained only partially declassified because they were conducted as part of nuclear weapons programs. What can be guessed is that they involved indirectly driving an inertial confinement implosion. The driver, in this case, was a massive dose of X-rays from a conventional nuclear weapon rather than X-rays indirectly generated by a laser. However, the X-rays took up the rest of the process as if it were laser fusion. The US wasn’t alone; the UK ran an independent version of the Halite-Centurion tests in 1982 that had exactly the same results. Even less is known publicly about these UK tests, apart from that they agreed with the US tests—and that one of the two designers was my PhD supervisor, Professor Steve Rose, now at the University of Oxford.

  What we do know about these secret inertial fusion tests is that they showed “excellent performance, putting to rest fundamental questions about the basic feasibility of achieving high gain” in inertial confinement fusion. High gain, at least ten times energy out for energy in, is a basic requirement for a fusion power plant. Inertial fusion has been shown to work experimentally, albeit with a huge input of energy—much greater than what NIF’s current laser can deliver. What we don’t know is how close to the upper bound given by these experiments the laser energy needs to be before net energy gain can be achieved. It’s exactly this question that NIF is now trying to answer.15

  Inertial confinement fusion with lasers, and magnetic confinement fusion with Russian-style tokamaks, have both been around since the 1960s. Early star builder John Cockcroft, who firs
t split the atom, thought it might be fifty years to a fusion power plant. That was in 1958. We know from John Lawson’s theory that star power on Earth can work in theory. The leading magnetic fusion machine, JET, has come close. Inertial star builders think that secret experiments have already shown that their approach works in principle. But despite a succession of bigger and better machines, star builders working at these big laboratories still haven’t managed to get to net energy gain. Maybe it’s time for a different approach?

  CHAPTER 7 THE NEW STAR BUILDERS

  “Ideas alone have little value. An innovation’s importance lies in its practical implementation.”

  —Werner von Siemens, cofounder of Siemens & Halske1

  Most star builders are looking to the heavens for inspiration. Not Dr. Nick Hawker, CEO and CTO of First Light Fusion. He’s thinking differently. He’s looking under the sea instead.

  The creature that set Hawker on the path to fusion is, by many accounts, the noisiest in the ocean: the pistol shrimp. Even though each one measures only a few centimeters in length, collectively they can drown out animals as big as whales. They are so noisy that, during the Second World War, US submarines hid from Japanese sonar among large colonies of them. The sound emanates from their claws. At a distance, mariners describe the clicking as resembling the crackling of burning tinder. Up close, the creature is much more deadly—at least, to other subaquatic denizens.

  One of the pistol shrimp’s two claws, the one that makes the noise, is about half the size of its body. The loud snap of a pistol shrimp is not, as scientists once thought, due to the sudden shutting of the large claw itself, but due to the bubble it makes as it closes. When that bubble of air pops, by imploding in on itself, it generates a shock wave that is heard as a CRACK. Pistol shrimp use the pressure of the shock wave to stun their prey before safely devouring them.2