The Star Builders

by Arthur Turrell

  But the biggest difference between inertial and magnetic confinement is how long the fusion plasmas are maintained: as opposed to magnetic fields holding plasma in place in perpetuity, the inertial approach traps it for just tens of billionths of a second, the length of time that the plasma’s own inertia holds it together. At NIF, that inertia comes from the momentum of the inward explosion, or implosion, of the fuel on itself.

  The implosion of the fusion fuel provides confinement, nothing else, and that only for the briefest of moments. But the briefest of moments is a long time in nuclear physics. When you pop a water balloon, the water retains the shape of the balloon for a few fractions of a second before it turns into a big wet mess. It’s the same with plasma. With nothing to hold the shape of a ball of plasma, inertia holds it there for the short time it takes a sound wave to cross the sphere of plasma. But fusion happens on timescales a thousand times shorter. Nick Hawker describes inertial confinement fusion plasmas as “transient phenomena, these very high-pressure, high-temperature, high-density states of matter, and they’re held together for a very short amount of time, simply by their own inertia. Once assembled, it just takes a certain amount of time for it to disassemble.”2 Each implosion is a race against time.

  In the prologue, we saw this race play out in a shot fired by NIF’s 192-beam infrared laser. Those beams are turned into ultraviolet laser light by crystals as they enter the reactor chamber and are focused on two small holes on either end of a small gold box, the hohlraum (German for “hollow room”). The beams hit spots on the inside walls of the hohlraum, generating high-energy X-rays. The X-rays bathe a small capsule of fusion fuel, expanding its outer layer and triggering a rocket effect whereby the remaining fuel capsule contracts rapidly. The inward explosion, or implosion, squeezes the fusion fuel into a ball with a radius the width of a human hair. With just the right combination of timing, laser pulse, hohlraum, and capsule, the fuel becomes a plasma with the right temperature and density for fusion. The fusion reactions radiate out from the center of the plasma like a wave, and engulf the rest of the fuel.

  It’s a series of extreme physics events strung together to make a spectacular finale. With a massive input of initial energy, fusion fuel can be collapsed in on itself for a moment of shining glory. At NIF, this all begins with the laser beam—which is fitting, because lasers are a big part of how this approach to fusion got started.

  The Light Fantastic

  Inertial confinement fusion began shortly after Edward Teller and his colleagues demonstrated the first hydrogen bomb in 1952. Teller had been contacted by politicians in Washington who had a new challenge for the brilliant but controversial physicist. “No sooner was it done,” Teller has said, “than every politician and every bureaucrat descended upon us saying, ‘Now you must solve the problem of controlled fusion.’ ”3

  Teller was skeptical of the magnetic confinement fusion schemes on offer in the pre-tokamak era. In 1954, he delivered a damning lecture in which he said that all magnetic schemes that had been tried were highly susceptible to plasma instabilities, the runaway processes that ruin confinement. He had an alternative. The plasma in magnetic fusion conditions is like the solar corona, the outer, wispy part of the Sun that becomes visible during solar eclipses. Physicists still don’t properly understand it, and even then Teller knew that working with similar plasmas would present tremendous difficulties because there’s so much going on: heat transfer between particles, light carrying energy away, and magnetic fields bringing all kinds of complex instabilities into the mix.4

  However, Teller thought, the inside of the Sun, where the density is much higher, is easier to model. The plasma is very pure: all electrons are stripped from helium and hydrogen, and the densities are high enough to make the plasma opaque to light. This plasma is much more similar to what is created in a hydrogen bomb, something that Teller understood better than anyone. So Teller had an idea.

  In 1957, a young scientist named John Nuckolls got the job of turning Edward Teller’s idea into a reality. Nuckolls’s instructions were crazy. He was asked whether it would be feasible to hollow out a mountain and drop hydrogen bombs in it to generate electricity. What wasn’t so crazy was that hydrogen bombs could already deliver huge net energy gains. But (and this will shock no one) exploding a series of hydrogen bombs has downsides. The mountain would eventually crumble into a radioactive mess, and the use of large numbers of nuclear weapons was an enormous nuclear proliferation risk.

  Nuckolls instead had the idea of taking the hydrogen bombs out of the design altogether. This got rid of the fission reactions, eliminating much of the radioactivity, as well as the proliferation risk. Instead of a few big explosions, he’d do many miniaturized fusion implosions using just ten milligrams (approximately four ten thousandths of an ounce) of fuel each time. The smaller yields of energy would be easier to deal with too: Nuckolls would replace an unmanageable “BANG!” with a controllable “pop”; instead of a single large release of energy, he’d have the setup work like a gasoline-powered engine, with a stream of pops as a succession of fuel capsules went off.

  It was a good idea, but it would only work if there was a way to compress and heat the tiny ball of fuel enough to initiate fusion. But no one had ever driven a fusion implosion, no matter how big or small, without using fission explosives.

  “The driver may have kilometer dimensions but must concentrate energy in space and time to energize a tiny sub-centimeter-scale radiation implosion,” Nuckolls wrote. “For power production, the driver focusing mechanism must be separated a safe distance from the fusion explosion. The driver must ignite billions of micro-explosions in a thirty-year power plant lifetime.”

  Nuckolls considered various schemes to trigger fusion in his capsules, including firing jets of plasma at them, smashing them with a very fast pellet gun, strafing them with charged particles, and hitting them with exploding foils. Nothing, however, was quite right. Livermore’s weapons designers weren’t impressed, calling Nuckolls’s prolific series of classified memos on small fusion capsules “Nuckolls’ Nickel Novels.”5

  Then, in 1960, a scientist named Theodore Maiman announced the creation of the first laser. Lasers are used in everything from the fiber-optic communications that enable the Internet, to eye surgery, barcode scanners, cleaning, playing DVDs and CDs, and cutting metals in industry. Although they’re ubiquitous, there’s extraordinary physics going on when a laser “lases.” The light that we see most of the time, from the Sun or a reading lamp, is made up of many different electromagnetic waves. The distance between the crests of light waves is what determines their color: pure sunlight has many colors, while sunlight reflected from grass is dominated by green light. But even if light appears in the wavelength of one color only, like the light from a red lamp, the crests and the troughs of that light aren’t all in sync. Laser light is special because the waves are the same shape, and so the same color, but they’re also in sync with each other. The difference between colored light and laser light is the difference between a crowd walking haphazardly with the same stride length, and an army marching forth in time together.6

  Because the light is in sync, a little laser light goes a long way. An ultramodern and efficient LED lamp might draw 5 watts of power and wouldn’t do you much harm if you glanced at it. Contrast this with a laser, which at just 0.01 watts can do serious damage to your retina if you look at it directly—and it’s never safe to look into a laser beam. The magic of lasers is that, with a lens, they can cram a lot of in-sync light energy into a tiny spot. They can focus energy in time and space. When Maiman first demonstrated the laser, it was the archetypal technological solution looking for a problem. John Nuckolls had just the problem.

  Nuckolls realized that lasers could drive enough energy into his fusion targets. Barely a year after their discovery, Nuckolls excitedly wrote a memo to the director of Livermore, explaining his idea of a laser driven “Thermonuclear Engine,” “the fusion analogue of the cyclic i
nternal combustion engine.”7

  More than five decades later, Nuckolls’s idea of using lasers to create mini-stars led to the construction of the National Ignition Facility. It’s not the only way that star builders are doing inertial confinement fusion, but it’s the way that has come closest to achieving net energy gain.

  To find out more about how Nuckolls’s idea has been translated into reality, I’m heading to the Optics Facility, a building dedicated to keeping NIF’s thirty thousand or so lenses, mirrors, crystals, and laser glass in good working order. These elements are key to delivering energy, via the laser, to the fusion fuel.

  As my guide, Operations Manager Dr. Bruno Van Wonterghem, leads the way, he points out where Edward Teller himself used to drive around the Lawrence Livermore site in his golf cart. I can imagine Teller, zipping along the quiet roads, saying hello to colleagues with the Hungarian accent that he never completely lost, despite living in the US for most of his life.

  When we arrive in the facility, we meet Dr. Tayyab Suratwala in a corridor full of scientific posters showing detailed schematics of experiments splashed with the bright laser colors of green, ruby, and violet. Tayyab is the program director for optics, the science of light and glass and how they interact. What that means here at NIF is that he is responsible for ensuring that the parts of this gigantic laser that manipulate light energy keep working. As well as directing NIF’s optics operations, Tayyab has found time to write more than one hundred scientific papers, file six patents, and write a book, all on optics.

  NIF, Tayyab tells me with warranted pride, is the only laser system that continuously operates above the damage threshold for the nine thousand or so more delicate optics directly in the laser’s path. What this means is that NIF’s laser is so energetic that it damages its own optics every time it fires.

  The damage manifests as frequent chips, fractures, and cracks in the most technologically advanced optics in the world. Because they’re so specialized, it can take an entire year for replacements to be manufactured and imported—precious experimental time that NIF’s scientists would lose if they waited around. So, instead, Tayyab and his colleagues have adopted a different approach: invent better optics (easy, right?) and, rather than avoid damage altogether, incorporate damage as a regular part of running the laser.

  Tayyab says that a shot by NIF with the quality of optics available back in 1997, when NIF was designed, would have created fifty thousand damage sites per optic, seeded by defects as small as ten-billionths of a meter within the glass. By inventing more perfectly polished optics, NIF’s staff has dropped that to fifty damaged sites per optic. And when they do see damage, they have amazing ways of repairing it, which Suratwala shows me. I watch, astounded, as a machine learning–driven robot takes thick slabs of glass, scans them for fractures, and then uses a small but powerful laser to smooth any sharp pits into gentle valleys that mean the optic can still function well.

  Even after the beams have entered the chamber, the risks to the optics aren’t over. “The second most energetic laser in the world is the scattered light from NIF,” Tayyab says. Any laser light that isn’t perfectly carried along the beam lines causes havoc elsewhere. And even when the light hits what it is supposed to, it doesn’t always do what it is supposed to do.

  The behavior of light and plasma is mostly determined by the electromagnetic force, and the two can interact, often in unintended and unhelpful ways. To name but a few that manifest with plasmas and laser beams, there’s cross-beam energy transfer (the plasma takes energy from one laser beam and puts it in another), self-focusing (the plasma acts like a lens), filamentation (the plasma extrudes smooth beams into strands like spaghetti), stimulated Brillouin scattering (the plasma acts like a mirror), fast electron heating, and a whole range of laser-driven instabilities.

  Even in Nuckolls’s day, the designers at Livermore knew that shining lasers directly onto a fusion capsule would risk all of these interactions complicating an implosion, and send you straight back into the realm of difficult plasma physics. When told about Nuckolls’s original scheme, Teller is said to have asked “Wait a minute! Are you telling me that laser fusion involves real plasma physics?”—to which the presenter responded, “Yes, sir, it does.” Reportedly, Teller replied, with some despondency, “Well… it will never work.”8

  Today, “real” plasma physics still presents a big challenge for laser-driven inertial fusion energy. But some of the problems are side-stepped when an intermediary is used to absorb the laser energy. That’s why Livermore’s scientists developed the hohlraum, the gold box into which the laser shines, entering through the two holes on each end. The bath of X-rays produced by the hohlraum when the laser hits its surface is smoother than the laser beams themselves, is less likely to cause plasma instabilities, and reduces the risk of preheating the capsule before it’s compressed to high densities. To distinguish it from directly shining laser light on a target, this approach is called indirect drive and it’s what’s used by NIF.

  While star builders haven’t completely given up on direct drive laser fusion (France’s smaller equivalent of NIF is still pursuing the direct drive approach), the indirect drive approach has been much more successful in getting fusion to work during the punishingly small window of confinement.9

  Targeting Net Energy Gain

  Even as the X-rays get to the capsule, there’s a lot more going on than you might think to make the tens of billionths of a second a NIF implosion lasts a success.

  So the next stop on my tour of NIF is the target fabrication laboratory, where I’m hoping to find out what role the targets play in this type of fusion. My guides are Becky Butlin and Dr. Michael Stadermann. Becky tells me that she caught the science bug in sixth grade when her teacher began assigning weekly mathematics puzzles, but she soon discovered that she enjoyed using that knowledge to make physical objects, and as a teen, she even set up a workbench in her garage. That desire to do science through building and creating objects has led to a career in fabricating targets for NIF. Becky and Michael talk to me about target fabrication as if it were the most important topic in the world. Their enthusiasm is infectious, and they have a point. As Michael tells me, the target (plus the laser pulse, he says in an aside) is the experiment. He’s right; although NIF is in a huge building, the most important parameters from experiment to experiment are the laser pulse and the target. My guides certainly have me hooked: target fabrication does seem to be key to the whole contraption ever hitting net energy gain, so tell me more! “Well,” Becky says, “would you like to go into the clean room?”

  You bet I would. We enter a changing area. There are so many things to put on: a sterile full body suit, sterile slip-over shoe guards, sterile gloves, hood, and face mask with nose pinch. Becky and Michael are in their clean armor in seconds; I am still trying to pull on a latex glove using another gloved hand when they come over to assist me. The clean-room gear isn’t too comfortable, and by the time they’ve finished, there’s barely an inch of flesh showing on any of us. It doesn’t help that I’m slightly nervous. Truth be told, I’m an applied mathematician at heart, and I’ve always tried to dodge laboratory work in favor of working with pen and paper or a computer. Given the submillimeter precision going into the laboratory’s products, I’m painfully aware that I am an overenthusiastic arm gesture away from causing hundreds of thousands of dollars’ worth of damage. Laboratory work is fiddly and hard, requiring saintlike patience. The satisfaction of having created something tangible that can help probe the most extreme conditions in the solar system is the reward. Following a Saturday spent building equipment, Rutherford once exclaimed to a colleague, “I am sorry for the poor fellows that haven’t got labs to work in!”

  We enter the target fabrication laboratory via a sticky patch of floor that captures any final pieces of the outside world. Inside, there are a few rows of scientists using machinery to minutely manipulate the tiny parts that go into a NIF target. There’s a constant gentle hu
m of filtered air-conditioning. Because any stray particles that adhere to the targets can interfere with the demanding uniformity of the sphere of fuel, dust particles greater than five-millionths of a meter in diameter must be extracted from the air. It’s a strange sort of office, and certainly the cleanest I’ve ever stepped into.

  Michael tells me that a typical target costs $100,000 or more. The outer box, the hohlraum, is made of gold, while the outer layer of the sphere is made of diamond, two of the most expensive materials on Earth. However, these account for just a few dollars of materials cost because there’s so little of either in each target. The hohlraum is only one centimeter long, with walls thirty-millionths of a meter thick. Working with such small sizes has its supply challenges. The lab’s staff once needed to buy twenty grams (just under one ounce) of specialist detergent to clean a component; the firm only sold it in drums of two hundred liters (approximately fifty-three gallons).

  The biggest cost of a target is human labor, because it takes hundreds of painstaking hours to build each target. In mass production, the costs would tumble, but for now, the targets are bespoke, tuned for specific experiments. Since NIF fires only four hundred shots a year, every single one counts and is cleverly designed to answer as many questions as possible. “Without targets, there is no science at NIF,” Michael says.

  He says that because even minute changes in the targets—and especially the capsules that sit within the hohlraum—can completely change the experiment. I’m standing in front of some of the two-millimeter-wide spheres. You can’t see this with the naked eye, but they’re like Russian dolls, each of the three layers precision-engineered to serve a different purpose in igniting fusion reactions.