Not everything, though, occurred to the Americans first, especially when it came to fusion reactors. In 1950, Sakharov was hard at work trying to figure out how to build a hydrogen bomb—an uncontrolled fusion reaction—when he began to ponder whether the reaction could be controlled. Like his American counterparts, he came up with a scheme using magnetic fields, but his idea was slightly different from the ones that would guide Project Sherwood. Sakharov’s device was neither a Stellarator nor a pinch machine. It was somewhere in between. It was a novel design, one that combined some advantages of a pinch machine with those of a Stellarator. It was just what scientists were looking for.
In a pinch machine, the plasma confines itself. The pinch begins by inducing a current of some sort in the plasma, forcing it to contract and to heat up. The Perhapsatron, ZETA, and Columbus all did this by running a current down the length of the plasma, while Scylla ran an electrical current around the circumference of the tube of plasma. In both cases, though, there is a current inside the plasma; this current induces a magnetic field, which squashes the plasma. Pinch machines were successful at making a plasma very dense and hot, even inducing a bit of fusion, but physicists couldn’t keep those conditions going for very long. The pinch was extremely short-lived. Once the current disappeared, so did the confinement of the plasma. Given the enormous energy it took to set up a pinch, and how little energy was generated by the brief fusion reaction, a pinch machine could not ultimately become a working reactor.
In a Stellarator, on the other hand, the plasma is confined from the outside. The machine uses carefully arranged electromagnets to generate intricate magnetic fields that bottle up the cloud. In theory, these fields would allow a Stellarator to confine the plasma for a relatively long time. Unlike a crush-and-release machine, a Stellarator attempted to maintain its hold on the plasma, keeping a fairly stable cloud. But scientists were having difficulty not only confining the plasma but also heating and compressing the cloud. It was much harder to warm and squash plasmas in a Stellarator than it was in a pinch machine.
The choice was bleak for American scientists in the late 1950s and early 1960s. They could get high temperatures and densities for a short time or lower temperatures and densities for a longer time, but not both. Yet scientists really needed a magnetic bottle that could heat the plasma to tens of millions of degrees, keep it very dense, and hold it for a relatively long time. The heating and density would ensure that the fusion reaction took place, while the confinement would ensure that the plasma reacted long enough to generate a significant amount of energy. Only then could physicists hope to turn such a magnetic bottle into a fusion reactor.
Sakharov’s scheme appeared to provide an answer. His bottle looked little different from the ones the Americans and British were proposing. It was donut shaped—toroidal—and used coils of wire to induce magnetic fields, earning it the cumbersome name
toroidalnaya kamera ee magnitnaya katushka
(toroidal chamber with magnetic coil). It was called the
tokamak
for short. But the tokamak was a bottle with a difference. Whereas the Stellarator used external magnetic fields to contain the plasma and the pinch machines used internal electric currents to squash it, the tokamak did both.
The tokamak has multiple sets of coils. One group of coils sets up a magnetic field that constrains and stiffens the plasma; it’s an external magnetic bottle, somewhat similar to the Stellarator’s, although not quite as sturdy. What gives the tokamak an extra bit of oomph is another set of coils that pinches the plasma. When scientists send a current through those coils, it induces a corresponding pinching current in the plasma circulating in the torus. This one-two punch of the external magnetic fields and internal current gave scientists a tool that, they hoped, would keep a hot, dense plasma stable for a long time.
Of course, the tokamak design had drawbacks as well. Unlike the Stellarator, which doesn’t require a plasma current at all, a tokamak absolutely needs one; its external magnetic bottle can’t by itself contain the plasma cloud for very long. But reducing this plasma current adds layers of complexity to the plasma, making it more unpredictable.
In some sense, the tokamak is something like a bicycle. Just as a bicycle is not stable until it is going relatively quickly, a tokamak’s plasma is not stable until the plasma current is up and running. The Stellarator is more like a tricycle. Just as a tricycle can be stationary, or can move forward or backward without any threat to its basic stability, a Stellarator can either have no plasma current or have one in either direction and still, theoretically, be stable.
Unfortunately, the current in a tokamak’s plasma is just one more thing that can fail. If an instability causes the current to drop momentarily, things get very bad very quickly. The plasma suddenly loses its pinch and explodes in all directions. This event is called a
disruption,
and it can be extraordinarily violent. It can even damage the machine. (One disruption at a modern British tokamak made the whole thing, all 120 tons of it, jump a centimeter into the air.) However, the disadvantages of the tokamak soon seemed small compared to the advantages of the design.
Sakharov was too busy working on nuclear weapons to spend a lot of effort on fusion reactors. But other Russian scientists, particularly the physicist Lev Artsimovich, took Sakharov’s design and put it to the test. By the mid-1960s, he was reporting spectacular results. His tokamak was confining a plasma at a given temperature and density ten times longer than could any other machine. Though confinement times were still on the order of milliseconds, Artsimovich’s results, if they were to be believed, indicated that Sakharov’s tokamak was blowing its competition away.
When Spitzer and the Americans first heard the Russian claims, they were skeptical, in part owing to American arrogance. The Stellarator was performing quite poorly, so they concluded that the problems they were encountering were likely due to a universal problem with magnetic confinement. If they weren’t succeeding, nobody was. The Americans were dubious that the Russians could do much better with their tokamak. Furthermore, Artsimovich’s measurements of the temperature of the plasma were rather crude. American scientists were relatively quick to disbelieve them. In the mid-1960s, Spitzer’s skepticism led him to conclude that tokamak performance was roughly the same as the Stellarator’s—underwhelming.
This conclusion was bad news for American fusion research. The enthusiasm of the 1950s had brought a downpour of funding from Congress. Since Project Sherwood’s inception, its budget had skyrocketed from almost nothing to nearly $30 million a year by the time of the 1958 UN conference. As the Stellarator began to choke, losing its plasma rapidly, a skeptical Congress began to wonder whether fusion reactors were possible at all, much less economically feasible. It didn’t help that the scientists, in their optimism, had consistently oversold their machines. They had promised Congress they would be building prototype reactors by the early 1960s, and the machines were nowhere near that stage. And in October 1957, the Russian surprise launch of Sputnik gave the Earth an artificial moon—and it gave Congress another Cold War scientific competition requiring truckloads of taxpayer money. The space race had officially begun. Fusion energy was no longer in the spotlight, and its budget stagnated, then dwindled.
The tokamak had to come to the rescue, but it would be several years before American and British scientists would accept that the Russian achievements were real. It was not for lack of data; Artsimovich continued presenting better and better results—dense plasmas heated to tens of millions of degrees and confined for handfuls of milliseconds. The tokamak results were still far from those needed for a realistic source of fusion energy, but they were certainly an order of magnitude better than anyone else’s. The work was getting harder to dismiss, but detractors still argued that the Russian temperature measurements were inaccurate. To settle the matter, in 1969 a British team visited Artsimovich’s lab in Moscow. They came armed with a sensitive instrument that could measure the temperature of a ten-million-degree plasma.
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At the heart of the instrument was a device that would change the face of fusion research, and not just because it confirmed the Russian claims. It would provide a new way of bottling a plasma without the use of magnets. The device was the laser.
Depending on whom you ask, the laser was invented at Columbia University in the late 1950s or at Hughes Research Laboratories in 1960. (There were competing claims and a patent battle.) But there’s no doubt that in 1960 a short paper in
Nature
gave the physics community a powerful new tool.
A laser is a device that produces an unusual beam of light. Even to the uninitiated, it is obvious that laser light is different from, say, the light that comes from a flashlight. If you shine a flashlight at a distant wall, you will see that it makes a large, circular spot. If you shine a laser pointer at the same wall, it makes only a tiny dot, barely larger than the hole from which the laser beam emerged. Laser light stays together in a tight beam rather than spreading out into a diffuse cone. A laser beam also consists of light that is a single, intense color,
44
unlike a flashlight beam, which is made of a whole bunch of colors mixed together and appears white.
There are many methods of generating light. If you heat something high enough, it begins to glow. When a substance is energetic enough, it emits visible light. (This is how an incandescent lightbulb works; the filament in the bulb is simply heated to a very high temperature.) It is a law of nature: the hotter an object is, the more light waves it emits. Or, if you prefer, you can think of the emissions as light particles rather than light waves. The laws of quantum theory say that light has both a particle-like and a wave-like nature, so physicists use whichever description is most suitable for the behavior they are attempting to describe.
A particle of light—a photon—can interact with matter in a number of different ways. It can strike an atom and give it a kick. It can make the atom rotate or move in other manners. If the photon is just the right color, the atom can absorb it. Absorbing a photon “excites” the atom, packing it full of the energy that once resided in the light particle. This excited atom will soon disgorge the photon, emitting a light particle of precisely the same color and relaxing from its excited state.
In 1917, Albert Einstein made a curious prediction about excited atoms. Such an atom is quivering with energy, looking for an excuse to spit out the photon it has absorbed. Einstein’s calculations showed that if a photon of the right color happens by—one precisely the same color as the one absorbed by the atom—then the atom will immediately disgorge a photon. This photon not only will be precisely the same color as the passerby but will also move with it in lockstep. The two photons will behave almost as a single object. This phenomenon is known as stimulated emission, and it is the mechanism the laser uses to produce its beam of light.
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Imagine that you have a hunk of material—a whole lot of atoms—that you want to turn into a laser. The first step is to excite all the atoms. You do this by “pumping” the material full of energy. It doesn’t matter how. Some lasers pump a material with electricity. Some lasers do it with light, and some do it with chemical reactions. It might even be possible to use nuclear bombs to pump atoms into an excited state.
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Once the atoms in the material are excited, they are primed to get rid of their energy—they want to emit light of a particular color.
This is where the clever part happens. Send a photon of that specific color into the material. The photon encounters an excited atom, which then disgorges a second photon of the same color through stimulated emission. These two photons move in lockstep. They encounter another excited atom, which emits another photon of the same color: three photons now in lockstep. Another excited atom, another photon: four photons, all the same color, all moving in precisely the same way. As the photons move through the material, they encounter more and more excited atoms, which emit more and more photons. The beam snowballs, growing bigger as it travels through the material. By the time it finally emerges, the beam consists of an enormous collection of light particles. It is an intense beam, and all the photons have the exact same color and are moving in lockstep, almost like one enormous particle of light. This is the secret to the laser’s power. It is why the photons in a laser beam don’t zoom out in different directions and have all sorts of colors as a flashlight’s do. The photons in an ordinary beam of light are like are an unruly mob; the photons in a laser beam are an army marching together with a single mind.
The laser’s unusual properties make it an incredible scientific tool. The tight beam allows it to travel great distances—to the moon and back, even—without scattering and dissipating too much. Because the beam is made of photons of the exact same color, it provides a great way to measure very, very hot temperatures.
Shine a laser at a plasma. The photons in the beam will begin with the exact same color. But as the photons strike the fast-moving particles in the plasma, the plasma gives the photons a kick, adding a bit of energy to them, shortening their wavelengths and making them slightly bluer. By looking at the color of a laser beam after it hits a plasma, scientists can calculate the energies of the particles in the plasma, which, in turn, reveals the temperature.
When the British scientists shined a laser beam at Artsimovich’s tokamak plasma, they saw that the Russians were not exaggerating. Their plasma was tens of millions of degrees, dense, and relatively well confined. The tokamak
was
performing much better than the other forms of magnetic bottles. This was wonderful news for the fusion community in the West, even though the Russians, rather than the Americans or the British, had done it. (One Atomic Energy Commission worker reportedly danced on a table when he heard the news.) Sakharov’s invention showed a way to bypass the troubles of the pinch machines and the Stellarators. Practically overnight, plasma physicists across the world scrapped their old devices and built tokamaks. Even Spitzer succumbed to tokamania. By January 1970, the model-C Stellarator was scrap metal. In its place, a mere four months later, a tokamak sprang up. The fusion community had been pumped full of energy once more.