Sun in a Bottle (8 page)

FUSION REACTIONS IN THE SUN:
Colliding protons release an electron and make deuterium, then deuterium and protons make helium-3, and finally helium-3s make helium-4, producing a lot of energy in the process.

The fusion energy released in the guts of the sun makes it shine. But it also threatens to blow the sun apart. The energy heats the hydrogen gas, making the nuclei slam together harder and harder, and the reaction speeds up, pouring more energy into the cloud. This would appear to lead to a runaway reaction; the furnace should run hotter and hotter and eventually get so energetic that the cloud explodes violently in all directions. However, it turns out that the hotter a cloud of gas is, the more it expands. So when the fusion engine runs hot, the star expands slightly. It becomes slightly less dense and the atoms slam into each other less and less often. The fusion engine slows, and the star cools. Gravity takes over once more, compressing the star, heating it up, and making the fusion energy run hot again. This means that a star is in a delicate equilibrium, caught between the force of gravity and the energy of fusion. The force of gravity tries to collapse the star while the energy of fusion tries to blow it apart.

EQUILIBRIUM IN A STAR:
Two forces compete for dominance in every star; gravity tries to crush the star to a point, while the fusion explosion tries to blow the star apart. In a normal star, these two forces are in balance.

When that delicate equilibrium fails, the star dies. A fusion engine, no matter how well-balanced, can only run for as long as it has fuel. As a star gets older, its hydrogen supply begins to run out; the hydrogen fusion cycles sputter to a halt. A large star then turns to other light elements to keep itself from collapsing. It begins to fuse helium, turning it into yet heavier elements, such as carbon and oxygen. As the helium runs out, the star fuses heavier and heavier fuels: carbon, oxygen, silicon, sulfur. The fusion engine is rolling further and further down the fusion hill. Soon it hits bottom. The valley of iron.

Fusion gets its energy by making light elements roll down the hill toward iron. Fission gets its energy by making heavy elements roll down the hill toward iron. Iron, already at the bottom of the hill, can’t yield energy through fusion or fission. It is the dead ashes of a fusion furnace, utterly unable to yield more energy. When a star runs out of other fuels, its iron cannot burn in its fusion furnace. The fusion engine has nothing that it can turn into energy, so it shuts off and the star abruptly collapses. Depending on the star’s nature, it can die a fiery death: the final collapse ignites one last, violent burn of its remaining fuel, blowing up the star with unimaginable violence. A supernova, as such an explosion is called, is so energetic that a single one will typically outshine all the other stars in its galaxy combined. The star spews its guts into space, contaminating nearby hydrogen clouds in the process of collapsing into new stars. This is what happened before our sun was born; it got seeded with the nuclear ash of a supernova explosion. All the iron on Earth, all the oxygen, all the carbon—almost all the elements heavier than hydrogen and helium—are the remnants of a dead fusion furnace. We are all truly made of star stuff.
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When Hans Bethe solved the riddle of the sun’s energy, the idea of a fusion bomb seemed absurd. To ignite a fusion reaction, you need to have a bunch of light atoms that are extremely hot (so they have enough energy to overcome their mutual repulsion and slam into each other) and extremely dense (so they are close enough to one another that they collide frequently). The laws of nature seem to conspire against having those two conditions at the same time: hot things expand, reducing their density. The only reason a star can keep a stable fusion engine going is because it is so massive. It is held together by the intense force of its own gravity, resisting the explosive force of the fusion engine in its belly. A cloud of hydrogen smaller than a star doesn’t have the benefit of that gravitational girdle keeping the reaction from puffing out. Even if you are somehow able to start a fusion reaction, it will blow itself up and snuff itself out in moments.

It is extraordinarily hard to get fusion going outside a star. Even the biggest explosion of all—the big bang—couldn’t get fusion going for more than a few minutes. In the first seconds after the big bang, all the matter in the universe—lots of fundamental particles, including a whole bunch of protons—was contained in a relatively small, intensely hot space. Protons—hydrogen nuclei—fused to create helium. But the universe was expanding rapidly because of its own explosive energy. After about three minutes, the universe had expanded so much that the matter wasn’t dense enough to fuse anymore. As hot and dense as the early universe was, it could only sustain fusion for a measly three minutes. After that, there wasn’t any fusion going on in the universe, not until it occurred again in the heart of a solar furnace. To get fusion going on Earth, you must create conditions that are as hot as the first few minutes after the big bang. And without the massive gravity of a star, it is nearly impossible to keep those conditions going for very long.

This was the major obstacle to designing the hydrogen bomb. Even with deuterium and tritium (or lithium) as fuel—deuterium and tritium are relatively easy to fuse—it is hard to make the fuel hot enough and dense enough to get the nuclei fusing. And if you can initiate fusion, you have to maintain the high temperatures and high density long enough to generate an appreciable amount of energy from it.

Teller’s first design would not have worked. Even with the power of an atomic weapon behind him, he would have found it difficult to get the hydrogen fuel hot enough and dense enough to ignite. As a substance heats up, it radiates energy more rapidly. In fact, the radiation goes up as the fourth power of the temperature: double the temperature of an object and it radiates its energy sixteen times as fast. To ignite fusion, the fuel has to get to tens or hundreds of millions of degrees (depending on the density of the fuel). Yet even then, it will radiate its energy away at a tremendous rate; it is almost as if everything in the universe is trying to cool it down. And if he had been lucky enough to ignite fusion, he would have been unable to keep the reaction from blowing itself apart with its own energy just as it got going.
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The Alarm Clock design was the simplest way to get around these problems. This bomb was to be like a spherical layer cake with alternating layers of heavy, fissioning material and light, fusionable hydrogen isotopes. By imploding the whole thing symmetrically, making sure that nothing squirted out, the hydrogen would get hot and dense enough to ignite. It would fuse for a tiny fraction of a second before the whole package blew itself apart. Teller dreamed up the concept in 1946 and discarded it as impractical. In Russia, the physicist Andrei Sakharov came up with a very similar design and called it the “sloika” after a Russian layer cake. The sloika was the basis of the Joe-4 test, but the design was eventually abandoned because megaton-size bombs became too large to use as weapons. Within a few years, Sakharov, like Teller and Ulam in the United States, figured out a much cleverer way to ignite a fusion reaction for a short time.

On the surface, it might seem that America’s first fusion bomb, Ivy Mike, was little different from Teller’s original “bomb at the end of a tank of hydrogen” design. But in fact, it was impossible to ignite a cylinder full of fuel in the way that Teller had hoped; more energy was radiated away by the expanding fireball than the fusion was producing, so the reaction would snuff itself out very quickly. The Teller-Ulam design had some subtle techniques to avoid this problem.

In a bomb like Ivy Mike, the fission bomb that starts the reaction (the primary) is a distance away from the cylindrical vessel containing deuterium and tritium (the secondary). As the fission bomb explodes, it radiates a huge number of x-rays in all directions. These x-rays, being light waves, travel at light speed and move much faster even than the blast wave coming from the fission bomb. As the atom bomb explodes, the x-rays course through a channel left in the casing that houses the primary and secondary. The x-rays then vaporize a plastic shell, turning it into a plasma, a hot soup of nuclei and electrons. This superhot plasma radiates more x-rays, which strike a heavy
pusher
surrounding the fuel, compressing the fuel from the outside. As the fuel cylinder compresses, it heats up, getting denser and denser. The compressing plasma soon ignites a small “spark plug
”
of fissionable material at the center of the cylinder, generating a second fission explosion that squeezes the fuel from the inside. The deuterium and tritium are caught between a pusher that is pushing inward and the spark plug explosion pushing outward. The fuel compresses even further and, whoomp! The fusion reaction ignites. It’s as if there’s a tiny hunk of the sun on Earth.

DETONATION OF A HYDROGEN BOMB:
(a) A fission bomb explodes at one end of the device, sending x-rays in all directions. (b) The x-rays strike the walls of the container, causing them to evaporate and radiate more x-rays. These x-rays hit the container containing deuterium and tritium, causing it to implode. (c) The compressing deuterium and tritium fuel heats up and ignites a fission “spark plug” at the center of the device, causing it to explode outward. Trapped between the imploding container and the exploding spark plug, the fuel ignites in a fusion reaction.

The reaction only lasts for a fraction of a second before it blows itself apart, but in the process it releases an enormous amount of energy. Ivy Mike was equivalent to ten megatons of TNT, but there was no reason why the whole device could not have been scaled up by adding a third stage . . . or a fourth. In 1961, the Russians detonated a (roughly) fifty-megaton whopper nicknamed the Tsar Bomba, the most powerful weapon ever built by man.

In theory, there was no end to the power of fusion. But the race to build ever-bigger hydrogen bombs crept to a halt, because it had diminishing returns. As early as 1949, scientists realized that after about 150 megatons, hydrogen bombs simply take a huge column of air and lift it into outer space, punching a hole in the atmosphere about fourteen miles across. Bigger bombs would not do much more than that. They would radiate most of their energy uselessly into space. So after 150 megatons, there was no point in getting bigger, unless you wanted to build a fusion device large enough to destroy the Earth. Not even the most rabid hawks were in favor of that.

Nevertheless, with Ivy Mike and its successors, the fusion bomb scientists had succeeded at creating a tiny star on Earth. For a fraction of a second, scientists were able to get a fusion reaction going. They had figured out how to use that energy for war. It would be much, much harder to harness that energy for peace.

I’ve felt it myself, the glitter of nuclear weapons. It is irresistible if you come to them as a scientist. To feel it’s there in your hands to release this energy that fuels the stars. To let it do your bidding. To perform these miracles, to lift a million tons of rock into the sky. It is something that gives people an illusion of illimitable power and it is in some ways responsible for all our troubles—I would say, this, what you might call “technical arrogance” that overcomes people when they see what they can do with their minds.

 

 

O
n January 15, 1965, deep inside the Soviet Union, a nuclear rumble shook the earth. The Americans were the first to spot the radioactive cloud as it floated over Japan and toward the Pacific Ocean beyond. The Soviets had detonated a fusion bomb, and the fallout was contaminating the atmosphere over the Japanese mainland. This particular explosion was different from all the other bombs exploded over two decades of nuclear testing. This one had the makings of a major diplomatic incident.