Free Sun in a Bottle by Charles Seife Page A

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. 19
    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.