When nuclear physicists were able to make the transition
from fissile atomic bombs to fusion-driven thermonuclear bombs in less than a
decade, it seemed that the harnessing of fusion power for the electrical grid
was imminent. At the time nuclear fusion was considered the energy source of
the future, and today it remains just that. It turned out that containing a
miniature star in a magnetic bottle was much harder than initially anticipated.
Taking in this melancholy state of affairs, by the 1970s researchers decided to try to make fusion easier by adding giant lasers to the reactor. The result was a serious effort toward using shock and swiftness rather than magnets to contain the fusing material that remains in work today. You can read more about the concept here:
The heavy nuclei required for fission reactors are naturally unstable, and only need a slight touch from their neighbors to cleave apart, releasing energy in the process. By contrast, fusion aims to bring nuclei together that are content to remain themselves, and the reactor has to push each nucleus up a long hill of electrostatic repulsion to bring them close enough to embrace. The energy release from each union of two hydrogen atoms is several times more potent per unit mass than that from each split of a uranium or plutonium atom, and either reaction is staggeringly powerful compared to the chemical reactions of molecules. Fusion also has the distinct advantage over fission of using fuel abundant in nature (or readily available in our nuclear stockpiles, at least) that leaves behind no vile radioactive residue. But how to bring the hydrogen together?
Three things are required for such nuclear wizardry - heat, density, and time. Stars, the natural reactors that light up the universe, have all three in abundance. Since waiting billions of years for two nuclei to fuse doesn't make many friends on the power grid, typical fusion reactor designs speed up the reaction by using heavier isotopes of hydrogen more amenable to attachment, and by cranking the temperature above that at the center of the Sun. Under such conditions, the average nucleus of deuterium or tritium will spend a matter of seconds or minutes in the reactor before fusing. Inertial confinement reactors aim to fuse everything in a matter of microseconds by exceeding the Sun in both temperature and density.
Inertial confinement fusion's principle of operation is charmingly simple. For a few trillionths of a second, the reactor delivers trillions of watts of laser light into a pinhead-size pellet of fuel. The flash transforms the surface of the fuel into an explosive shock wave of plasma rocketing both outward and inward toward the fuel's center. On the way in, the wave sweeps up fuel along with it, knocking atoms closer and closer until the pellet, or what's left of it, is up to a hundred times denser than lead and six times hotter than the center of the Sun. At this point, the fuel ignites, and if all goes well it releases about the amount of energy of a barrel of oil without all those pesky carbon dioxide emissions.
Predictably things are a bit more complicated than that, but although inertial confinement reactors quickly advanced to the efficiency of the best magnetic confinement reactors of the time could do in the 1980s, progress since has been slow. Physicists found that while the push of the shock front is considerable, it isn't overwhelming, and turbulence in the wave tends to cause mixing before full compression. Like engine knock in a car, some of the fuel burns to early and fouls the reaction, keeping it from producing more power than the lasers provide. The tolerances required of the laser beams and pellet geometry are formidable, and make fueling the reactor for more than a few experimental shots a severe challenge. Modern lasers are horridly inefficient, and it's unclear if it's possible to unleash more energy from the fusion reaction than the lasers deliver without a fundamental breakthrough in the physics of coherent light. None of this means that power from inertial confinement fusion is impossible, but it leads the curious to consider other options.
Since the harnessing of gravity seems beyond the scope of our current understanding of physics, the only credible alternative is magnetism. Most experimental fusion reactors use intense magnetic fields to bottle up the plasma like a battery of lightning. My friend who does research on plasmas and their application to spacecraft propulsion likens this problem to "holding up a ball of water with a whole bunch of sharpened pencils." Magnetic boxes are really more like nets, and the hottest atoms tend to leak out first, leaving the bulk too cold to light up. This is an exotic physics regime where the outlands of heat, nuclear forces, magnets, and fluid dynamics share an uneasy border. Nobody quite understands what happens here, and the reactors still don't unlock more energy than it takes to light them.
Will fusion, magnetically- or inertially-confined, always be the power source of the future? Kim Stanley Robinson thought not. In Blue Mars he imagines great spaceworthy liners powered by inertial-confinement fusion plying back and forth between the planets in a matter of days, their engines doubling as gravity simulators since the ships never stop accelerating. The imagery bewitches the mind, but for now it's a mirage. Some day, maybe with lasers, maybe with magnets, I'm confident that we'll master the ways of hydrogen and helium. The solar system will seem a little more human-sized then, but for now it remains as vast as the Pacific to those who first set foot on the islands of Polynesia.
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