Sun in a Bottle: The Strange History of Fusion and the Science of Wishful Thinking - Charles Seife (2008)
APPENDIX: TABLETOP FUSION
Thiago Olson was an ordinary teenage boy, for the most part. He had one oddity. There was something mysterious about what he did after school. The seventeen-year-old’s friends had nicknamed Olson “the mad scientist.” For good reason: in his basement, he was building his own fusion device.
On June 20, 2006, Olson told his fellow fusion enthusiasts about his homemade “fusor,” cobbled together from parts taken from a defunct x-ray machine. “Yesterday I input power into my fusor for the first time,” he wrote, adding that he was happy to see “the familiar purple plasma” glowing away through a viewing window. Over the next weeks, Olson steadily improved his six-foot-tall device, upgrading the system that handles the deuterium gas in the machine. Three months later, Olson was making national news. “Michigan Teen Creates Nuclear Fusion,” the headlines blared.
Olson had, in fact, done it. A neutron counter implied that Olson’s fusor was producing about 200,000 neutrons a second. And though a fusion device might seem like a scary thing to keep in one’s basement, the fusor was perfectly safe. Once the headlines broke, two government radiation-safety officers and a fire marshal visited his home and gave the fusor a clean bill of safety.
On the surface, it seemed that Olson had succeeded where Pons and Fleischmann had failed. He had come up with a cheap “tabletop” device that actually achieved nuclear fusion. The public reacted with astonishment, because the cold-fusion debacle seemed to prove that tabletop fusion was impossible. However, it is not that hard to build a tabletop fusion device; people have been doing it for decades.
Indeed, Utah’s history with tabletop fusion goes back decades before the Pons and Fleischmann fiasco. The first person to achieve fusion with a cheap device was Utah born, a young man who grew up on a farm. His name was Philo Farnsworth.
Farnsworth is best known for inventing electronic television. As a young boy, he was plowing a potato field—back and forth, back and forth—when he was inspired with an idea. He could use the same back-and-forth motion to “dissect” a photographic image with a stream of electrons. Though it took years for him to perfect the device itself, at age fourteen Philo Farnsworth had invented a rudimentary television camera.
Farnsworth’s device, known as the image dissector, first turned a picture into a set of electrons. Light causes a peculiar material—cesium oxide—to emit electrons, so an image shining on a plate of cesium oxide will change from a pattern of light and dark spots into a pattern of electrons streaming from the plate. Electrons, unlike photons, are strongly affected by electric and magnetic fields, and Farnsworth exploited this property by using electromagnetic fields to move the electron image back and forth, plowlike, over a detector. This allowed Farnsworth to convert an image into an electronic format that could then be transmitted over the airwaves. Though it was a relatively crude device, it worked. The age of electronic television had begun.81 Unfortunately, a nasty patent battle ensued. Farnsworth won, but he never got rich from his invention. (Just the opposite; it nearly drove him to madness. At one point, he committed himself to an insane asylum and received shock therapy.)
Farnsworth was a brilliant inventor, particularly when it came to manipulating charged particles—like electrons—with electric and magnetic fields. So when he first heard about attempts to create a fusion reactor in a magnetic bottle, he came up with the design for a device that he thought would do it. In the 1960s, he mortgaged his house and borrowed against his life insurance to make his dream a reality. The result was the Farnsworth fusor.
While Farnsworth’s television camera manipulated electrons, his fusor manipulated deuterium nuclei. Stripped of its electrons, a deuterium nucleus has a charge equal and opposite to the electron’s; though deuterium is much more massive than an electron, it, too, can be guided and accelerated by a powerful electric field.
Over the years, Farnsworth and his colleagues patented a number of slightly different designs for the fusor, but in principle they were all relatively simple. A fusor takes deuterium nuclei (or the nuclei of other elements) and injects them into a vacuum chamber that contains a pair of charged metal electrodes. The electrodes have to be shaped to allow the nuclei to pass through them; for example, they might be two concentric wire-mesh spheres, a big positively charged sphere surrounding a smaller negatively charged one. When a deuterium nucleus is squirted past the outer sphere, it is repelled by the positive charge and attracted to the negative charge of the inner sphere, so it zooms inward with ever increasing speed. If the spheres are kept at high voltages, the ions will be moving so fast that they will overshoot the inner sphere and plunge toward the center of the device, where they might strike other ions that have fallen inward from other directions. They might even fuse, releasing energy.
The fusor wasn’t tough to build. The inventors had to be able to create a decent vacuum inside their chamber, construct electrodes designed to handle a very high voltage, and of course, get themselves some deuterium to inject into the device. Other than that, building the fusor was really pretty easy, thus well within the reach of a dedicated amateur. A small one can fit on a tabletop. And it works, too. Farnsworth got neutrons right away. Soon he and his colleagues were producing so many neutrons they had to run the device in a large pit, using the ground to shield them from the flood of particles.
Unfortunately, the Farnsworth fusor, as well as later devices that use electric fields to confine plasmas, will probably never be able to produce more energy than it gobbles up. Clever as the fusor design is, it is not a very good bottle for a star. Its electric fields let particles escape, and the motion of electrons in the plasma radiates energy away at an alarming rate. Nevertheless, fusors have acquired something of a cult following.
Young Thiago Olson’s fusor—fundamentally the same as Farnsworth’s device—is just one of more than a dozen that have sprung up in amateurs’ basements around the country. Olson was the eighteenth amateur to achieve fusion on his own, according to a roll of honor on a Farnsworth fusor aficionado Web site that Olson regularly visited. (In fact, he wasn’t the first high schooler on the list. The fifth amateur to achieve fusion, Tanhui Li, was also a high-school student; his fusor won him a scholarship in the 2003 Intel Science Talent Search.)82 Though Olson doesn’t make any claims that his device will solve the world’s energy problems, many die-hard fusor fans are convinced, hoping against hope, that fusors will soon lead to a fusion reactor—a source of unlimited energy.
On November 9, 2006, just days before the Olson story broke, the fusion physicist Robert Bussard gave a talk at Google about his research with a modified fusor. He had been working for the navy, but after a number of years he had run out of money for the program. The scientist told his audience that if he could only get his hands on $200 million, he would be able to produce a working power plant within four to five years. Bussard was deceiving himself if mainstream scientific thought is any guide. The equations of plasma physics strongly imply that fusorlike devices are very unlikely ever to produce more energy than they consume. Nature’s inexorable energy-draining powers are too hard to overcome.
Luckily, the fusor is not the only tabletop fusion device around. Plenty of researchers are building small, cheap fusion machines. Scientists without huge budgets have gotten fusion to work with inexpensive lasers, and by even stranger means.
A major hurdle with laser fusion is that electrons tend to absorb the light beam’s energy better than the heavy nuclei they are attached to. But hot electrons are pretty much useless for inducing fusion, which requires hot, fast-moving nuclei instead. In an ingenious experiment, Todd Ditmire, a Livermore physicist, figured out how to turn this liability into an asset.
Ditmire injected microscopic droplets of deuterium into a vacuum chamber and then zapped them with a cheap infrared laser. Ordinary laser fusion scientists had long since abandoned infrared lasers because infrared light heats electrons too much. However, this effect was precisely what Ditmire was looking for. When he shot the laser at the deuterium microdroplets, the laser heated up their electrons, boiling them off in a fraction of a second. The positively charged nuclei left behind, stripped of their negatively charged electrons, began repelling their neighbors. All the nuclei immediately tried to escape from one another, and the droplets exploded with great force, spewing deuterium nuclei at high speeds in all directions. Ditmire’s laser did the exact opposite of what traditional laser fusion was trying to do: instead of compressing and confining a dollop of deuterium plasma, he was causing it to blow apart. Ditmire discovered that on occasion, though, the fragments from exploding droplets—fast-moving deuterium nuclei—collide with each other and fuse. For every laser shot, he got about 1,200 neutrons from fusion. Considering that the energy of the laser was so low, less than what’s put out by a Christmas light in a second, this was an impressive fusion yield. Even so, the energy produced by the fusion was ten million times less than the energy the laser poured in. Ditmire’s scheme might be useful for studying fusion on a very tiny scale, but it will never lead to a reactor that produces more energy than it consumes.
Seth Putterman, working at UCLA, came up with an even more innovative way to induce fusion. He and his team created a device whose heart was made of a crystal of lithium tantalate, a compound with a very curious property. It is pyroelectric: when you heat it, electrons in the crystal rearrange themselves so that one side of the crystal is positively charged and the other side is negatively charged.83 Attached to the crystal was a fine tungsten needle. When the researchers heated the lithium tantalate, the crystal’s charges rearranged themselves and ran down the tungsten. The crystal and needle acted as a giant focusing device; all the energy of heating the crystal got turned into an extremely strong electric field right at the needle’s tip.
Putterman and his colleagues put this device—about the size of a coffee can—in a chamber filled with deuterium. When they turned on the heater, the device worked as advertised, creating a huge electric field near the tip of the needle. When a deuterium atom ventured near the tip, the electric field immediately stripped off its electrons and sent the nucleus zooming away at tremendous speed toward a target filled with deuterium. On occasion, the flying deuterium would fuse with one in the target. All in all, the device produced about eight hundred neutrons per second. Again, it was an impressive display and might even lead to a commercial device to produce neutrons; however, it will always consume more energy than it produces. Beams of deuterium nuclei lose energy whenever they strike a target, and on average the amount of energy lost by the deuterium nuclei that don’t fuse will outweigh the energy produced by those that do.
After the cold-fusion debacle, the idea of tabletop fusion seemed impossible—a pipe dream sought after only by cranks. (The first reaction of Michael Saltmarsh, the bubble fusion debunker, upon seeing Putterman’s pyroelectric fusion paper was, “Oh, God, not again.”) But in fact, tabletop fusion—fusion reactions carried out cheaply in a small piece of laboratory equipment—is real, It just isn’t yielding any more energy than it consumes, so it is useless as a source of power.
It is an unfortunate fact of nature: unless you are creating fusion in a hot, dense plasma, you are extraordinarily unlikely to produce excess energy. Too many phenomena conspire against you. Tabletop fusion is an interesting curiosity, but not a path to unlimited power.