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“Thermonuclear fusion” is a big phrase with big connotations. Related phrases that immediately come to mind include “the Sun” and “H-bomb”. For those following the field, “Tokamak” and “Shiva” might pop up. What about the word “benchtop”? Throwing this in instantly brings up the 80’s “cold fusion” fiasco. But some of the frontrunners in the race to achieve controlled nuclear fusion are humble devices that could fit on your kitchen table. The newest, introduced in April of 2005, could fit in your pocket. And we’re not talking about cold fusion.
In our workaday lives on Earth, atoms merely rearrange themselves in a relatively mellow way. Nuclear fusion is the process of jamming two atoms together so hard that they form a new, different atom. The nugget in the center— the nucleus— is the part that gives an atom its personality, and this is the part involved in a “nuclear” reaction. If the starting nuclei are light, fusion yields a huge dividend of energy. Fusing together the lightest nuclei, hydrogen, gives the greatest yield. This is the reaction that keeps the Sun glowing. Since hydrogen is the most abundant element in the universe, most experts agree that fusion is THE energy source for humanity’s needs.
Many scientists, thinking about the Sun and the scale of the energies involved, designed appropriately humongous machines to attempt fusion. The “classic” device is the Tokamak, Russian for “torus”. These are building-sized donut-shaped things designed to hold hot gasses by electric and magnetic manipulation. Their track record is decidedly so-so. The next “big science” design is laser fusion. Such “megalasers” have 20 or more hefty lasers all focused on the same point. These have names like “Nova” and “Shiva” (the Hindu goddess of destruction). Such toys are used on off-hours to simulate nuclear explosions.
Although both of these “big” designs DO produce fusion (and both are improving all the time), the real Holy Grail is “breakeven”, where one gets as much energy out as one puts in. No invention has yet achieved this milestone, except for nuclear weapons, which are hard to work with. However, a few smaller devices have been achieving measurable fusion for decades.
Debatably, the first working device was the Farnsworth Fusor, created by electronic television inventor Philo T. Farnsworth himself. This Utah-plowboy-turned-engineer became the world’s expert on complicated vacuum tube designs. One design he experimented with focused electrons at points that didn’t touch anything. Philo reasoned that this would attract protons to high densities, possibly causing fusion. It worked. The beauty of the design was that heat lost from individual protons tended to go to other protons, rather than to the outer walls of the device. The ball didn’t really get rolling, however, until Robert Hirsch joined the lab and made some critical improvements to form the currently popular Hirsch-Meeks Fusor (pictured here). One in working mode is the opening picture of this article.
The design is simple enough that a dedicated high school class could build one, and folks continue to tweak the design in search of the elusive breakeven. In the meantime, the Fusor has found a commercial application as a laboratory neutron source. Flick a switch, deuterium and tritium (heavy hydrogens) fuse, and neutrons come out. Turn off the switch, neutrons stop. Far more convenient than sources based on radioactive material, fission reactors, or giant particle accelerators.
An equally old device is called the “Z-pinch”. This device is based on currents within the hot gases generating magnetic fields that interact with external fields to squeeze the plasma. The plasma compresses until it sort of explodes, but not in a good way. Incremental improvements gave demonstrable amounts of fusion, but nothing to write home about. In the 1980’s, folks made a new “staged” design using a cylinder of wires that vaporized to start things off. Last I heard, improvements on this design have brought it within a factor of two of breakeven! By this time, however, “benchtop” kind of fails to describe this one. (See above picture of the “Z Machine” at Sandia National Labs.)
The next-to-latest thing is “bubble fusion”, in which ultrasound makes tiny bubbles in heavy water. The bubbles collapse, supposedly achieving tremendous temperatures and fusion. The validity of these experiments is still hotly debated. One undeniable fact is that the bubbles emit tiny bursts of light, known as “sonoluminescence”.
Now they’ve all been outdone. In the April 29 2005 Nature, Seth Putterman (who also works on bubble fusion) and others reported the creation of a tiny device (pictured) that can put out a stream of neutrons. The key is a “pyroelectric” crystal that generates a large voltage when modestly heated by, say, a nine-volt battery. The voltage is concentrated into a tiny tungsten needle coming out of the crystal, and the whole thing is immersed in deuterium gas. The voltage ionizes a deuterium atom, it shoots out at top speed, hits a deuterium-coated target, and— Voila`! Fusion.
The device isn’t set to break any records, but it may prove useful as a portable neutron generator. Neutrons are useful in seeing through solid objects, killing cancers, analyzing material composition, and detecting nuclear material, among other things.
And it’s got no moving parts.