A crystal with a strange property is at the heart of a clever method for inducing nuclear fusion in a tabletop-sized device. The inventors of the machine--which works by firing fine beams of atomic nuclei at other atoms--are not billing it as a possible source of energy, but they say it could serve as a portable source of neutrons and of x-rays for medical therapies. Although the field of room-temperature fusion is littered with scandals and dubious discoveries, the device has already won over some skeptics.
In this week's issue of Nature, Seth Putterman, a physicist at the University of California, Los Angeles, and colleagues describe the fusion device, which is about the size of a small bucket. At its heart is a small crystal of lithium tantalate--a material that has a peculiar property: pyroelectricity. Heat a pyroelectric crystal up, and its electrons rearrange themselves, making it act something like a battery.
Putterman's group cooled the pyroelectric lithium tantalate crystal and put it in a chamber full of deuterium gas. When they warmed the crystal with a heater, the pyroelectric effect created a huge electric field near a tungsten needle attached to the crystal. The crystal and needle essentially focused all the energy of the crystal's heating to the very tip of the tungsten spike. When deuterium (atoms of heavy hydrogen, with a proton and a neutron in the nucleus) ventured near the tip, the field stripped off their electrons and shot the charged nuclei into a deuterium-loaded target. Some of those deuterium ions struck deuterium in the target and fused, releasing protons, neutrons, and energy.
"Neutrons were everything to this experiment," says Putterman, whose data show about 900 neutrons per second flying away from the target with the energies one would expect from a fusion reaction. "If you look at the raw data, we maintain that it's incontrovertible," he adds.
"My first reaction was, 'Oh, God, not again,' " says Michael Saltmarsh, a physicist and neutron expert at Oak Ridge National Laboratory in Tennessee. "But upon reading the paper, I thought that it was really neat; it's such a cute way of making an accelerator." While Saltmarsh cautions that the device is unlikely to be useful for generating energy and would have to be significantly scaled up to be useful as a neutron generator, he finds it to be quite innovative. "There [are] no moving parts," marvels Saltmarsh. "Just heat it up."