In the December-1989
issue of Analog, I wrote an
AV Column entitled
“Cold Fusion, Pro-fusion, and Con-fusion” that described and gave my opinions
about the recently announced “discovery of cold fusion” by Stanley Pons and
Martin Fleischmann. These
By contrast, in this column I want to report on a well executed experiment performed by B. Naranjo, J. K. Gimzewski, and S. Putterman (NGP) of UCLA that demonstrates the successful production of the nuclear fusion of deuterium with a relatively simple tabletop experiment. It was reported in the April 28, 2005 issue of the science journal Nature. The announcement of this breakthrough produced hardly a blip in science-based news reports, perhaps because many science reporters had previously been burned by the overblown Pons and Fleischmann affair,.
So what is d+d nuclear fusion? Let’s review the process. Nuclear fusion is the primary energy source of the Sun. High temperatures and pressures near the Sun’s center drive the fusion of hydrogen into helium, releasing lots of energy. Here on Earth, we would also like to use fusion as our primary energy source, but, with the exception of thermonuclear bomb explosions, we have yet to master the trick very well. One must bring two deuterium nuclei (mass-2 hydrogen containing a proton and a neutron) close enough together that they can fuse. This fusion could, in principle, form a single helium nucleus (2 protons + 2 neutrons), and a gamma ray, liberating about five million times more energy than could be obtained from any chemical reaction between two atoms.
However, there are several problems with achieving this. First, both deuteron nuclei have a positive electrical charge. When they get close, these charges repel, producing a large electrical force that pushes the nuclei apart. One must overcome this force with either high temperatures or acceleration to bring the deuterons close enough to fuse.
The second problem is that a fusion process must simultaneously obey the law of energy conservation and of momentum conservation. Because of this dual requirement, the d + d fusion reaction makes helium-3 plus a neutron or hydrogen-3 plus a proton with much higher probabilities than it makes helium-4 plus a gamma ray. Therefore, any d + d fusion reaction should be a prodigious source of fast neutrons, i.e., neutron radiation. As someone said in 1989, Stanley Pons’ own announcement refuted his claims, because if his experiment had actually worked, he would have died of neutron exposure before reaching the microphone.
On the other hand, the NGP experiment does proceed by the d+d → 3He+n reaction and does make lots of neutrons. It gets the deuterons close enough to fuse by accelerating one deuteron to a kinetic energy of about 115,000 electron-volts and slamming it into another at-rest deuterium atom by using a heated 1 cm thick ferroelectric crystal of LiTaO3 (lithium tantalate) to produce a very large electric potential (~115,000 volts).
What?? A 1 cm thick crystal producing a 115 kilovolt electric potential??? That sounds like magic! Yes, I suppose it is magic in a way, the kind of quantum magic that occurs in some crystals and is called the ferroelectric process.
To understand it, let’s first consider ferromagnetism. The atoms of the metals iron, cobalt, and nickel contain little built-in magnetic compasses, each with its own little needle with a north and a south pole, called its magnetic dipole moment. Many materials have such magnetic moments, but in the ferromagnetic materials they like to line up to form a permanent magnet, with most of the atoms pointing their little compass needles in the same direction, so that a bar of the material has a definite north magnetic pole at one end, a south magnetic pole at the other end, and a fairly strong magnetic field around the bar. This occurs because, for subtle reasons involving quantum mechanics, a ferromagnetic system with the magnetic moments of its atoms lined up has a lower net energy than does one with the magnetic moments pointing in random directions. We have now seen this quantum magic in magnets so frequently that we take it for granted and use it to stick notes and pictures to our refrigerator doors without thinking about it.
A ferroelectric crystal works the much same way. Individual molecules of the crystal have an electric dipole moment, with a net positive electric charge on one end of the molecule and a negative charge on the other end. Again, for subtle reasons involving quantum mechanics, a ferroelectric crystal with its electric moments lined up has a lower energy than one with the electric moments pointing in random directions, creating a sheet of positive charge on one end of the crystal and a sheet of negative charge on the other. This is not a new discovery. It was first described by the Greek natural philosopher Theophrastus in 314 BC, it has been studied for many years by condensed-matter physicists and chemists, and it is already the basis for a commercial device that produces low-intensity X-rays.
In analogy with the poles of a permanent magnet, the ferroelectric crystal has a definite positive charge end and a negative charge end. Under the right circumstances, particularly when it is heated (pyroelectricity), it develops a sizable electric potential between them. However, in air the electric field of a ferroelectric material is not so easy to observe because it is rapidly dissipated by polar molecules (e.g., water vapor) attracted to the surface and neutralizing the net charge. In a vacuum, however, this does not happen, and significant electric fields can be produced by the pyroelectric process as the crystal is heated.
The UCLA group placed a cylindrical 3 cm diameter by 1 cm thick ferroelectric lithium tantalate crystal in a vacuum vessel to which deuterium gas at a pressure of 0.7 Pa (0.0001 psi) had been admitted. The crystal was mounted with its negative end attached to a temperature-variable copper block and its positive end supporting a copper disc with a sharp tungsten spike at its center. The spike was 0.080 mm in diameter, 2.3 mm long, and had a tip radius of 100 nanometers. When the positive end of the crystal reached its maximum electric potential, the electric field near the tip of the tungsten spike was very large ( greater than 25 volts per nanometer), strong enough to pull loose the electron from a deuterium atom and sent the positively charged nucleus in the other direction. Thus, a beam of ionized deuterium nuclei was given an energy of up to 115,000 electron-volts and directed against a grounded target plate placed opposite the crystal. The target plate supported a sheet of deuterated polyethylene, providing deuterium atoms with which the beam of deuteron ions could collide.
The crystal was temperature-cycled, first dropping its temperature to 240 K (-33 C) using liquid nitrogen and then progressively raising the temperature with electrical heating while observing the results with neutron and X-ray counters. After 100 seconds of heating, X-rays were observed from free electrons in the gas hitting the positive copper disc and crystal. After 160 seconds the neutron signal rose above background and increased rapidly until 220 seconds, when the heater was shut off. Neutron emission then began to drop as the deuteron beam bled off charge faster than the pyroelectric current could replace it, but strong neutron emission continued until 393 s when a spark discharged the system.
The reported measurements show clear evidence that the nuclear reaction d+d → 3He+n had been produced, that the system had produced a deuteron beam ion current of 5 4.2 nA and had produced about 900 neutrons per second. Subsequent measurements at Rensselear Polytechnic Institute have confirmed the UCLA measurements. Therefore, a tabletop experiment has successfully produced controlled d+d fusion. This was accomplished using well established physical phenomena and has required no “visits from the Tooth Fairy” to make the process work.
There are some obvious improvements that could be made to the fusion demonstration configuration used by the UCLA group. First, there is no reason to attach only one sharp spike to the copper disc. As long as spikes on the disc are separated by distances greater than their length, they can operate as independent sources of deuterium ionization. Thus, one can imagine a “bed of nails” configuration using a copper disk equipped with perhaps 100 such tungsten spikes. This would in principle increase the fusion reaction rate and neutron yield by two orders of magnitude. Second, The Rensselear group has demonstrated that by using two pyroelectric crystals mounted in opposing positions with oppositely charging faces, the electric field can be doubled. For example, in the NGP configuration the deuterated target could be located on the negative face of a second pyroelectric crystal, with both crystals cooled and heated together, to produce a deuteron beam with a kinetic energy of 230 keV for the reactions, allowing the deuterons to travel further into the target and make more fusion reactions before losing too much energy to react. One could even think of stacking many crystals to achieve significantly larger potential differences. Third, if the target was made of a material loaded with tritium (mass-3 hydrogen) instead of deuterium (mass-2 hydrogen), the fusion reaction rate and neutron production rate would be about 100 times larger, and would produce neutrons that are 6 times more energetic (about 15 MeV instead of 2.5 MeV). We also note that by replacing the low pressure deuterium in the vacuum vessel with low-pressure helium-3, one could produce “radiation-free” energy with the d+3He→4He+p fusion reaction, which produces no neutrons and would be very easy to shield in a power-production context.
Thus, tabletop controlled fusion is now a reality! What does that mean? Are we on the brink of the new controlled fusion age of pollution-less and virtually free energy? Should we hold off buying a new car until the fusion-powered models become available? Sorry, it’s not that easy.
The NGP demonstration experiment, even with the improvements suggested above, is far from the “break-even” point of reliably producing more energy than it consumes. Further, the amount of energy it does produce is very small, and the system reliability for long duration operation, depending as it does on the robustness of lithium tantalate crystals against radiation damage, is not at all clear. Like any new technology, it needs to be explored further and is likely to encounter unforeseen problems and produce unforeseen applications.
But in any case, it represents a small, inexpensive, and convenient method of producing a beam of neutrons. This has applications for material studies and for medical cancer treatment and imaging. In the latter context, the NGP group has already demonstrated that it is possible to “tag” the direction an emerging neutron by measuring the direction of the recoiling 3He nucleus that was produced in the same reaction, since the two particles are emitted back-to-back in the system center of mass frame. Thus, if a neutron was scattered by or produced a nuclear reaction in some material, one would know the direction the neutron was traveling before the event occurred. This could have important element-analyzing imaging applications, both in medicine and in areas like homeland security and nuclear weapons safeguards. A space-based variant of the deuteron ionizing setup could also provide the basis for a new type of ion thruster.
Seth Putterman has stated that with this new technology, one could construct a sealed egg-size device, place it in ice water for a while, then hold it in your hand to bring it up to body temperature, and this would cause it to emit enough neutrons to give you a dangerous radiation dose. That does not sound very useful, but it illustrates the power of this new fusion technology.
“Observation of nuclear
fusion driven by a pyroelectric crystal”, B. Naranjo, J. K. Gimzewski, and
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