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Earlier this year, a team of scientists based at NASA’s Glenn Research Center reported on a novel, promising method for generating nuclear fusion reactions in solid-state materials they termed “lattice confinement fusion.”
Apart from their great scientific interest, the results demonstrate a novel approach to realizing nuclear fusion as a practical energy source, one that exploits the so-called “electron screening” effect to drastically increase the rates of fusion and other nuclear reactions.
As I shall explain below, “electron screening” is a well-known phenomenon in the fusion field but the magnitude of the effect obtained in the reported experiments is much larger than previously expected. The reason evidently lies in the special physical environment created inside the crystal lattice structure of certain elements.
The experiments at the Glenn Research Center utilized gamma rays to trigger fusion reactions in crystals of titanium or erbium that had been loaded to high density with nuclei of the fusion fuel deuterium.
Such “deuterated” materials can store deuterium at high density, with deuterons trapped in each of the cells of the crystalline lattice. Below I shall use the standard term “deuteron” for the deuterium nucleus.
Penetrating into the crystal, the gamma rays trigger cascades of energy-liberating nuclear reactions, including not only deuterium-deuterium fusion reactions but also nuclear transmutation processes involving nuclei of the host material.
The results presented in Physical Review C suggest, although they do not prove, that the method could be optimized to produce useful amounts of net energy in the future. Most importantly, they show how it is possible to drastically lower the energy threshold at which fusion reactions can occur and boost reaction rates by a large factor.
Lattice confinement fusion could be regarded as a kind of intermediate between conventional “hot” fusion and the anomalous phenomenon of so-called low energy nuclear reactions (LENR) in deuterated materials, better known under the blanket name of “cold fusion.”
In the lattice confinement fusion experiments, the host material titanium or erbium remained at room temperature, while a small portion of the deuterons inside the lattice was heated up to “hot fusion” temperatures via interactions with the high-energy gamma rays. I shall describe the process in more detail below.
While LENR remains controversial and a satisfactory explanation for the observed LENR phenomena is still lacking, lattice confinement fusion has a solid theoretical basis within the framework of standard nuclear and solid-state physics.
In fact, the investigation of “hot” lattice confinement fusion may shed useful light on what is actually going on in LENR experiments. A bit of background may be useful before turning to some details of the lattice confinement fusion process.
The Coulomb barrier
As is well known, the chief difficulty with fusion lies in the fact that atomic nuclei are positively charged and therefore repel each other. This follows from the laws of electrostatics established by Charles Augustin de Coulomb back in the 18thcentury.
To make two nuclei react they must be pushed extremely close together. That means doing work against the force of repulsion, whose strength grows rapidly as the nuclei approach each other. Fusion scientists speak of the phenomenon as the “Coulomb barrier.”
It is only at distances on the order of a fraction of a millionth of a millionth of a centimeter, comparable to the diameters of the nuclei themselves, that the so-called nuclear strong forces take over, pulling the nuclei together and causing fusion to occur. These forces are orders of magnitude stronger than the electrostatic forces but have much shorter ranges.
This suggests two ways to overcome the Coulomb barrier. One is to apply such enormous pressures to the fuel that its nuclei are compressed down to the critical distance where the nuclear forces take over. Something like this happens in the interiors of some stars, but it is far beyond human capabilities at present.
The second main approach is to set the nuclei into such rapid motion that when they collide their momentum will be sufficient to overcome the repulsive forces. The standard method to achieve the necessary velocities in a portion of fusion fuel is to raise it to a very high temperature, hence the name “hot fusion.”
Present-day laser and so-called magnetic confinement fusion devices work with temperatures of 100 million degrees or more. So far they have not been able to produce more energy by fusion reactions than is consumed by the devices themselves.
Fortunately, the situation with the so-called Coulomb barrier is more complicated than just described. I say “fortunately” because the complexities provide opportunities to make fusion happen much more easily. That is what lattice confinement fusion is all about.
The strategy involves making better use of two well-known physical mechanisms, which are involved in all forms of fusion: the “tunnel effect” and “electron screening.”
Impossible made possible
The tunnel effect is a consequence of the laws of quantum physics.
According to classical physics – physics as known prior to the discovery of the quantum theory in the 1920s – the force of repulsion between two nuclei defines an absolute barrier against fusion in the following sense: If two nuclei collide at velocities below the threshold value calculated from classical principles, then in literally 100% of cases they will simply bounce off each other.
Thus fusion reactions will never happen.
In reality, experiments detect a small number of fusion reactions even for velocities well below the classical limit. Some nuclei seem to cheat the laws of classical physics. They somehow “tunnel” through the Coulomb barrier, behaving as if it were not there at all.
Quantum mechanics interprets this tunneling effect as a consequence of the wave-like nature of physical processes on the microscopic scale. Leaving scientific rigor completely aside, here is a glimpse of the principle involved:
The wave associated with a particle propagates throughout space and no region is 100% opaque to it in the sense of being able to block its passage. According to quantum mechanics, a particle can show up anywhere where the wave’s amplitude differs from zero.
When two nuclei move toward each other, their waves propagate ahead of them. As in the case of light propagating through a thick piece of clouded glass, the Coulomb barrier can attenuate the waves but not block them completely.
As a result, the waves will inevitably come to overlap. This translates into a non-zero probability for the nuclei to pop up together in close proximity to each other inside the critical distance for a fusion reaction to occur.
Quantum mechanics also predicts that the probability of tunneling is extremely sensitive to the height of the barrier, which in turn depends on the strength of the repulsive force acting on the nuclei.
Electron screening effect
This sets the stage for the so-called electron screening effect. In the real world, whether in stars or in fusion experiments, there are always electrons lurking around in the environment around nuclei.
This applies even at high temperatures, where the electrons are no longer bound to nuclei in atoms but wander or swarm about more or less freely in the plasma state. Electrons are dense and plentiful in the interior of crystals, a fact immediately relevant to lattice confinement fusion.
Being negatively charged, electrons tend to balance out the electric fields of the positively charged nuclei. For example: If an electron happens to be close to a deuteron that has an equal and opposite electric charge, then their fields will tend to cancel out. As such, an approaching deuteron will experience much less repulsive force.
The electron in this case has screened off the electric field of the first nucleus to a greater or lesser degree and the two can be united much more easily. The same principle applies to more complicated real-life situations, for example when deuterons are embedded in the lattice structure of a crystal.
Here the degree of screening depends on the distribution of electrons, their mobility and other factors. Under favorable conditions electron screening can substantially lower the Coulomb barrier for fusion, greatly enhancing the tunneling probability and increasing the reaction rate, potentially by orders of magnitude.
The existence of the screening effect has been known for a long time. While screening plays a role in practically all fusion processes, mainline approaches to fusion fail to make full use of it. Meanwhile, various experiments have shown that the screening effect can be unexpectedly large when the nuclei are embedded in a crystalline lattice.
Choosing optimum materials and crystal structures is key to the strategy of lattice confinement fusion. It appears that titanium and erbium crystals, used in the NASA experiments, provide an ideal environment for electron screening to take place.
It is important that titanium and erbium are metals and conductors of electricity. This implies that a portion of their electrons is highly mobile, enhancing the potential for effective screening.
Turning to the NASA experiments, a particularly novel feature lies in the indirect method used to heat up a portion of the deuterons in the lattice, imparting sufficient energy to cause a first wave of fusion reactions.
The process is kicked off by gamma rays, produced with the help of a high-energy electron beam generator and a tantalum metal target.
When the electrons collide with the target, they give off part of their energy in the form of electromagnetic radiation, referred to by physicists as Bremsstrahlung, or “breaking radiation.” At the electron energies used for the experiments – millions of electron volts – this radiation takes the form of gamma rays.
Samples of deuterated crystals are positioned under the gamma ray source, which is applied in a pulsed regime.
The gamma radiation causes some of the deuterons inside the crystal lattice to split up into their nuclear constituents, which fly off at high velocities. Each deuteron consists of a single proton bound to a single neutron.
The high-energy protons and neutrons produced in this way go on to collide with other deuterons, accelerating them to high velocities. As it turns out, the neutrons are particularly effective in transfering energy to deuterons, “heating” them in that sense.
This is only the beginning of the story. Thanks to the electron screening effect, collisions between “hot” deuterons and other deuterons in the lattice result in a significant number of fusion reactions. Among the products of these reactions are high-energy neutrons, whose subsequent collisions generate even more “hot” deuterons, which in turn generate more fusion reactions and more neutrons, and so on.
Thus, absorption of gamma rays by a small number of deuterium nuclei induces an entire cascade of energy-producing fusion reactions. For various reasons the process is not self-sustaining and comes rapidly to an end when the source of gamma rays is turned off.
There is an additional, interesting turn to the story. Analysis of data from neutron detectors revealed several distinct energy peaks. Two of them corresponded to the expected energy spectrum of neutrons produced by primary and secondary D-D fusion reactions.
A third higher-energy peak reveals that other types of nuclear reactions were taking place in the samples, so-called Oppenheimer-Phillips reactions, which transmute nuclei of the host material into other elements. Here, also, the electron screening effect plays a crucial role.
The presence of transmutation reactions suggests a potential application of lattice fusion to producing rare isotopes for medical and other uses. The main interest of NASA, naturally, is to develop fusion power sources for long-duration, deep-space missions and for space propulsion.
A final remark
Apart from their evident scientific value, the reported results once more demonstrate the importance of a broader-based approach to fusion, one that would give more support to innovative approaches and modest-sized experiments.
These can no doubt bring us much faster to realizing safe and inexpensive fusion energy than the giant projects that have been absorbing the majority of funds. Further information on Lattice Confinement Fusion, including videos and technical reports can be found on the website of the Glenn Research Center.
Jonathan Tennenbaum received his PhD in mathematics from the University of California in 1973 at age 22. Also a physicist, linguist and pianist, he’s a former editor of FUSION magazine. He lives in Berlin and travels frequently to Asia and elsewhere, consulting on economics, science and technology.