This is no table top idea still confused about chemical or physics properties – the muon catalyst fusion method is well understood and has been until a decade or so ago, widely researched. The idea came before 1950 from Andrei Sakharov and F.C. Frank, who predicted the phenomenon of muon-catalyzed fusion on theoretical grounds. Luis W. Alvarez et al., analyzed the outcome of some experiments with the muon’s incident on a hydrogen bubble chamber at Berkeley in 1956, observed muon-catalysis of exothermic p-d, proton and deuteron, nuclear fusion, which resulted in a helion, a gamma ray, and a release of about 5.5 MeV of energy.

The Alvarez experimental results, in particular, spurred John David Jackson to publish one of the first comprehensive theoretical studies of muon-catalyzed fusion in his ground-breaking 1957 paper. (“Catalysis of Nuclear Reactions between hydrogen isotopes by μ−-Mesons”. Physical Review 106: 330) This paper contained the first serious speculations on useful energy release from muon-catalyzed fusion. Jackson concluded that it would be impractical as an energy source, unless the “alpha-sticking problem” could be solved, leading potentially to an energetically cheaper way of producing the catalyzing muons.

By 1957 the idea to bind atoms using muons seemed a rich research field. Until 5–10 years ago, muon-catalyzed nuclear fusion experiments were conducted in muon facilities in the United States, Switzerland and Russia, and all achieved an energy balance of about 40%. These countries withdrew from these studies because of the decommissioning of muon facilities or a lack of experts who could deal with the tritium fuel, a radioactive material.

That left The RIKEN Foundation’s Nishina Center for Accelerator-Based Science and the RIKEN–RAL Muon Facility at the Rutherford Appleton Laboratory in the United Kingdom. The RIKEN–RAL Muon Facility generates the most powerful pulsed muon beam in the world, and the center has taken a leading role in applied studies of muons.

Teiichiro Matsuzaki, director of the RIKEN-RAL Muon Facility says, “Using muons, we can achieve nuclear fusion in a comparatively small facility at reasonable cost.” Matsuzaki and scientists at the facility have been conducting unique experiments as part of fundamental research into the use of muons to develop industrially viable nuclear fusion technology.

The RIKEN–RAL Muon Facility creates, artificially generated, the most powerful pulsed muon beam in the world, and the center has taken a leading role in applied studies of muons. Muon-based nuclear fusion does not require such ultrahigh temperatures or superdense states, says Matsuzaki. Compared to magnetic field confinement fusion and inertial confinement fusion, muon-base nuclear fusion could allow stable nuclear fusion to be induced in a smaller facility at lower cost for a longer period of time. The issue is then how muon-based nuclear fusion can be induced.

The fuel medium, a mixed gas of deuterium and tritium, is cooled to temperatures below around −250°C, causing the gas to form a liquid or solid. Using negative muons, the injection of a beam of muons into the fuel medium then generates muonic tritium atoms (tµ), which are similar to hydrogen atoms.

As muons are 207 times heavier than electrons, the muon orbits the nucleus at a distance much shorter than that for electrons. Thus, tµ atoms are extremely small, and because the tµ atoms have no charge, they collide with deuterium atoms without being affected by repulsive electrical force. This process produces muonic deuterium–tritium molecules (dtµ), which are also similar to hydrogen atoms, and which have a nucleus consisting of a muon, a deuterium nucleus and a tritium nucleus. Similar to the tµ atom, the dtµ molecule is extremely small, which allows the deuterium and tritium nuclei to come into very close proximity, thus inducing the desired d–t nuclear fusion.

At the end of the fusion instance, the muon in the dt molecule is liberated and becomes available for the creation of a new dtµ molecule. Thus a chain of nuclear fusions occurs. This reaction is called ‘muon-catalyzed nuclear fusion’ because the muons act like a catalyst that drives nuclear fusion. This scenario has gotten to 40% efficiency without the deep fuel cooling.

In the RIKEN–RAL Muon Facility, a single muon is capable of inducing d–t nuclear fusion 120 times before it decays, producing 2 GeV of energy. Generally about 5 GeV of energy is required to produce one muon, corresponding to an energy balance of 40%. For scientific break-even the efficiency level required is estimated to be 3–10 times higher where a single muon needs to induce d–t nuclear fusion 1,000–3,000 times before decaying.

The fusion waste is helium, where about 1% has a muon still stuck aboard and preventing the liberated muons from catalyzing subsequent nuclear fusion reactions. Matsuzaki has learned the muons can be stripped from the muonic helium as the atoms collide with deuterium and tritium atoms in the fuel, the “Reactivation” step seen in the graphic.

Gaining efficiency depends on improving two points: how to strip the muons from the muonic helium atoms efficiently, and how to create dt molecules more efficiently. When the fuel is made much denser by cooling, the muonic helium atoms can be stripped of their muons more easily because of the increased probability of collision with deuterium or tritium atoms. Also denser fuel will contribute to an increased probability of creating dtµ molecules by more frequently bringing muonic tritium and muonic deuterium atoms into close proximity, as well as increasing the frequency of nuclear fusion induced by a single muon and increasing the efficiency of the nuclear fusion cycle.

That offers a tantalizing possibility, if muonic helium atoms can be completely stripped of their muons, at the present level of dtµ creation efficiency, a single muon could induce nuclear fusion 340 times before decaying, which will be close to the scientific break-even condition.

Matsuzaki and his team have shown that the efficiency of the nuclear fusion cycle improves as the temperature of the solid fuel is increased from 5 to 17 K (0 K = –273.15°C) and will attempt to increase the temperature of the solid fuel further to investigate how the efficiency of the nuclear fusion cycle might increase. If the temperature is increased too far, the solid fuel will melt and become a liquid. The fuel, however, stays solid up to temperatures of 30 K provided that the pressure is maintained above 1,000 atmospheres.

Matsuzaki and his team have finished designing the laboratory equipment for this experiment, which will be a world first as it has never been attempted before. The new equipment allows for other ideas for experimentation. Controlling the molecular-excited state of the deuterium in the fuel by irradiating it with a laser beam may enhance the production of dtµ molecules. Applying an electric field to the fuel may increase the efficiency of stripping the muons from the muonic helium atoms.

In 2008, a new muon facility was constructed in Japan. This facility, the Japan Proton Accelerator Research Complex (J-PARC), produces an intense beam of muons.

It seems Matsuzaki has the tenacity to get much closer to a breakeven fusion and could get beyond. The research he leads is full of strong points and potential. Even with the innovation to super cool the fuel and the coming experiments, one thing just jumps out at the casual observer – that 5 GeV of energy of required to produce one muon – is a gold mine looking for an engineer. Cut that 5 GeV by half and Matsuzaki might already have the breakeven technique.

Thanks to Brian Wang at nextbigfuture for noticing the Riken Foundation news.

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