Muon catalyzed fusion for energy production
RIKEN Facility Office at Rutherford Appleton Laboratory (RAL)
RIKEN Nishina Center for Accelerator-Based Science
The nuclear fusion that takes place in the core of the Sun, where temperatures reach 15 million degrees Celsius, liberates enormous amounts of energy. We see the result of this energy liberation in the Sun’s glare. The elementary particle known as a muon, however, provides a means of achieving nuclear fusion at sub-zero temperatures. “Using muons, we can achieve nuclear fusion in a comparatively small facility at reasonable cost,” says Teiichiro Matsuzaki, director of the RIKEN-RAL Muon Facility. 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
In 1935, Hideki Yukawa predicted the existence of then undiscovered particles called ‘mesons’, and presented a ‘meson theory’ stating that the strong nuclear force that binds the nucleus together could be attributed to the emission and absorption of mesons by the protons and neutrons constituting the nucleus. Yukawa was awarded the Nobel Prize in Physics for this achievement in 1949.
Mesons decay into particles called 'muons' and 'neutrinos' . Muons were first discovered in cosmic rays in 1937 by a group that included Yoshio Nishina, then chief scientist at RIKEN and a well-known pioneer of Japanese nuclear physics. We are very proud of the fact that we can conduct muon research at the RIKEN Nishina Center for Accelerator-Based Science, which was named after Dr Nishina, says Matsuzaki.
What kind of particle is a muon? Spread out the palm of your hand; on average, one muon passes through your palm every second, says Matsuzaki. Mesons are produced by collisions between atomic nuclei in the atmosphere and protons arriving from space, and these mesons immediately decay into muons and neutrinos, which bombard the Earth continuously.
One major research theme at the RIKEN–RAL Muon Facility is the establishment of muon-based nuclear fusion technology. Nuclear fusion is a reaction in which the nuclei of very light elements such as hydrogen fuse together to form the nuclei of heavier elements. The reaction involves the release of large amounts of energy. The glare of the Sun, for example, is the result of nuclear fusion in the Sun's core.
If nuclear fusion can be induced using hydrogen, which is abundant on Earth, and if the energy released can be applied in the form of practical nuclear fusion for power generation, it may be possible to solve the world's energy problems.
Nevertheless, nuclear fusion is a very difficult reaction to achieve because it requires two nuclei to be brought very close together, within a distance of about 10-13 cm, to allow the nuclear force to act. Individual nuclei, however, are positively charged and strongly repellent. The challenge is therefore how to overcome this repulsive electrical force and bring nuclei close together.
The Sun is a giant gaseous ball, with a mass about 330,000 times that of the Earth. In the Sun's core, hydrogen nuclei move violently due to the extreme temperature, and the ultrahigh density resulting from the Sun's massive gravity causes the hydrogen nuclei to be forced together to within one ten-trillionth of a centimeter, inducing a chain of nuclear fusions.
How, then, can we achieve nuclear fusion on Earth? Deuterium (d) and tritium (t) nuclei are used as the fuel in place of hydrogen, as these nuclei are more readily induced into nuclear fusion and the reaction releases greater energy. Whereas a hydrogen nucleus consists of just one proton, a deuterium nucleus consists of one proton and one neutron, and a tritium nucleus consists of one proton and two neutrons.
Two methods of nuclear fusion have been developed. In one method, called 'magnetic field confinement fusion', a magnetic field is used to confine a plasma of completely free electrons and nuclei, and the plasma is allowed to reach ultrahigh temperatures. In the other method, called 'inertial confinement fusion', a laser beam is used to rapidly compress the fuel into a superdense state (1,000 times denser than a solid). In both cases, very large facilities are required to achieve the ultrahigh temperatures or superdense states necessary to induce nuclear fusion.
In contrast, 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.
Achieving muon-based nuclear fusion at very low temperatures
The muon belongs to the lepton group of elementary particles, which includes electrons. It has a lifetime of 2.2µs, and a mass one-ninth that of a proton and 207 times that of an electron. There are positively charged muons and negatively charged muons. In a material, the positive muon acts as a 'light' proton, while the negative muon acts as a 'heavy' electron.
Muon-based nuclear fusion is conducted using negative muons. A mixed gas of deuterium and tritium is cooled to temperatures below around −250°C, causing the gas to form a liquid or solid. The injection of a beam of muons (µ) into the 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 d–t nuclear fusion.
After the occurrence of d–t nuclear fusion, 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.
The only experimental muon-catalyzed nuclear fusion facility
At the RIKEN–RAL Muon Facility, a beam of muons is injected into about 1 cc of fuel to induce d–t nuclear fusion at a rate of about one million times per second (Fig. 4). In general, 5 GeV of energy is required to produce one muon. 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. In other words, 5 GeV of energy is required to generate 2 GeV of energy, corresponding to an energy balance of 40%.
The scientific break-even point for achieving 100% energy balance will be achieved when a single muon can induce d–t nuclear fusion at least 300 times before decaying. Economic nuclear-fusion power generation will clearly require improved efficiency far exceeding the scientific break-even point. The efficiency level required is estimated to be 3–10 times higher than that for scientific break-even. This means that a single muon needs to induce d–t nuclear fusion 1,000–3,000 times before decaying.
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 tritium, a radioactive material. Thus, we are now the only research institute in the world that continues to perform fundamental experiments on muon-catalyzed nuclear fusion. Our aim is to achieve scientific break-even conditions in an effort to put nuclear fusion into practical use, says Matsuzaki.
Muon-based nuclear fusion involves the production of helium nuclei (á particles), which are positively charged. About 1% of the negatively charged muons liberated by the nuclear fusion reaction become stuck to these helium nuclei, forming muonic helium atoms (áµ) and preventing the liberated muons from catalyzing subsequent nuclear fusion reactions. The muons, however, may be stripped from the muonic helium as the atoms collide with deuterium and tritium atoms in the fuel. Our experiments have shown that the creation of muonic helium atoms is very difficult to prevent. The key to successful research therefore lies in two points: how to strip the muons from the muonic helium atoms efficiently, and how to create dt molecules more efficiently.
If the fuel is made much denser, the muonic helium atoms could be stripped of their muons more easily because of the increased probability of collision with deuterium or tritium atoms. Furthermore, 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.
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. Furthermore, if the fuel were five times denser than liquid hydrogen, a single muon could induce nuclear fusion as many as 1,200 times. For example, we could easily create a new fuel that is 5–10 times denser than liquid hydrogen by combining our process with laser-based inertial confinement fusion. I would very much like to undertake a joint study because there have been very few conventional studies on nuclear fusion involving the combination of different methods.
Furthermore, recent studies by 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). We 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. We have finished designing the laboratory equipment for this experiment, which will be a world first as it has never been attempted before.
Matsuzaki and his team also have many other ideas. 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. Furthermore, 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. The facility was jointly constructed by the High Energy Accelerator Research Organization and the Japan Atomic Energy Agency, and is now jointly managed by these two organizations. If it achieves target performance, J-PARC will become a unique muon-beam facility that boasts the most intense muon beam in the world. The Japan Atomic Energy Agency employs technical experts who are capable of dealing with radioactive tritium. Thus J-PARC is the perfect facility for advancing research into the practical application of muon-catalyzed nuclear fusion technology. I hope that the RIKEN-RAL Muon Facility, which has been working on the fundamental research, will be able to collaborate with J-PARC to advance research on the practical application of muon-catalyzed nuclear fusion technology.
About the Researcher
Teiichiro Matsuzaki was born in Tokyo, Japan, in 1952. He graduated in 1979 from the Graduate School of Science and Technology at the Tokyo Institute of Technology, and obtained his Doctor of Science degree from the same institute in 1980. He became a research fellow of the Japan Society for the Promotion of Science for one year in 1979, and then served as research associate at the Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, from 1980 to 1982. He started his career in muon science in 1982 as a research associate in the Muon Science Laboratory, Faculty of Science, University of Tokyo. He later moved to RIKEN as a research scientist in 1986, and was promoted to senior scientist in 1994. He was involved in the construction of the RIKEN RAL Muon Facility in the United Kingdom, and has been progressing muon-catalyzed fusion research toward energy production. He has acted as director of the RIKEN Facility Office at RAL since 2004.
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