Cold fusion is alive and doing well in, of all places, the heart of the English countryside. In a small lead-lined room at the Rutherford Appleton Laboratory (RAL) in Oxfordshire, behind protective glass and hidden among a tangled mass of pipes, is a small box. Inside, against all the odds, atomic nuclei are fusing at the rate of half a million times a second and at a temperature close to absolute zero. 鈥淭his,鈥� says Kanetada Nagamine 鈥渋s real cold fusion鈥�.
Nagamine is the head of the Japanese team which is using this exotic chemistry set to create outlandish atoms in which the electrons have been replaced with elementary particles called muons. Muonic atoms behave very differently from ordinary ones. They are several hundred times smaller and form into tiny molecules. Inside these molecules, atomic nuclei are squeezed so tightly that they fuse, releasing huge amounts of energy. So impressive is this output that Nagamine hopes a large-scale version of his experiment could one day generate cheap energy for all.
The work has astonished theoreticians who say that this process can never produce more energy than it consumes, let alone enough power for domestic consumption. But Nagamine鈥檚 experiment has already produced more energy than the theorists predicted. Now he is tantalisingly close to the breakeven point-the all-important level at which his experiment produces more energy than it takes to create the muons. Breakeven cannot be far away now, he says.
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Although Nagamine calls his work 鈥渃old fusion鈥�, it is a far cry from the experiments of Stanley Pons and Martin Fleischmann who in 1989 claimed to have produced fusion in a simple electrochemical cell (鈥淲hatever happened to cold fusion?鈥�, 19 January 1991, p 46). Pons and Fleischmann鈥檚 work is now regarded with scepticism by mainstream science. Nor is Nagamine鈥檚 work anything like the conventional techniques which require giant magnetic confinement chambers or huge lasers to heat nuclei to billions of degrees.
On paper, nuclear fusion is straightforward enough: squeeze two suitable atomic nuclei together and they fuse to produce energy which can be harnessed, at least in principle. The main players in fusion are the two isotopes of hydrogen: deuterium and tritium. Like hydrogen, these have a single electron and so are chemically identical. But they differ from hydrogen in their atomic mass. The nuclear core of hydrogen is a single proton, but the nucleus of deuterium contains one proton and one neutron, while tritium has one proton and two neutrons.
Nagamine and his team are fusing nuclei of deuterium and tritium. Each union creates an alpha particle-two protons bound to two neutrons- and a neutron. Between them these by-products share 17.6 mega-electronvolts (MeV) of energy which sends them careering apart at high speed.
Deuterium and tritium nuclei combine by virtue of the strong nuclear force which is responsible for holding together the components of every nucleus. Since this force is the most powerful in the Universe, once the nuclei are in its grip, they cannot escape. 鈥淚f we can get two nuclei close enough, then the strong nuclear force becomes effective. Then the two nuclei fuse,鈥� says Nagamine.
Brute force
But there鈥檚 a catch. The strong force is strictly short range and works only over distances of a million-billionth (10-15) of a metre , or 1 fermi, roughly the size of a hydrogen nucleus. That鈥檚 about one fifty-thousandth of the radius of a hydrogen atom. And there is another force at work which acts over much larger distances. Because nuclei are positively charged, they repel each other. Though this repulsion is weaker than the strong force at short distances, it dominates over larger distances. Before the nuclei can fuse, this force, known as electrostatic repulsion, must be overcome.
One way to do this is to heat a gas of nuclei to extreme temperatures so that they collide with enough force to overcome electrostatic repulsion. The temperatures involved are hundreds of millions of degrees Celsius, several times hotter than the Sun. At these temperatures the walls of any reaction vessel would be vaporised, so powerful magnetic fields are used to confine and compress the gas. Another approach uses intense lasers to heat and implode tiny pellets of deuterium-tritium fuel to create immense temperatures and pressures for a fraction of a second.
Nagamine and his group are doing something radically different. They rely on muons-elementary particles which are more than 200 times heavier than electrons but have the same electric charge, so it is possible to dupe atoms into accepting them in place of electrons. The result is the exotic form of matter known as a muonic atom.
According to the laws of quantum mechanics, all particles can be thought of as waves with a wavelength that depends on their mass. Because muons are heavier than electrons they have a shorter wavelength. This shorter wavelength wraps much more tightly around the nuclear core so that muons tend to orbit some 200 times closer to the nucleus than electrons do. The result is 鈥渁 very tight, and a very small atom鈥�, says Nagamine.
Muonic atoms have a chemistry all their own. Just as hydrogen atoms prefer to combine into pairs to make hydrogen molecules, a muonic tritium atom can combine with an ordinary deuterium atom to form an unstable deuterium- tritium-muonic molecule containing both electrons and muons, and written as (dtm). Because muonic atoms are small, so are their molecules. The nuclei in an ordinary hydrogen atom are more than 100 000 fermi apart. But in (dtm), the nuclei are only 500 fermi apart. At this distance, quantum mechanical fluctuations in their position ensure that the nuclei come close enough to combine. The result is fusion-creating an alpha particle and a neutron while liberating the muon, which is then free to induce another fusion event. In effect, muons act as catalysts for fusion reactions.
This is just as well. The energy released in each fusion is much less than it takes to create the muon in the first place. Breakeven can only be achieved if the muon goes on to catalyse 300 fusion events.
But the clock is ticking because unlike electrons, muons are unstable. After a couple of microseconds they transform into electrons, and the magic is lost. So each muon has only a couple of microseconds to take part in 300 fusion events. This is why scientists have always believed that muonic catalysed fusion could not be a viable energy source.
Chain reaction
The idea of muon-induced fusion was first conceived by the Russian and British physicists Andrei Sakharov and Charles Frank in the 1950s. In 1957, David Jackson, then at McGill University in Canada, realised that muons can take part in a chain reaction in which they act as a catalyst. That same year, Luis Alvarez and his team at Berkeley became the first to detect muon-induced fusion.
The fusion reaction itself is very fast-all over in a fleeting billionth of a second. But it is the creation of the (dtm) molecule itself which holds things up. Early Russian experiments appeared to show that this process is so slow that a muon has no chance of catalysing the necessary number of fusion events before it decays. The prospects for muon-induced fusion seemed doomed.
The Russians were not so easily detered, however. In the late 1970s, Russian theorists Sergei Gerstein and Leonid Ponomarev developed a theory showing that (dtm) molecules should form much more rapidly. The mechanism for this was a quantum mechanical phenomenon called resonance in which a muonic tritium atom and a deuterium molecule combine rapidly to form a loose association with a deuterium atom and a (dtm) molecule. This idea was confirmed in 1983. It turns out that (dtm) molecules can be created in around a thousand millionth of a second which is fast enough to allow each muon to catalyse 1000 fusion events before it expires. Muonic fusion had another chance.
But there is yet another obstacle. The alpha particles created in the fusion event have a double positive charge and so tend to capture negatively charged muons and take them out of circulation before they decay. This stops muon-induced fusion in its tracks. 鈥淭his phenomenon we call `sticking鈥�,鈥� says Nagamine.
Sticking is measured as the probability that a given muon will end up stuck to an alpha particle. 鈥淚f muon-to-alpha sticking is 1 per cent, then when muon fusion takes place the cycle only continues a hundred times. If 10 per cent, then 10 times,鈥� says Nagamine. In these terms, breakeven occurs only if the sticking probability can be reduced to 0.3 per cent. 鈥淚f this sticking process is lower than 0.3 per cent, then we have a chance,鈥� he says.
Nobody really knows just how strong the sticking effect is. Nagamine鈥檚 experiments at RAL are designed to find out. When a muon is captured by an alpha particle, it emits X-rays as it becomes tightly bound. Nagamine鈥檚 idea is to look for this characteristic radiation. But it is no easy task. Tritium is highly radioactive and produces X-rays when it decays. Separating the muon-produced X-rays from this background noise is difficult. 鈥淥ther people said this was an almost impossible experiment,鈥� he says.
And indeed, it has only been possible because of the unique facilities at RAL. The laboratory is the proud owner of a circular particle accelerator called ISIS. This device generates pulses of protons with energies up to 800 MeV. When high-energy protons smash into a graphite target they produce particles called pions. These, in turn, decay into muons which Nagamine can use. The result is the world鈥檚 most powerful muon beam.
Ironically, the Pons-Fleischmann cold fusion episode catalysed funding for Nagamine鈥檚 work. Construction of his muon-induced fusion experiment was completed in June last year at a cost of 拢10 million, paid for by the Japanese Institute of Physical and Chemical Research, or RIKEN, which is based just outside Tokyo. RAL is the landlord and supplies high-energy pulses of protons in exchange for access to part of the muon facility. It鈥檚 an arrangement which offers Nagamine the dubious perk of commuting from Tokyo to Oxfordshire once a fortnight.
The fact that this facility produces pulses of muons rather than a continuous beam is crucial. Every 20 milliseconds a pulse lasting 70 nanoseconds and containing over 10 000 muons enters the target, a mixture of either solid or liquid deuterium and tritium at between 15 and 20 kelvin. The signature of muon-to-alpha sticking is an X-ray spike synchronised with this muon pulse, whereas the tritium X-rays form a continuous background. Other muon facilities have continuous and relatively dim muon beams, which make muon-to-alpha sticking almost impossible to spot.
Sticking spike
Over a cup of Japanese green tea, Nagamine holds up a raw X-ray data plot showing a spike protruding above the shoulder of the tritium background. 鈥淲e are very proud of this picture,鈥� he says. By measuring the number of neutrons produced, Nagamine can determine how many fusion events took place. 鈥淎nd the X-rays give us the number of stickings that are happening,鈥� he says.
The results are astonishing. Nuclear theory predicts that the probability of sticking should be about 0.6 per cent, a level that would doom each muon to less than 200 fusion events. But Nagamine has already done better. 鈥淪ticking is close to 0.4 per cent,鈥� he says. This allows around 200 fusion events for each muon, but Nagamine believes that it can be improved by optimising his setup. The trick will be to find the optimum mix of tritium and deuterium in the target.
One thing is for certain-the theorists will definitely have to revise all of their ideas about alpha sticking. Nagamine points to the difficulties in understanding how alpha particles may be stripped of their muons as they move through the target. 鈥淭hat may explain the gap between theory and experiment,鈥� he says.
But even if Nagamine reaches breakeven, he cannot be sure that the process will ever be viable as a power source. Commercial viability will not be an option unless Nagamine can demonstrate 900 fusion events for each muon. Just how this might be achieved, Nagamine is reluctant to say.
One idea is that adding hydrogen to the deuterium-tritium mix might enhance (dtm) molecule formation. Nagamine also hopes to increase the density of the target by increasing the pressure to 10 000 atmospheres. But just how the measurements can be made safely in these conditions is not yet clear. 鈥淭he experiment itself is not very easy,鈥� admits Nagamine.
In the longer term, more highly concentrated beams of muons might help, but these would need a new accelerator. 鈥淲e need a more advanced muon beam generator,鈥� says Nagamine.
Meanwhile, governments are becoming increasingly reluctant to fund research into high-temperature fusion. The next stage in that programme is to build a prototype reactor that can produce more energy than it uses. It is called the International Thermonuclear Experimental Reactor and has a price tag of $6 billion. Just where this money will come from, nobody knows.
All of which leaves the field wide open for a dark horse in the form of muon-induced fusion. At a cost of 拢10 million, the muon facility at RAL starts to look cheap, but there is the small matter of the need for a 拢250 million accelerator to provide protons.
Fusion research is certainly a game for high rollers. 鈥淔usion is normally so difficult to achieve that any process which can produce it at the laboratory scale just has to be followed up,鈥� says Graeme Hirst, a researcher at RAL who specialises in laser-induced fusion.
One simple fact is so irresistible that it will spur researchers to keep on searching for viable fusion. With a mere 10 grams of deuterium and 15 grams of tritium, fusion could produce enough power to supply the average inhabitant of the industrialised world with electricity for life.
