For his eighth birthday, Richard Hull’s mother bought him a Geiger counter. It was 1955 and the United States was testing nuclear weapons on its own soil. “They would always announce a test in the newspaper,” Hull remembers. “The material that went into the stratosphere drifted with the prevailing winds. The radioactive fallout particles came down with rain, as far north as New York and as far south as Georgia.” Hull lived then, as now, in Virginia, squarely in the path of the fallout that blew east from the bombs in the Nevada desert. “We would have days when we couldn’t have milk,” he remembers, “because of the strontium-90.”

Hull wanted a Geiger counter not because he was afraid of radioactivity, but because he was enthralled by it. He pointed his new toy at anything that might make it tick, from wristwatches to rocks, and he collected fallout from the bombs. “I would take bird-bath water, or water that I gathered in pails from the downspouts of the house, and I would slowly evaporate that water on my mother’s stove, and that would leave the solids behind. And they were highly radioactive,” he says, with evident satisfaction.

For Hull and others like him, radioactivity is not a poison but a thrill, a kind of life within materials. The uranium we find on Earth, he explains, has been ticking away since before the planet itself was formed. “It has been decaying since the supernova that blew the material off and [it] slowly accreted into the Earth, billions of years ago. And it’s still going today. That fascinated me,” he says. “It still does.” This fascination has led Hull to experiment with radioactivity for more than 60 years. Under Eisenhower’s educational “Atoms for Peace” programme, a schoolchild in the 1950s could order small quantities of radio isotopes to their home, so Hull wrote to the US nuclear facilities for free samples of caesium-137, sulphur-42 and cobalt-60. Following a guide in Scientific American, he contaminated clover plants with radio-phosphorus and laid them on photographic paper, creating radiographic pictures of the veins within the leaves.

He grew up to become an electronics engineer, and in his spare time he built Tesla coils – huge electrical transformers, capable of producing millions of volts. Then, one day in 1997, he decided to build his own nuclear fusion reactor, at home, in his garage.

Nuclear fusion is the reaction that powers stars. It is so energetic that you can suffer serious burns from the fusion happening in the sun, despite being separated from it by several miles of atmosphere and 93 million miles of freezing vacuum. It releases so much energy that you can see it happening, without a telescope, a thousand trillion miles away. It is not a force that most people would think to produce and contain in their sheds.

Since Hull first tried, however, about two hundred people across the globe have followed his example. While governments spend billions working on fusion science, amateur “fusioneers” are drawn to tinker in the field by curiosity, thrills – and the possibility that they might stumble across the solution to the global energy crisis.

There are two kinds of fusioneer, as listed on the website fusor.net (which Hull runs, offering advice and resources to those who want to try fusion for themselves). The first category is the Plasma Club, which consists of people who have built a “fusor” and created plasma – the glowing, gas-like matter found in neon signs – within it. A shorter list names the few people who have recorded the emission of neutrons from the fusor – proof that real nuclear reactions are happening within the chamber. These are the members of the Neutron Club.

Humanity has known how and why the sun shines for less than a hundred years. Before fusion was discovered, it was suggested that our star might be made from burning coal, or molten metal. Yet physicists had also shown that fire – even at the sun’s immense size – could burn for just a few thousand years before its fuel ran out. The problem was solved only when scientists began to understand the structure of the atom.

A hydrogen atom consists of a proton orbited by an electron. If you made a basic scale model, with a tennis ball as the proton, the electron would whizz around it at an average distance of two miles. Everything else is empty space; atoms are more like clouds than building blocks. All the chemical reactions we see on Earth, from fire to digestion, involve electromagnetism – the force that holds these clouds together. Only in extreme conditions, such as the immense heat and gravitational pressure inside a star, are atoms ever forced so close together that the unimaginably small chunks inside the clouds actually touch. When this happens, the “strong force” involved is much more powerful than electromagnetism and the energy released is much greater. This is why a few kilograms of material can release, in a nuclear reaction, the same energy as blowing up a million tonnes of TNT.

In the Nineties, Hull heard about a way to perform controlled nuclear fusion at home, using something called a Farnsworth fusor (after Philo T Farnsworth, one of the inventors of television). The assembly seemed simple enough, to him: “I just needed 40,000 volts, and a six-inch sphere.” The 40,000 volts came from “an old, discarded X-ray transformer, from a dentist’s office”, while the stainless-steel hemispheres could be ordered “for about $60 each. So for $120, I had my six-inch steel sphere.”

A pump emptied all the air from the chamber, creating a vacuum within, into which Hull added a small amount of deuterium – a non-radioactive isotope of hydrogen, in the form of a gas – which he’d also bought via mail order. With 40,000 volts humming in the electrodes, the powerful electric field accelerated the deuterium ions towards the metal cage in the centre of the sphere, forcing them together so hard that their nuclei touched and fused. Or at least, that was the idea. “The first time,” he recalls, “I didn’t get any fusion.” But as Hull persisted over the months that followed, a pinkish-purple light began to emanate from within the cage in the middle of the sphere. After six months of learning to operate the fusor, the pink glow became a glare, with bright beams of violet plasma coming from the centre. Fusioneers call this “star mode”.

The moment Hull remembers most fondly was when he used the neutrons generated in his fusion reactor to make a piece of silver radioactive. This, for him, is the power fusion offers – to change elements, in the most fundamental way, to breathe life into them. “I had taken a dead metal,” he remembers, “and I’d altered its atoms.”

One of the few British people who qualifies for Neutron Club membership is Dr Jonathan Howard, a 47-year-old scientist from London. Like Hull, Howard was drawn to fusion by a fascination with high-voltage electricity. In his early twenties, Howard attached a weather balloon to a very long coil of copper wire and released it into the sky one prickly, charged evening just before a thunderstorm. As the wire unspooled, it began to glow blue. “I accidentally brushed my leg against the wire coil on the ground, just bumped it very gently,” he remembers, “and the shock threw me several feet.”

Electrocution is the greatest risk that fusioneers face. A fusion reactor, Howard points out, “is only radioactive when it’s turned on” – and even then, in a home experiment, not dangerously so – but 40,000 volts can kill instantly.

That first shock, however, did nothing to dampen Howard’s spirit of inquiry. His office at the laboratory where he works is filled with things he has made: the learning robot he built for his PhD in computational neuroscience; the 3D printer he made from scratch. In his day job he works with cyclotrons – massive particle accelerators, used to make radioactive medicines.

Howard has also built his own fusion reactor at home, but he sees fusion differently to Hull. For Hull, fusion is a science experiment, but for Howard, as for many other fusioneers, the excitement comes from improving his fusor’s efficiency, gradually edging his equipment towards fusion’s holy grail: net power gain. If someone can find a way to make a fusion reactor produce more energy than the electricity that powers it, this technology could power the world.

The fusion reaction itself produces no radioactive waste. It cannot go critical, as a fission power station can (all current nuclear power stations use fission, not fusion), and the most common fuel, deuterium, is easily extracted from seawater. Twelve ounces of deuterium can release as much energy as a thousand tonnes of coal. There is enough deuterium in the world’s oceans to power human civilisation indefinitely.

Howard thinks we will see fusion power stations within 50 years, and beyond that, smaller units that will provide distributed power to hospitals, villages, vehicles, even electronic devices. And for a long time, a group of quiet optimists in the government and the scientific establishment have shared this view.

****

About ten miles from Oxford, just off the A415 between Abingdon and Burcot, is the hottest place in the known universe. Within the Joint Experimental Torus (Jet) reactor, a set of giant electromagnets holds a puff of fuel – a hundredth of a gram – inside an invisible “bottle” made from a hugely powerful magnetic field. Bombarded by particles, massive amounts of electricity and powerful radio waves, the plasma inside the bottle is heated to between 150 and 200 million degrees. This is more than ten times hotter than the core of the sun.

Two hundred million degrees does not look as you would expect. Watching the Jet reactor’s most recent test on monitors in the control room, Tony Donné, the programme manager at Eurofusion (the EU’s fusion research programme) explains that the bright streaks, flowing like pink fire down one wall of the reactor’s massive, doughnut-shaped chamber, are not fusion plasma. In fact they are the exhaust from the fusion; within the magnetic bottle, the material is so hot that it does not emit any light in the visible spectrum. On the display, surrounded by jets of searing pink light, the hottest part of the reaction chamber is dark.

It begs the question: is the sun, too, dark inside? If you could survive the journey to the interior of a star, would it appear hollow, like the dull purple oval wavering on the monitor?

Despite its vast energies, however, not even Jet has ever produced more power than has been put into it. The first reactor projected to break this barrier is the International Thermonuclear Experimental Reactor (Iter), currently being built in south-east France. A $14bn joint project of 35 countries around the world (including Britain; our new isolationism, according to the Eurofusion team, has not yet extended to this area of science), Iter is expected to produce more than ten times the energy that is put into it. If Iter works, a demonstration power station could be built by 2050, and by the end of this century, fusion power could become widespread.

Then again, the world may decide to make better use of the unlimited supply of clean energy that comes from the vast, self-sustaining fusion reaction in the sky. An area of solar panels 5 per cent the size of the Sahara desert – or 0.3 per cent the size of the Pacific – would be enough to meet all the world’s energy requirements, but the political and economic effort shift needed to move to a renewable-energy world may be even more difficult than the science of fusion.

For now, however, it is easy to see why people want to understand fusion, to make it happen, and to control it. Fusion is the most primal force in our universe. It made us: all of the elements in your body were fused in the atomic forge of a star that died billions of years ago. For people such as Richard Hull, Jonathan Howard and the men and women of the Neutron Club, it is this – more than the possibility of fame, or fortune, or even solving the global energy crisis – that drives them to devote so much time to fusion: the idea that a person may kindle, in their garage or their shed or their spare bedroom, the same spark that lights every star in the sky.

Will Dunn is editor of the New Statesman’s policy supplement, Spotlight