A long-running joke asserts that nuclear fusion is the power supply of the future, and always will be. Call it bottled sunshine, star power or the impossible dream - scientists have been chasing the tantalising vision of harnessing thermonuclear energy to run power stations for the past 60 years, and are still probably decades from success.
Hope that the sun's own power source - the energy released when small atomic nuclei are squeezed together to form larger ones - could be tamed and contained to generate electricity bloomed brightly in the optimistic milieu of post-war America. That was the culture that spawned The Jetsons, the prime-time animated sitcom set in a futuristic utopia in the year 2062. It also delivered, on November 1, 1952, the first detonation of a thermonuclear weapon, the hydrogen-bomb, replacing Enewetak Atoll, a Pacific island west of Bikini, with a crater 175 metres deep.
Strictly speaking, the awesome power of that 10.4 megaton nuclear test explosion was unleashed more by nuclear fission than fusion. Fission, the splitting of large atomic nuclei into smaller ones, also releases a lot of energy, and is the power source of modern nuclear reactors. The Enewetak bomb used a fission reaction to heat a mixture of "heavy" hydrogen isotopes - hydrogen atoms with extra neutrons in their nuclei - to an immense temperature enabling spontaneous fusion to occur, liberating a brief flash of energy that intensified the slower-burning fission reaction.
But that fusion flash was enough to fire lasting dreams of generating enough clean electricity from the hydrogen available in a bucket of water to supply the world's energy needs for millennia. Nuclear fusion holds the promise of virtually limitless energy supplies from a fuel source available to everyone, without the carbon emissions of combustion, the intermittency problems and huge land requirements of most renewable energy sources, and with fewer safety and security issues and much less radioactive waste than fission-fired atomic power.
Nuclear fusion releases 10 million times as much energy per unit mass as burning petrol. If it could be harnessed on earth, it would solve the planet's energy problems. That "if", however, is huge. Despite decades of high-priced tinkering, no one has yet produced a sustainable fusion reaction by a method yielding more energy than it took to produce. "Nuclear fusion as currently understood occurs only in the core of stars, in nuclear weapons, in high-temperature plasmas or in inertially confined high-energy collisions," states a recent analysis report by the US Defence Intelligence Agency.
By all indications, a so-called practical fusion reactor is still a far-off figment of futurists' imaginations. Even so, the vision refuses to fade. That is why the EU, US, Russia, China, Japan, South Korea and India are collaborating to develop the International Thermonuclear Experimental Reactor (ITER) in the south of France at a projected cost of ?12.8 billion (Dh64.34bn). The ITER website says the world's biggest fusion machine should start test operations in 2018 and could inject power into the grid "as early as 2040".
"Iter" means road in Latin, but the ITER project is not the only possible path to commercial fusion power. Here is an inventory of some potential routes to clean energy's holy grail: Tokamaks and stellarators The Joint European Torus (JET) sounds like a logical employer for George Jetson, but it is the world's largest experimental fusion reactor. It works on the principle of using a doughnut-shaped magnetic field to confine a hydrogen plasma, or a cloud of charged particles consisting of hydrogen nuclei and free electrons, so it can be heated to a temperature high enough for fusion without the plasma flying apart. Tokamaks, invented in Russia, and stellarators, the US version, both use the principle and face the same basic problem: maintaining the powerful magnetic field is extremely energy-intensive. ITER will eventually be the largest machine of this type. Its designers hope its huge size will make the project commercially viable.
Magnetic levitation An experiment last month with a giant levitating magnet turns the tokamak design inside out. Using an electromagnetic field, scientists at the Massachusetts Institute of Technology suspended a half-tonne, doughnut-shaped magnet made of superconducting wire in midair. They used it to control the motion of a surrounding plasma. The magnet created turbulence in the plasma that caused it to condense into filaments ? a phenomenon that astronomers have observed in space plasmas. The coalescence could allow fusion to take place at a lower temperature than in a tokamak, but a much larger magnet will be needed to test the theory.
Laser enlightenment Using the most powerful laser system ever built, scientists at the National Ignition Facility in California have heated a hollow gold pellet the size of a large bean to millions of degrees Celsius, causing it to implode. For one tenth of a billionth of a second last month, the laser pulse produced more power than flows through the entire US electricity grid at any moment. The test confirmed that a technique called inertial fusion ignition could trigger nuclear fusion in a pellet filled with the right combination of hydrogen isotopes, an experiment planned for this year. The trick, not previously achieved, was to deliver the uniform heating needed to squeeze the pellet to less than the size of a pinhead, briefly simulating conditions at the centre of a star. A stream of pellets per second would have to be imploded for power generation, and the cost of the pellets would need to come down greatly for the process to be commercially viable.
Shock therapy General Fusion, a start-up company in Vancouver, Canada, is trying to build a prototype fusion power plant on the cheap. For less than US$1bn (Dh3.67bn), the company will build a metal sphere with 220 pneumatic pistons designed to ram its surface simultaneously. The resulting acoustic wave would accelerate through a sheath of molten lead and lithium, generating a shockwave to compress a plasma target at the sphere's centre. To produce power, the process would be repeated every second. The relatively low-tech idea was proposed in the 1970s, when the advanced digital control technology required to synchronise the pistons did not yet exist. US fusion experts call the Canadian experiment a long shot, but well worth a try.
Doing the bump Scientists at the University of Florida are using a particle accelerator to fire hydrogen and boron nuclei towards each other at immense velocities, creating fast-moving helium nuclei when some of the particles smash into each other. The kinetic energy of the helium can be converted into electricity. Unlike the hydrogen isotope mixtures used in most other fusion research, this reaction does not produce neutrons, which are problematic because they would make the materials encasing a reactor radioactive. The Florida approach, while still costly and experimental, holds the potential for nuclear power generation free of radioactive waste.