The power source of tomorrow (and tomorrow and tomorrow)

under the microscope Scientists behind the huge National Ignition Facility laser in California believe it could provide a limitless source of safe energy.

Technicians inside the National Ignition Facility target chamber. At right is the target positioner, which holds the target fuel capsule.

It sounds like some kind a doomsday machine: a laser the size of three football pitches whose radiation beams pack a punch equivalent to 500 times the power consumption of America. But according to its creators, the National Ignition Facility (NIF) in California can help save our planet, not destroy it. Later this year, its almighty power will be used to ignite nuclear fusion reactions, the power source of the stars. If it succeeds, say NIF scientists, it could pave the way to a source of safe, carbon-free energy, using fuel found in virtually limitless amounts in seawater.

And so far, they seem to be on track: the team has just announced that their $4 billion (Dh15bn) leviathan has fired the most powerful burst of laser light ever produced: over a million joules of energy. Lasting a few billionths of a second, its 192 separate beams packed a punch with the power of more than 100,000 nuclear power stations. According to the NIF team, that should be enough to create temperatures of more than 100 million °C and pressures exceeding the 100 billion atmospheres needed to trigger fusion reactions.

All very impressive - but just how relevant is laser fusion to the future energy needs of our planet? There are certainly grounds for scepticism. For decades physicists have been promising to "harness the power source of the stars" using doughnut-shaped machines known as tokamaks, in which intensely hot fusion fuel is gripped in magnetic fields while it releases energy in the form of fast-moving particles.

Yet not one of these machines has come close to producing more energy than it needs to trigger the fusion reactions - a pretty crucial criterion for any viable energy source. The team at NIF are confident they can do better than this "old-style" fusion, and sometime around May they aim to be the first to get more energy out than they put in. That will undoubtedly be a major scientific accomplishment - and also a necessary condition for making laser fusion a viable source of fusion energy.

But it is not sufficient: the energy must also be produced at reasonable cost. And NIF will not come close to answering that question. The reason is simple: despite all the PR hype, the huge machine was never designed to investigate energy production. Its principal role is testing thermonuclear weaponry. Best known as the hydrogen or "H-bomb", these devastating nuclear weapons work by triggering uncontrolled fusion in hydrogen-like fuel using X-rays, exactly the same method used in NIF.

Despite appearances, the laser will not heat fusion fuel directly, but instead focuses its beams on a gold "cell" holding the fuel pellets. This releases the X-rays needed to make the pellets implode and heat up to the 100 million °C temperatures. The result is like a tiny thermonuclear explosion, ideal for studying the physics behind these most destructive of all weapons of mass destruction. The use of NIF for simulating H-bombs has led to protests from peace campaigners, who claim the machine violates international treaties controlling the development of such weapons.

But the attempts to portray NIF as a curtain-raiser for real fusion power have also drawn protests from scientists, who argue the whole process is fundamentally flawed. The most obvious problem is that laser fusion produces power in bursts, rather than continuously. Even running flat-out, NIF will only be capable of only a few blasts per day, well short of the rate needed for a power station. Attempts are being made by European scientists to address this problem. They have formed a 10-nation consortium known as the High Power laser Energy Research facility (HiPER), which aims to begin its own fusion experiments in the early 2020s.

Unlike the NIF approach, HiPER will heat up the fuel in two stages. First, a bank of lasers will blast each fuel pellet until it implodes, after which electrons are injected into the debris heating it to fusion conditions. The process is akin to that of a car engine, in which fuel is first compressed using pistons, and then ignited sparkplugs, releasing the fuel's energy. As with NIF, the energy will then emerge in the form of fast-moving neutrons, which slam into a "blanket" of special material surrounding the pellet. This converts their kinetic energy into heat, which is used to generate electricity by the standard means of heating water to drive steam turbines.

The HiPER team claims their approach is both more efficient and more suitable for running at the firing frequencies needed for commercial power generation. While NIF aims to generate around 10 to 30 times more energy than is put in, HiPER could reach values as high as 100. However, some physicists remain deeply sceptical of the whole idea of laser fusion. They include William Nellis of Harvard University, who last summer produced a report warning that the sheer violence of the laser process could be its own downfall. Put simply, the blast of radiation striking the pellet causes turbulence within the fuel, cooling it below the temperatures needed for fusion. "Based on what is known today," argues Prof Nellis, "it is unlikely that NIF will produce practical amounts of fusion energy."

Ultimately, the problem with laser fusion is the same one which has dogged fusion research since its inception: no one really knows what will work, and finding out is punitively expensive. That, in turn, means that those holding the key to the completion of the quest to harness the energy source of the stars are not scientists but politicians. For only they can decide to sign the colossal cheques needed to put an end to the old joke about nuclear fusion: that it is the power source of the future - and always will be.

Robert Matthews is visiting reader in science at Aston University, Birmingham, England