Force hopes

6 min read

UK-led research uses lasers to mimic the sun’s energy-forming reactions in an effort to make fusion a practical and credible source of electricity. Stuart Nathan reports.

It sounds like something out of a James Bond film. A tiny pellet is fired into the centre of a steel sphere, where it is hit by a barrage of super-powerful lasers. A fraction of a second later, the pellet blooms into a huge explosion, releasing a wave of energy.

In a Bond film, of course, that energy would be used for some nefarious purpose. But this is a real concept — part of the research effort to make nuclear fusion a practical and credible source of electricity. A facility that aims to test this concept is now in the advanced planning stage. Called the High Power laser Energy Research facility (HiPER) it is likely to be sited in the UK.

From roots in the world of nuclear weapons development, laser fusion — also known as inertial confinement fusion — is developing on a parallel track to magnetic confinement fusion, the system being used by ITER, an experimental magnetic fusion plant being developed in France.

Experiments in inertial confinement are soon to reach a practical phase in the US, at the National Ignition Facility (NIF) at California’s Lawrence Livermore National Laboratory, due to be completed within the next two years. HiPER is designed to take the NIF research on to the next level, said project leader Prof Mike Dunne, director of the Central Laser Facility at Oxford’s Rutherford Appleton Laboratory.

Inertial confinement fusion is precisely analogous to the processes that take place inside the sun. In the heart of the star, the intense pressure of gravity forces hydrogen atoms together so hard that their nuclei fuse to form helium, which releases a huge amount of energy in the form of a fast neutron. The nuclei are positively charged, but the gravitation force is so strong that it overwhelms the repulsion of two like charges approaching each other.

Recreating those forces is the challenge of fusion. Magnetic confinement uses a plasma containing the nuclei of two types of hydrogen — deuterium, which contains a proton and one neutron, and tritium, which has an extra neutron — and uses a magnetic field to squeeze it together. Inertial confinement takes a different approach. It uses a fuel pellet — a ball bearing sized sphere, containing deuterium and tritium — and hits it with an intense laser pulse. This vaporises the casing of the pellet, which causes a shock wave that compresses the fuel.

‘The density gets higher, the temperature goes up and at some point fusion starts to happen,’ said Dunne. ‘The trick is to make sure you burn up enough of the fusion fuel before its inertia — from the movement imparted by the vaporising pellet — is overcome by the energy of the fusion, and the whole thing blows itself apart.’

NIF and HiPER both work on this principle, but their mechanisms differ. NIF, Dunne explained, is a proof-of-principle project. ‘Within the next couple of years, NIF hopes to prove that a laser can compress matter to such a degree that you get more energy out than you put in, by a factor of 30 or so. The laser itself can fire every few hours, and they’ll perhaps do one fusion experiment a month.’

But such a plant would not be suitable for generating electricity. ‘You’d need a plant that operates a bit like a car engine: inject the fuel, compress it, it gives off energy, you have an exhaust phase, and you repeat. That would have to happen about five times a second to get gigawatt-scale power out of it.’

HiPER, which will test the concept of using inertial confinement nuclear fusion energy to create an electricity source, is likely to be sited in the UK

HiPER is being designed for a faster repetition rate, and with a technique that will allow it to be built to engineering tolerances, rather than those of a more precise scientific instrument. This means using a slightly different technique, known as fast ignition, which has not yet been attempted.

For NIF, compression, heating and ignition all come from a single laser, split into 192 beams, delivering some 2MJ of energy. ‘You need incredibly tight tolerances and well-behaved laser pulses: the perfect laser and the perfect fuel pellet,’ said Dunne.

Returning to the engine analogy, NIF works like a diesel engine: compress the fuel enough and it will ignite. HiPER, on the other hand, will work like a petrol engine, with the fuel pellet hit by two laser pulses in rapid succession. The first vaporises the pellet casing and compresses the fuel to a degree; the second, more powerful, acts like a spark plug and heats up the compressed fuel so that fusion reactions and ignition occur. ‘We believe that loosens the tolerances on the quality of the fuel pellet and the laser, so it’s more amenable to converting into a conventional engineering reality.’

In theory, it should also produce more net energy. Direct heating of the fuel is more efficient than using compression to generate heat. HiPER’s lasers will be about a third of the power of NIF’s, even though it needs to compress the fuel inside the pellet from a density of about 0.1g/cm3 to 300g/cm3, 30 times denser than lead.

NIF converts its laser energy into X-rays, the form of the energy that strikes the pellet. This, said Dunne, is a hangover from the early days of inertial fusion research, which was closely connected with nuclear weapons. In the late 1980s, he said, the US tested whether the power from a nuclear bomb could be used to implode and cause fusion in pellets containing deuterium and tritium. ‘The full results are classified, but we know it worked; that gives the US great confidence that the physics behind NIF are sound,’ he said. But it means NIF will use the same type of energy, although it generates it in a different way.

‘In generating X-rays, they lose about a factor of six in efficiency,’ said Dunne. ‘We don’t care about X-rays in a civilian energy programme, so we just use the light directly to implode the pellet. That gives us an efficiency gain, and it also breaks the link to defence research. We gain on both counts.’

Dunne admits that inertial fusion’s roots in weapons research can be a hindrance, a problem that does not affect magnetic fusion research. But it is also an advantage. ‘This is swords into ploughshares writ large,’ said Dunne. ‘The defence programmes have invested trillions of dollars into the underlying physics, and we’re picking up that investment and pointing it directly at a civilian application. That allows Japan, Canada and many European countries to get actively involved, and they wouldn’t countenance that if there were a direct link to a weapons programme.’

HiPER is somewhat behind ITER; Dunne estimates the target date for first fusion on HiPER will be 15 years after ITER. But both programmes are necessary, he claimed. ‘In the same way that there are a variety of solutions for fossil fuel energy generation and for renewables, we think it’s inconceivable that mankind would rely on only one method for fusion generation of energy supply.’

In terms of physics, he added, inertial fusion is ahead of magnetic, but magnetic is well ahead in engineering. ‘They have to have an integrated approach to all the systems involved, because without that the tokomak, the fusion device, won’t work at all,’ he said. ‘For us, we can divide off the laser development for high repetition rates, the study of the behaviour of the pellet, and how to make the pellet, but we haven’t got around to integrating them yet.’

Dunne also believes that magnetic and inertial fusion could be complementary. ‘Magnetic fusion reactors look as though they’d operate like continuously burning furnaces, which would provide constant baseload power. With our approach, you can imagine turning the output up and down by changing the repetition rate, which would cope with peaks and troughs of demand.’

So, for the moment, HiPER remains a series of concepts and development projects. The most important of these is probably the Petawatt Aquitaine Laser, under construction near Bordeaux. HiPER needs high-efficiency lasers, and is considering laser diodes to provide the light that is amplified by the main laser material, a neodynium-doped glass.

The concept is also closely coupled with the magnetic fusion research effort in terms of material science, and with the systems needed to extract the energy and generate tritium. ‘These are exactly the same problems,’ said Dunne. ‘We surround the target chamber, which is roughly spherical and about 6m in diameter, with a blanket that heats up when the fast neutrons hit it, and a cooling system extracts that energy to a heat exchanger to raise steam. The blanket also contains lithium, which is converted to tritium when the neutron hits it. We’re working with [the JET fusion team at] Culham on common technologies.’

HiPER’s schedule is tied to NIF’s results, but Dunne anticipates that construction is likely to start in 2015, and the UK is strongly tipped as the site for the facility, due to its heavy investment in the project and the existing presence of JET. The budget, shared between 11 nations, is likely to top £1bn. ‘Given that there is a need for abundant clean energy, the fact that we can take advantage of all the previous investment in this area seems to be eminently sensible,’ said Dunne.