Force hopes

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.’