The glittering prize

Nuclear fusion could one day provide a reliable source of energy thanks in part to researchers from two UK universities who aim to use a diamond lining to solve the problem of wall erosion in fusion reactors.

The plasma materials group at Heriot-Watt University and materials modellers at University College London are focusing on a tokamak reactor. First invented in the 1950s, a tokamak produces a toroidal magnetic field for confining and manipulating plasma — the raw material for nuclear fusion.

Fusion power would take advantage of the enormous numbers of energetic neutrons that are released when helium is produced by the fusion of different types of hydrogen isotopes, deuterium and tritium at high temperatures.

But fusion is not an easy phenomenon to produce. A large amount of energy must be expended for it to happen inside a reactor. The hydrogen nuclei are positively charged and repel each other, so they must be brought close enough together to fuse. All the while the plasma must be kept away from the reactor walls because there are high temperatures and high fluxes of charged and neutral species where ionised gases meet the container walls. This environment attacks them, producing dust that absorbs the tritium fuel.

The researchers intend to mitigate this problem by coating the plasma-facing components with diamond, which has high temperature stability, radiation resistance, high atomic density and unsurpassed chemical stability in the presence of hydrogen plasmas. If successful, this approach would enable reactors to operate for longer periods before maintenance stops and reduce the amount of tritium held in the reactor.

The teams propose using diamond that is synthetically created through chemical vapour deposition (CVD). CVD diamond growth typically occurs under low pressure and involves feeding varying amounts of gases into a chamber, energising them and providing conditions for diamond growth on a substrate. The gases always include a carbon source and, typically, include hydrogen as well. The energy source, which can include hot filament, microwave power or arc discharges, is intended to generate a plasma in which the gases are broken down and more complex chemistries occur.

Heriot-Watt's prof John Wilson, a co-investigator in the project and a CVD expert, said his past research in synthesised diamonds led him and his colleagues to take on this project.

'When we saw the problems that people were expressing with materials in tokamaks — that its walls are based on carbon, which suffer a hydrogen etch problem — it seemed to be directly related to the growth of diamond,' he said.

The researchers will spend the next three years putting their materials under the gun of high-flux laser beams and high-flux ion and electron beams to separate and determine the different effects on diamond.

They will also be given time to test their material within an existing tokamak, Joint European Torus (JET), the world's largest nuclear fusion experiment at Culham in Oxfordshire. The tokamak is lined with tiles made from a carbon-fibre composite but its successor, ITER, to be built at Cadarache in the south of France, will use beryllium, which has a higher melting point, is lighter than aluminium and stiffer than steel.

While beryllium is deemed to be a better material than carbon-fibre for fusion reactor walls, Wilson argues that beryllium is still not good enough for reactors of the future.

'ITER was designed with materials that were the best at the time but those involved with ITER realise those materials might not be good enough,' he said. 'There's obviously limited materials they can use. They don't want to use heavy metals because if they erode, they contaminate the plasma. So they are down to light elements like beryllium and carbon.'

He added: 'The reason we are interested in diamond is because we know it erodes far more slowly than graphite in hydrogen plasmas. What we don't know is, given the extreme conditions in the tokamak with all sorts of other fluxes of particles and ions and charges and temperatures, whether that difference will be maintained.

'But the measurements that have been made by various people over the years still suggest that diamond will etch away far more slowly than a graphite surface.'

Wilson said his research team will be working with other Scottish universities to determine how to measure the erosion process inside the tokamak. 'Obviously, what you don't want to do is to put something in and take it out some days later and then try and guess what happened to it,' he said. 'Our colleagues will be attempting to develop an optical probe that will be put inside the tokamak to measure what's going on.'