One of the biggest question marks hanging over the ITER fusion reactor project - a giant international collaboration under construction in France - is over what material to use for coating its interior wall.

After all, the reactor has to withstand temperatures of 100,000°C and an intense particle bombardment.

Researchers have now answered that question by refitting the current world's largest fusion device, the Joint European Torus (JET) near Oxford, in Britain, with a lining akin to the one planned for ITER.

JET's new "ITER-like wall," a combination of tungsten and beryllium, is eroding more slowly and retaining less of the fuel than the lining used on earlier fusion reactors, the team reports. "This was very good news, because it means that our choice of materials for ITER was the right one," says physicist Peter de Vries, task force and session leader at JET.

Fusion is the process that powers the sun and stars, and, potentially, it's the perfect energy source.

The necessary fuels are easily accessible and virtually inexhaustible, and the process doesn't produce any greenhouse gases or long-lived nuclear waste.

For fuel, it requires deuterium and tritium (forms of hydrogen with one and two extra neutrons, respectively, in their nuclei).

These have to be heated so that they form plasma-an ionized gas-and when they reach about 150 million°C, the nuclei collide with such force that they overcome their mutual repulsion and fuse into a new, larger nucleus.

The products of the reaction are a helium nucleus and a very energetic neutron, whose energy is later harvested in the form of heat.

But the harsh truth is it's not at all easy to run this fusion process in a controlled way.

The favoured technique is to use a reactor called a tokamak, which employs powerful electromagnets to confine the plasma inside a doughnut-shaped reactor vessel.

The magnets aim to hold the plasma away from the walls of the vessel long enough for the nuclei to fuse but plasma can often shift around in unpredictable ways.

If the plasma touches the wall, this can cool it to below reaction temperature and also scour off atoms of the lining material that poison the fusion reaction.

And tritium is a radioactive isotope that reactor operators have to account for very carefully. Any tritium that embeds itself in the reactor wall has to be painstakingly extracted.

No fusion reactor has yet produced more energy than was put in to heat the plasma in the first place.

But researchers have high hopes for ITER, the massive reactor with an estimated price tag of as much as $20 billion that is now being built in the south of France by China, the European Union, India, Japan, Russia, South Korea, and the United States.