Is your fusion reactor getting clogged with garbage? Try using some coconut. It's not the most obvious ingredient for such a high-tech device, but it will play a vital part in the ITER reactor in France.
The coconuts will be used to generate 10,000 cubic metres of nothing - the vacuum essential to ITER's operation. Some of this vacuum, in the central chamber, separates the plasma from the surrounding solid walls and allows fusion to proceed unhindered by air molecules. A whole lot more nothing is needed to fill the vacuum jackets that insulate ITER's supercooled magnets.
The vacuum pumps that will suck air out of ITER will also need to hoover up the waste helium from the fusion reaction, along with other debris created when hot plasma smashes into the reactor wall. "This can only be done with very large cryogenic pumps," says Christian Day of the Karlsruhe Institute of Technology in Germany, who is building ITER's vacuum pumps.
A cryopump works by capturing gas molecules on a cold surface. The pumps in ITER will have two stages, chilled to 80 and 5 kelvin respectively. While most gases will freeze on contact at these temperatures, the helium and hydrogen isotopes emerging from ITER are less easily caught. They can only be captured by adsorption, a process that involves atoms of the gas sticking loosely to a solid surface - and the greater the surface area, the better. "We wanted a material that behaves like a sponge, with lots of internal surfaces," says Day.
He and his team have spent 20 years searching for the ideal adsorber. Their quest was wide-ranging, taking in sintered metals and porous minerals called zeolites, but eventually they found that the material with the greatest capacity to adsorb gases was charcoal. They tried charcoal made from industrial polymers, from different varieties of wood, from peat, and from textiles such as wool and cotton, and more.
Once source stood out. "We found that coconut-shell charcoal is the best," Day says. "It is somehow strange that you need this very natural material to make a fusion device."
To make coconut charcoal, you first char the shells in a low-oxygen atmosphere, then wash them in acid and bathe them in steam to clean out a network of fine pores. The walls of these pores form a huge internal surface - about 1200 square metres per gram of charcoal. "We found the decisive point is pore size distribution," says Day. "We need pores of all types, because different pore sizes work best for different gases."
The provenance of the coconut matters too. The best fusion coconut turned out to come from one particular Indonesian island, Day found, and the quality of the resulting charcoal even depends on the year. "2002 was an excellent year for coconuts," he says.
So the team bought up most of the vintage 2002 Indonesian coconut-shell charcoal. "We have a few tonnes in a garage in Karlsruhe," Day says. That will be enough to supply ITER's cryopumps a few times over, so there should be a surplus - perhaps destined for the first commercial fusion power stations some decades from now. Unless 2035 is an even better year for coconuts.
