11 July 2012
The scientific journal Nuclear Fusion lists it as one of its highlights for 2011: diamond might be a valid wall material for a fusion power plant. The wall of such reactors will encounter enormous heat and aggressive erosion processes. Together with international colleagues, physicist Greg De Temmerman at FOM Institute DIFFER subjected boron-doped diamond to extreme heat loads. The team discovered that the material outperforms other forms of carbon and even the extremely heat resistant metal tungsten.
Thermal shocking diamond with electron gun
Can boron-doped diamond resist the intense plasma conditions in ITER? The team lead by De Temmerman used the JUDITH electron gun at Forschungszentrum Jülich in Germany to find out. The device is capable of 5 millisecond pulses which deliver 2.5 GW per square meter, comparable to the most devastating disruptions. Afterwards, a scanning electron microscope was used to track changes to the surfaces they blasted. "In all cases, we found that diamond resists extreme heat loads better than tungsten", says De Temmerman. "In the best case, we could see no damage or surface erosion after even a hundred cycles of exposure to heat fluxes that are three times higher than the melting threshold of tungsten. Graphite or CFC, on the other hand, showed significant surface erosion under such conditions."
Scanning Electron Microscope picture of a boron-doped diamond sample with no surface processing after exposure to 1000 cycles at 141 MW m−2 s1/2. Panels (b) and (c) are close-ups of panel (a), showing the undamaged and damaged regions observed under similar exposure conditions. Panel (d) shows a similar sample after 100 cycles at 176M W m−2 s1/2.
Against the odds
With the new results, De Temmerman hopes to reignite interest in diamond as a material for the walls of a fusion power plant. But the going is tough. "I have been pushing this project against almost all odds - the community thinking that the idea of diamond made no sense. Seeing that Nuclear Fusion considers this work one of the highlights of the year is quite nice." Tungsten is the chosen material for the first ITER divertor, but replacements are foreseen halfway through the experimental exploitation phase and the material choice for those replacements is still undecided.
After ITER, the largest European fusion reactor is JET nearby Oxford. JET is the current plasma record holder in fusion research. After a 1.5 year upgrade campaign, JET now has a full-tungsten exhaust (the trench in the bottom) to make it as close a comparison as possible with the upcoming ITER reactor.
Background - fusion reactors face punishing conditions
Fusion researchers are trying to mimic the process powering the sun as a sustainable power source on Earth. The international ITER project aims to build a reactor capable of producing ten times more power from fusion than it consumes itself. It is scheduled to produce its first plasma in 2020. ITER's forebears commonly used graphite or fibre reinforced carbon (CFC) as their wall materials. ITER has decided to make the first version of its reactor wall out of tungsten. This metal can better resist the energy eruptions known as ELMs (Edge Localized Modes) that occur in the fusion plasma. The power released scales with the reactor size and in a commercial fusion power plant, the largest ELMs could deposit gigawatts of power on the exhaust of the reactor.