A microwave plasma is activating the gas mixture to deposit a diamond coating on to a substrate below.

Researchers from two UK universities currently aim to test a diamond lining to solve the problem of wall erosion in fusion reactors. The plasma materials group at Heriot- Watt University and materials modelers at University College London have developed methods of growing diamond film at relatively low pressure and temperature, using gases to lay down the material in a plasma.

The group is led by Professor Phil John and Professor John Wilson. “In prototype fusion reactors the internal walls are lined with carbon composite tiles similar to those found on the edges of space shuttle’s wings or the brakes of jet aircraft”, Professor Phil John explains. “Even this material may not be sufficient to withstand the enormously hot plasmas envisaged for the next generation of fusion reactors, and erosion of the tiles would mean frequent close-downs to replace eroded tiles. To prevent this, we intend to coat the tiles with diamonds, a material unique in its ability to withstand high temperatures, be resistant to radiation and maintain chemical stability in the presence of hydrogen plasmas. This would allow the prototype reactor to operate for longer periods before it would need to be shut-down for maintenance.“

At Heriot-Watt-University, Diamond film research began in the late 1980‘s, when the first UK plasma chemical vapour deposition (CVD) growth system was established. The CVD process generally uses energy, traditionally thermal, to drive a reaction in a gaseous medium, producing reactive species that can form a deposit on any adjacent surface. “We use a plasma discharge to provide the energy, which allows a lower temperature for the gas, because the reaction is driven by energetic electrons.” The surface is usually heated as well, to ensure the species have some mobility when they arrive on it, and so can make a dense deposit, and also sometimes to drive off any unwanted by-products. The gas may be at low pressure or atmospheric pressure.

“We went on to study deposition on a range of materials for applications in optics and electronics”, John Wilson explains. “Of course, the synthetic route that uses microwave plasma to decompose a hydrocarbon gas mixture uses the inverse process of hydrogen erosion that occurs in fusion plasma experiments. The plasma in this CVD process provides energetic electrons that dissociate both the hydrocarbon gas (typically methane) and the predominant hydrogen in the mixture: atomic hydrogen is an excellent etchant for any nondiamond component, thus leaving a pure diamond film on the heated substrates. Although it is known that diamond etches far more slowly than graphite in hydrogen plasma we don‘t know what additional effects will arise in the more energetic plasma of a tokamak device. Carbon is of course a more preferred plasma facing component than heavy metals, if it can be prevented from eroding. It should then cause less contamination of the plasma. We believe that Carbon Fibre reinforced Carbon (CFC) is the best carbon-based material available at the moment, but diamond may extend its performance.”

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 MAST at Culham. “The reason we are interested in diamond is because we know it erodes far more slowly than graphite in hydrogen plasmas”, John Wilson added. “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.”