These bundles of short upright fibres are key to plasma stability, effectively providing an array of lightning rods so that the plasma filaments from each coalesce giving a uniform and extremely high temperature. Some of the carbon fibres woven into the felt naturally lie flat while others stand upright. The researchers used carbon fibre felt at either side of their atmospheric pressure plasma cavity. The plasma reactor with carbon-fibre-tip electrodes visible Source: © Hua Xie et al/Springer Nature Limited 2023 ‘We have tips so we have high temperature, but they support each other so we have stability.’ The carbon advantage ‘What we do here is we have a combination of both,’ says Liangbing Hu at the University of Maryland. The temperature difference between the plasma filament and the gas around it makes these plasmas very unstable. However, here the temperature only increases along the filament. Using a sharp tip that concentrates the electric field around it can create a plasma filament at atmospheric pressure by providing a preferred route for electrons to take – the principle behind lightning rods. To generate high-temperature plasmas, higher pressures are required so that collisions between the more closely packed ions will increase their velocities. As the velocity of the ions left behind is barely changed, the temperature of the gas itself remains low. However, the low mass of electrons means that even at high velocities they don’t have the momentum to affect chemical reactions. The stable plasmas familiar in neon signs and televisions exploit very low pressures, where an applied voltage strips electrons from a gas, which then hurtle towards the positive electrode.
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