Sub-60 °C atmospheric helium–water plasma jets: modes, electron heating and downstream reaction chemistry

For plasma treatment of many heat-labile materials (e.g. living tissues) that either are moist or contain a surface layer of liquid, it is desirable that the gas plasma is generated at atmospheric pressure for process convenience and with a gas temperature ideally no more than 60 °C for mitigating permanent damage to the integrity of the test material. This implies that the liquid-containing plasma needs to be of low dissipated electrical energy and that plasma treatment should be based largely on non-equilibrium reaction chemistry. In this paper, a class of sub-60 °C atmospheric helium–water plasma jets is studied in terms of their main physiochemical properties. It is shown that there are five distinct modes appearing in the sequence of, with increasing voltage, the first chaotic mode, the plasma bullet mode, the second chaotic mode, the abnormal glow mode and the non-thermal arc mode. Its chaotic modes may be sustained over a wide range of water vapour concentrations (0–2500 ppm). Compared with other liquid-containing plasmas, the He–H2O plasma jet operated below its non-thermal arc mode has several distinct advantages, namely very low energy consumption (2–10 µJ per pulse), sub-60 °C gas temperature, electron-modulated production of He, N2, , O*, H and OH(A–X), and low ozone production (0.1–0.4 ppm). These results provide a first attempt at the landscape of the physiochemical characteristics in atmospheric He–H2O plasma jets.