Unravelling reactive oxygen species production in low temperature, atmospheric pressure plasmas

Low temperature plasmas (LTPs) are efficient sources of reactive oxygen and nitrogen species (RONS) at atmospheric pressure. Understanding how different parameters influence RONS production is essential for optimising LTP devices for biomedical applications. Several plasma sources are investigated here. First, a numerical model is used to study the influence of pulse repetition frequency on the plasma chemistry of a nanosecond-pulsed, He+H2O plasma. Increasing the pulse repetition frequency increases the density of H, O, and OH radicals. H2O2 and O3 are formed throughout the afterglow, and their density is instead found to depend on the afterglow duration, gas temperature, and radical densities. Next, cavity ring-down spectroscopy is used to determine the spatial distribution of H2O2 in the effluent of a COST-Jet and a kINPen-sci in He+H2O. The H2O2 density near the jet nozzle is 2.3×10^14~cm^-3 for the kINPen-sci and 1.4×10^14~cm^-3 for the COST-Jet. The average H2O2 density in the effluent of the kINPen-sci is a factor of two higher than for the COST-Jet. The H2O2 distribution in the COST-Jet effluent is uniform near the jet nozzle but diluted beyond 15 mm. For the kINPen-sci, the H2O2 density near the nozzle has a comparatively pronounced radial spread, while dilution is more gradual at further distances. Finally, O3 production is investigated for a He+O2, radio-frequency plasma jet driven with tailored voltage waveforms. Increasing the number of harmonics in the driving waveform for a fixed peak-to-peak voltage enhances the O3 density but significantly increases the gas temperature. Increasing the number of harmonics for a constant RF power allows the O3 density in the effluent to be increased by a factor of 4, to a maximum of 5.7×10^14~cm^-3, without significant change to the gas temperature. It is hoped that the results presented in this work may help to optimise RONS production by biomedical plasma devices.