Spatially and temporally resolved H and OH densities in a nanosecond pulsed plasma jet: An analysis of the radical generation, transport, recombination and memory effects

Cold atmospheric pressure plasma jets (APPJs) enable the generation of a large flux of highly reactive species to heat sensitive substrates at near ambient temperatures. Nonetheless, the experimental determination of reactive species inside APPJs, while exceedingly important, has not been reported in significant detail. In this work, we report the time-resolved axial density of hydroxyl (OH) and hydrogen radicals (H) in an APPJ device and its effluent operating in a He-H2O mixture using 1D laser induced fluorescence (LIF) and two-photon absorption LIF (TALIF). The results show that H and OH are mainly generated between the electrodes in the APPJ rather than by the guided streamer. The produced H and OH inside the jet are convectively transported to the jet effluent and determine the H and OH densities in jet effluent. The dominant production and destruction mechanisms of H and OH were obtained from a 0D model. The different production mechanisms of H and OH can explain the change in memory effect observed for OH (and not for H) for varying pulse repetition rates of the plasma generation.