Production of reactive oxygen species in an atmospheric-pressure pulsed He+H₂O plasma: Effect of pulse repetition frequency

The controlled production of reactive oxygen species (ROS) in atmospheric-pressure plasmas is key in many of their applications. In this work, the production pathways of ROS are studied numerically in a nanosecond-pulsed pin-pin He+H₂O discharge as a function of pulse repetition frequency (1-500 kHz). The plasma is simulated using the 0D plasma-chemical kinetics model GlobalKin. The pulse shape (80 ns duration, 3 ns rise time, 2.3 kV amplitude) is kept constant in all cases, such that the afterglow duration is dependent on the repetition frequency. Analysis of the bulk plasma chemistry shows that short-lived species such as atomic hydrogen and oxygen increase with increasing repetition frequency, because these species are predominantly produced during the plasma pulse and decay in the subsequent afterglow. With decreasing afterglow time, their density at the start of the pulse increases, increasing the overall density within the plasma pulse. However, for some long-lived species such as H₂O₂ and O₃, there is a decrease of density at high repetition frequencies because these species are mainly produced during the afterglow from a conversion of short-lived species. A careful consideration of both pulse and afterglow period needs to be made when optimising the production of long-lived ROS through pulse repetition rate.