Control of the chemistry of a humidified He RF atmospheric pressure plasma jet by pulse modulation

Pulse modulation of RF atmospheric pressure plasma jets can be used to control the production of short-lived and long-lived species. This behaviour has been studied in the production of H 2 O 2 by a humidified He atmospheric pressure plasma jet (APPJ) operating at 13.56 MHz. In addition to H 2 O 2 , several species involved in the plasma chemistry were measured in the effluent by various diagnostics: OH by LIF, H 2 O 2 by FTIR and mass spectrometry and HO 2 , H 2 and O 2 by mass spectrometry. The highest species densities were found for H 2 and O 2 with values of 6 × 10 15 cm −3 and 9 × 10 15 cm −3 , respectively. H 2 O 2 and HO 2 densities in the order of 1 × 10 14 cm −3 were found and the trends of the two species were closely coupled. The lowest density was found for the OH radical in the order of 2.5 × 10 13 cm −3 . For all species, the feed gas humidity and the dissipated plasma power can be used to control the densities of the species. Under pulse modulation of the RF signal, comparable densities of OH, H 2 O 2 and HO 2 to the unmodulated case were found at a duty cycle of 50 %. At a lower duty cycle of 10 % the long-lived species accumulated and the short-lived OH radical was suppressed. The molecular species H 2 and O 2 showed remarkable differences when the pulse modulation was applied as a three-fold reduction in H 2 density and an almost five-fold increase in O 2 density was present.