Behavior of Atomic Oxygen in Pulsed Barrier Discharge in Sub-atmospheric Pressure Oxygen*

 Atmospheric-pressure cold plasma (APCP) exhibits high chemical reactivity and valuable applicability to various targets including liquids and living materials. APCP produces high-density radicals, whereas the radical's lifetime is short due to a frequent collisional deactivation. To address this issue, we focus on the plasma under sub-atmospheric pressure, which is slightly reduced pressure from the atmosphere. In sub-atmospheric pressure plasma, the radical's lifetime will be extended maintaining the variety of the targets.  In this study, we measured the behavior of atomic oxygen (O) in needle-to-sphere pulsed barrier discharge in sub-atmospheric pressure oxygen, using TALIF spectroscopy. The half-value period of atomic oxygen was extended by a factor of 10 by reducing pressure from 95 kPa to 20 kPa, whereas the peak O density at 20 kPa was 70% of that at 95 kPa. Therefore, radical flux in sub-atmospheric pressure plasma will be significantly higher than that in the atmospheric-pressure. The relatively high production rate of O compared with atmospheric-pressure indicates that the electron energy enhancement owing to reducing pressure led to an increase in the O2-dissociation rate, in spite of small amount of electrons and feedstock O2 gas in the reduced pressure.