Investigation into the Role of Singlet Oxygen in Positive Corona in Air

Corona is the partial breakdown of air in a divergent electric field and is particularly evident around high voltage equipment. Air comprises of nitrogen and oxygen and the corona process involves many complex phenomena including ionisation, attachment and excitation and the interaction of the ions and excited molecules generated by these phenomena. A particular state of excited molecular oxygen, singlet oxygen, has the characteristic that it remains excited for a relatively long period of time. Singlet oxygen plays a role in the detachment of electrons from the negative oxygen ions and its role in corona discharges has been accounted for through theory and models, but due to the difficulties of measurement of the singlet oxygen, the influence it does have on corona discharges is not entirely clear. On this basis the gas processes associated with the corona discharge in air have been explored, where a Boltzmann equation solver, the electron energy distribution function, transport coefficient and Townsend coefficients are used to understand the phenomena and provide input to a gas discharge model, where the gases are representing as species in a drift-diffusion model. The model indicates the presence of negative ions and singlet oxygen, but clearly illustrates how the space charge plays the critical role in positive corona due to the collapse of the electric field. An experiment that involved altering the environment with air-flow produced an unexpected result in the positive onset streamers where the repetition rate changed considerably, initially it was thought that this was due to the removal of singlet oxygen. In an effort to determine if their was a relationship between repetition rate and singlet oxygen, experiments detecting the emission from singlet oxygen and exciting oxygen through a laser were undertaken. There was no evidence to suggest that the repetition rate of the positive onset streamers could be related to singlet oxygen. There were no emissions detected from various configurations, whilst visible corona was clearly seen. The experiment where laser induced excitation showed no change in repetition rate. It could be inferred that a relationship between repetition rate and that the detachment due to negative ions and singlet oxygen is not a dominant process in the corona discharge.