atmospheric-pressure plasmas using tailored voltage waveforms*

Ozone (O₃) plays a key role in many medical applications of atmospheric-pressure plasmas. However, at high concentrations it is toxic, which means its production must be carefully controlled. Electrical control mechanisms are favoured over mechanical ones because of the flexibility and control speed that can be achieved. In this work, tailored voltage waveforms (TVWs) are employed to drive a radio-frequency He+O₂ plasma in a COST-like source. The TVWs consist of up to 5 harmonics (N), and form peaks, valleys and sawtooth waveforms. The density of O₃ is measured in the far effluent using Fourier Transform Infrared Spectroscopy.

When different waveforms of constant peak-to-peak voltage are applied, including extra harmonics in the driving waveform considerably enhances O₃ production until an upper limit of N=4. At this limit, an increase in power deposition likely increases the background gas temperature and drives the decomposition of O₃ into O₂. When a constant power is supplied to the plasma rather than a constant voltage, O₃ production can also be enhanced but the effect is not as strong as with constant voltage. The increased O₃ production at constant power is linked to a power deposition that is more localised in time, due to increased potential gradients in the plasma.