Production of ozone in an He+O2 radio-frequency atmospheric pressure plasma jet driven by tailored voltage waveforms

Atmospheric pressure plasmas are efficient sources of reactive oxygen and nitrogen species with applications in materials processing, gas conversion, medicine and agriculture. Selective control over the production of reactive species presents an ongoing challenge. This study therefore investigates the production of ozone by a radio-frequency plasma jet driven with tailored voltage waveforms composed of up to five consecutive harmonics, with a fundamental frequency of 13.56 MHz. The plasma is operated in helium with small admixtures (0.1%– 1.0%) of oxygen. In the far efflient, the ozone density is measured with Fourier transform infrared spectroscopy and the gas temperature in the plasma channel is determined with optical emission spectroscopy. Voltage waveform tailoring is found to enhance the ozone density in comparison to operation with single-frequency voltage waveforms. Increasing the number of applied harmonics in the driving voltage waveform for a fixed peak-to-peak voltage enhances the ozone density but also significantly increases the gas temperature within the plasma channel. Increasing the number of applied harmonics while maintaining a constant RF power deposition allows the density of ozone in the effluent to be increased by up to a factor of 4 relative to single-frequency operation, up to a maximum density of 5.7×10¹⁴ cm⁻³, without any significant change to the gas temperature. In conclusion, this work shows that tailored voltage waveforms can be used to control the density of ozone delivered through the plasma effluent, marking an important step towards realising the potential of these plasmas for applications.