Atmospheric Pressure Plasmas Driven By Tailored Voltage Waveforms

This thesis presents the dynamics of capacitively coupled atmospheric pressure plasmas driven by tailored voltage waveforms (TVW). They are composed of a fundamental frequency (13.56 MHz) and a number of harmonics varied from 2-5. This work concentrated on studying the electron excitation dynamics obtained experimentally in helium-nitrogen admixtures and helium-oxygen electronegative plasmas. Peaks-type waveforms and sawtooth-type waveforms operate plasma with different numbers of harmonics. The electron impact excitation dynamics are extracted from the emission of plasma photon through the use of phase-resolved optical emission spectroscopy (PROES). As the electron dynamics are influenced by the transition of the plasma boundary sheath, however, a field reversal builds up in the plasma because the reduction of electron motion due to the collisions. Consequently, the electric field initiates an acceleration of electrons toward the electrode and therefore contribute to ionisation or excitation mechanisms. This electric field was observed through the application of peaks or valleys waveforms, either via helium-nitrogen or helium-oxygen. Electron impact excitations are influenced by two parameters used in this work. First, with more harmonics applied, the excitation rates increase. This is applicable on all types of TVW used here. Therefore, the plasma density increases as observed from the thickness of the plasma sheath being smaller with more harmonics applied. The second factor affecting electron dynamics is the variation of the molecular gases used (nitrogen and oxygen). In electro-negative plasma (He-O2), the transition from sheath expansion-dominated excitation to sheath collapse-dominated excitation occurs when adding more admixtures of oxygen. However, this kind of transition is not fulfilled in the case of helium-nitrogen plasmas. When operating the helium-oxygen plasma, the resultant effluent was exploited to measure the ozone density. In all cases ozone densities rise with more concentrations of oxygen.