Characteristics of atmospheric pressure glow discharges generated using HF & VHF excitation

Summary form only given. Recent studies have shown that atmospheric pressure radio- frequency glow discharges (RF-APGD) have the potential to replace costly vacuum plasma processing systems. Unlike low pressure discharges the highly collisional nature of atmospheric pressure plasma introduces an inherent plasma instability RF-APGD exhibits a tendency to undergo a glow-to-arc transition (GAT) when the input power exceeds a critical level or when a reactive gas is added to enhance plasma chemistry. One possible means to overcome such instabilities is to increase the excitation frequency beyond the conventional HF band (3-30 MHz) and into the VHF band (30-300 MHz). This study reports the characteristics of RF-APGD generated between 3 MHz and 100 MHz. Space and time resolved optical diagnostics are employed to show the evolution of the discharge as frequency increases whilst maintaining a constant input power. It is demonstrated that increasing excitation frequency significantly enhances plasma stability, permitting greater addition of reactive gases and higher input powers to be achieved. It is also observed that an increase in excitation frequency impacts upon ionization efficiency, both fluid and PIC modeling is employed to provide further insight in to the experimental findings. To compensate for a reduced ionization efficiency whilst maintaining the beneficial properties of VHF excitation a dual frequency scheme is considered. The mixing of HF and VHF frequencies introduces several new parameters that can be tailored to further enhance discharge stability and chemical reactivity. Considerable work conducted at lower gas pressures indicates that the phase of the two frequencies and the input power ratio impacts upon the excitation dynamics, and vibrational/rotational temperatures. Very few studies have investigated dual frequency operation at atmospheric pressure and the effects remain poorly understood.