Investigation of power balance in micro dielectric barrier glow discharge with ultra-high driving frequency

Recently, atmospheric pressure micro plasmas attract lots of interests for the useful applications such as industrial surface modification and bio-medical treatment. Among many plasma devices, a dielectric barrier discharge structure (DBDs) is widely used for the simplest device which can sustain the glow discharge with a sub-millimeter gap length with ultra-high frequency (UHF). However, it is not still well-known how to apply theoretical study for discharge characteristics to engineering optimization from RF to UHF in atmospheric pressure micro DBDs. In this study, a particle-in-cell simulation has been utilized to understand the helium and argon discharge characteristics of a planar micro dielectric barrier discharge in a 80 µm gap for the variation of driving frequency from 5 MHz to 500 MHz. The optimal condition for the efficient generation of high density plasmas with minimized power is obtained when the ratio of ion transit time to the RF period is about a quarter, so that both ion flux and secondary electron flux are accelerated in the same phase with the sheath potential. At this condition, the plasma density as well as the ion current is maximized because power loss by ionization is maximized among the total power loss. At higher frequency, the collisional electron heating increases by the induced bulk electric field, more frequent excitation and elastic collisions, and thus ionization per total power loss decreases while excitation power efficiency increases.