A diagnostic study of a dual frequency–driven atomic oxygen plasma source in the context of contactless cleaning

Abstract

This paper presents a diagnostic study of a dual frequency–driven atomic oxygen (AO) plasma source designed for the contactless cleaning of thermally sensitive artworks. Optical diagnostics, including ICCD imaging and actinometry, is used to evaluate the source performance. In plasmas driven at a base frequency (f0 = 13.56 MHz), both the AO density and gas temperature scale linearly with operational power. The admixing of the harmonic component (f1 = 27.12 MHz) reduces these scaling slopes synchronously. Importantly, by shifting the relative phase (θ) between harmonic and base waves, the AO density can be independently modified at a constant gas temperature. The 1-D plasma simulation results demonstrate that the phase modulation arises from the combined effects of geometrical-electrical asymmetry. The geometrical asymmetry induces a constant DC bias between electrodes to balance the charge flux, whereas the electrical asymmetry compensates or enhances this bias. These combined effects modify the spatial distribution of AO generation kinetics. For gas heating, ion Joule heating is sensitive to phase modulation but localized in narrow sheathes, whereas electron elastic collisions in plasma bulk are stable and dominate the gas heating in plasma bulk. This study provides novel insights for improving AO generation in cold plasma, especially for its applications in artwork restoration.