Exploring the Effects of Particle Size Distribution and Gap Distance on a Nanosecond Pulsed Discharge in Air with Fluorapatite Microparticles

Dusty or complex plasmas are rich in reactive species and micro-/nano-particles, and they are present in different systems such as interstellar clouds, Tokamak, and plasma-based microfabrication, among others. Depending on the experimental conditions, the particles may affect the plasma properties, which in turn affects the particle characteristics. So far, the interdependent plasma–particle system remains ambiguous, particularly in the case of non-thermal plasmas at atmospheric pressure. Herein, we study the properties of a pulsed nanosecond discharge in ambient air in contact with fluorapatite microparticles of variable size distributions (d1: 20–38 μm, d2: 38–106 μm, and d3: 106–150 μm), at different gap distances. The obtained results demonstrate that the discharge electrical and optical properties are strongly dependent on the size distribution of particles, but not on the gap distance. For instance, homogeneous discharge emission is observed in the presence of d1 particles; however, the emission obtained with d2 and d3 particles is filamentary. Based on emission spectroscopy and other analyses, Ca, Ca + , and F elements are present in the plasma (band intensities vary depending on the conditions), which indicates that the plasma–particle interactions are strong. Furthermore, the morphology of the processed particles shows signatures of heating and melting in the case of d1, while in the case of d3, discharge processing induces matter ejection from the particles by electro-erosion-like phenomenon. These changes in particle morphology are consistent with the emission data, as d1 particles are suspended in the plasma, whereas d3 particles remain at the electrode surface. Overall, the findings reported here contribute to the understanding of plasma–particle interactions, as well as to the development of applications related to particles processing.