An investigation into the nonthermal behavior of a microwave surface-wave sustained plasma through alumina bead topology

Abstract The present work is a 2D numerical investigation of the non-thermal properties of a microwave (MW) surface-wave sustained plasma source incorporating a dielectric bead assembly (BA), with a focus on material-processing applications. The MWs are launched into the discharge tube via a reduced-height waveguide (RHW), which generates a locally enhanced electric field ( E -field) inside the discharge tube. The waves are launched in the middle of the tube ( y = 0) from a radial direction such that the E -field is perpendicular to the wave propagation. Inside the tube, it propagates in both directions from the launching position. Without BA, the discharge profile, investigated at 1 Torr of argon gas pressure, exhibits two overdense plasma regions sustained symmetrically about y = 0. The BA is introduced into the discharge tube so that the beads are aligned along the inner surface of the tube. The dielectric (alumina) bead topology followed in the study ensures that (a) the inner surface of the discharge tube is filled with BA to have polarization along the wave propagation direction, and (b) some of the assembly falls along the line-of-sight of the MWs. Among the three topologies investigated, it is observed that keeping the BA symmetrically about the MW launching position ensures the highest (i) ion current density, and (ii) metastable density near the plasma outlet. The presence of the BA increases the strength of the local E -field near the MW launcher; nevertheless, the plasma source exploits the excess MW energy effectively through stepwise ionization to increase the metastable density rather than increasing the electrons’ thermal energy. Excessive electron energy causes metastable density to exceed the electron density, making the current source suitable for generating non-thermal plasmas.