Void dynamics in low-pressure acetylene RF plasmas

Low-pressure acetylene plasmas are able to spontaneously form (under certain conditions) dust particles, resulting in a cloud of particulates up to micrometer sizes levitated in the plasma. We studied a capacitively coupled RF plasma with constant flow of argon and acetylene. After the dust cloud has been formed, an ellipsoid-shaped dust-free zone (void) develops. Concurrently, the dust particles grow in size. During its expansions the void suddenly stops growing and even shrinks, to shortly thereafter resume its expansion. We dubbed this the ‘hiccup’. We infer this is induced by coagulation of a new batch of dust particles inside the void. The processes are periodical and reproducible. Several techniques that are time resolved (microwave cavity resonance spectroscopy, plasma impedance monitoring) and in addition spatially resolved (Mie scattering, emission spectroscopy) are used to characterize the plasma and/or dust particles.