Spatiotemporal characterization of dust-forming acetylene RF plasmas

In low-pressure hydrocarbon plasmas, dust particles spontaneously form under certain conditions. The whole process occurs in a matter of seconds to minutes after igniting the plasma and results in a cloud of particulates up to micrometer sizes levitated in the plasma. We studied a capacitively coupled radio-frequency plasma with constant flow of argon and acetylene. After the dust cloud has been formed, an ellipsoid-shaped dust-free zone – termed the void – develops. Concurrently, the dust particles grow in size. During the void’s expansion it suddenly stops growing and even shrinks, to shortly thereafter resume its expansion which we refer to as 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. We probe the plasma by several measurement techniques, most notably microwave cavity resonance spectroscopy (MCRS) for which the discharge chamber acts as resonator. This yields the electron density weighted by the electric field of the excited resonant mode. We studied several resonant modes to get a better understanding of where changes of the electron density happen. The associated electric field patterns inside our cavity are obtained by locally and systematically disturbing the medium (without plasma) by a small PVC cube. MCRS is complemented by plasma impedance monitoring, laser light scattering and emission spectroscopy.