On-Ground measurement of dust residual charge in a dusty plasma afterglow using thermophoretic gravity compensation
Résumé
Dusty or complex plasmas are partially ionized gas composed of neutral species, ions, electrons, and charged dust particles. In laboratory experiments, these particles can be either injected or grown directly in the plasma. Injected dust particles are usually micron-size particles. Due to their mass, they are confined near the bottom electrode where the electric force counterbalances gravity. The microgravity condition is necessary to study dust clouds of micrometer-size particles filling the whole plasma chamber. In the laboratory, dense clouds of submicron particles light enough to completely fill the gap between the electrodes can be obtained using reactive gases such as silane or using a target sputtered with ions from plasma. Dust-particle charge is a key parameter in complex plasma. It determines the interaction between a dust particle and electrons, ions, its neighboring dust particles, and electric field. However there are only a few papers devoted to dust charging, or decharging to be more specific, in the discharge afterglow. In this paper, we report an on-ground experiment on the residual charge measurement of dust particles after the decay of a dusty plasma. The experiment was performed in the PKE-Nefedov reactor where the dust particles were physically grown in the plasma. A temperature gradient was introduced in the chamber to create an upward thermophoretic force to balance gravity in order to maintain the dust particles in the volume of the chamber when the discharge is switched off. The residual charges were determined from an analysis of dust oscillations, which were excited by applying a sinusoidal bias to the bottom electrode. It was shown that dust particles do keep a residual charge of few electrons and that both positively and negatively charged dust particles coexist in the late plasma afterglow. In order to understand these results, measurements on electron density decay have been performed into other dusty discharges such as argon/methane dilution plasma and sputtering discharge.