In-situ measurement of dust charge density in nanodusty plasma

Abstract

Abstract A dust grain immersed in a low-pressure gas discharge obtains a permanent negative surface charge due to the high mobility of electrons compared to that of ions. This charge essentially governs all fundamental processes in dusty and complex plasmas involving dust grains, neutrals, (an)ions and electrons and—consequently—virtually all industrial applications of these types of plasmas are affected and steered by it. In this work, we have measured the surface charge by application of laser-induced electron detachment from nanosized dust grains in concert with microwave cavity resonance spectroscopy and laser light extinction. The main result is that the electron release is governed by photodetachment rather than by thermionic emission, and that recharging of the dust grains occurs on timescales that are well in agreement with the orbital-motion-limited (OML) theory. The total surface charge density residing on the dust grains inside the laser volume follows from the saturation of the photodetachment signal, which was used in combination with dust density values derived from extinction measurements to estimate the mean dust charge. The negative dust charge on the 140 nm (average) diameter dust grains in this work is obtained to be in the range of 273 – 2519 elementary charges, of which the lower bound matches well with analytical predictions using the OML theory.