Abstract
Dusty plasmas comprise a complex mixture of neutrals, electrons, ions and dust grains, which are found throughout the universe and in many technologies. The complexity resides in the chemical and charging processes involving dust grains and plasma species, both of which impact the collective plasma behavior. For decades, the orbital-motion-limited theory is used to describe the plasma charging of dust grains, in which the electron current is considered collisionless. Here we show that the electron (momentum transfer) collision frequency exceeds the electron plasma frequency in a powder-forming plasma. This indicates that the electron current is no longer collisionless, and the orbital-motion-limited theory may need corrections to account for elastic electron collisions. This implication is especially relevant for higher gas pressure, lower plasma density, and larger dust grain size and density.
Highlights
Dusty plasmas comprise a complex mixture of neutrals, electrons, ions and dust grains, which are found throughout the universe and in many technologies
We report a time-resolved trace of this effective collision frequency throughout the three earlier mentioned phases, revealing insight regarding the importance of elastic electron collisions in dusty plasmas
The irrelevance of the electron-dust cross section for momentum transfer implies that the electron loss channel due to attachment of electrons to dust grains is insignificant up to this point, and this implication is reflected by the stability of the electron plasma frequency and effective collision frequency
Summary
Dusty plasmas comprise a complex mixture of neutrals, electrons, ions and dust grains, which are found throughout the universe and in many technologies. The field, in general, assumes that the electron plasma frequency greatly exceeds the electron-neutral momentum transfer collision frequency at low-pressure (typically, 500 MHz and 50 MHz, respectively, for pristine argon plasmas at 0.1 mbar (i.e., 75 mTorr)), and the electron-neutral (momentum transfer) collision frequency has been neglected when developing theoretical frameworks for plasma-dust charging This assumption applies to the orbital-motion-limited (OML) theory, introduced by Irving Langmuir[20], which is the widely accepted theoretical framework for predicting the surface charge of dust grains in dusty plasmas. We provide experimental evidence (see Fig. 1) that the electron-neutral collision frequency should be included in the dust charging theory by showing its dominance over other timescales in a dusty plasma For this purpose, we measured—in addition to the electron plasma frequency fpe (via the free electron density ne) which is widely used in literature to indicate and frequency eff and fpe (MHz)
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