Influence of grain charge gradients on the dynamics of macroparticles in an electrostatic trap
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Y PLASMA
Influence of Grain Charge Gradients on the Dynamics of Macroparticles in an Electrostatic Trap O. S. Vaulina* Joint Institute for High Temperatures, Russian Academy of Sciences, Moscow, 125412 Russia *e-mail: [email protected] Received June 23, 2016
Abstract―An analytical model of anomalous heating of charged dust grains (macroparticles) caused by their stochastic motion in a bounded plasma volume is proposed. Analytical expressions allowing one to describe the pumping (heating) of interacting grains with additional stochastic energy due to grain charge gradients are derived. The analytical results are verified by numerical simulation of the problem. It is shown that spatial variations in the charges of dust grains can lead to their anomalous heating in laboratory plasma. DOI: 10.1134/S1063780X17030163
1. INTRODUCTION Studies of energy exchange in inhomogeneous systems of interacting particles are of significant interest for different fields of science and technology (plasma physics, medicine, biology, physics of polymers, etc.) [1–8]. Dusty plasma is an ionized gas containing charged dust grains (macroparticles) of micrometer size. Such plasmas are widespread in nature and are also created in a number of technological processes [1–3]. Most laboratory experiments with dusty plasmas are carried out using various types of gas discharges [9–13]. In weakly ionized plasma, massive dust grains efficiently dissipate their kinetic energy in collisions with gas atoms and molecules. Therefore, it is often assumed that, as a result of energy exchange between dust and neutrals, they are in thermodynamic equilibrium. However, experiments have shown that, under certain conditions, the stochastic kinetic energy (temperature) of macroparticles in laboratory plasma can substantially exceed the temperature of the ambient gas Tn. This phenomenon is usually called “anomalous heating” of dust grains, while their stochastic kinetic energy is called the “kinetic temperature” [1, 2]. In the absence of external energy sources, the kinetic temperature of dust grains corresponds to their surface temperature and is equal to Tn. Dust grains in gas-discharge plasma can acquire a stochastic kinetic energy of ~1–5 eV. Anomalous dust heating is usually attributed to temporal and spatial variations in the grain charges [14–19]. However, in some cases, the existing theoretical models fail to explain high kinetic energies acquired by dust grains under typical experimental conditions [12, 13, 20–23].
It should be remembered that the dust grain charge is not fixed but is determined by the local parameters of plasma near the grain [1, 2]. There are two main mechanisms leading to random variations in the grain charge and, as a consequence, to the development of irregular stochastic oscillations of dust grains. The first mechanism is related to the random nature of the ion and electron currents onto the grain surface [14, 15], while the second is determined by stochastic dust motion in a bounded plasma volume [24–26]. Since the dust grain char
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