DIAGNOSTICS OF DUST PARTICLES ACCELERATION IN DUSTY

The dust grain dynamics in plasma of a capacitively coupled radio frequency discharge and in the post-discharge have been studied using the laser sheet imaging method. The decay of dust structures is investigated in the plasma afterglow and the numerical simulations are performed taking into account balance of forces acting on dust particles for estimation of their size and residual charge after the extinction of plasma. INTRODUCTION Investigations of dust-particle dynamics in dusty plasma is of great importance for solving fundamental problems in the physics of nonideal plasma and solid state [1], as well as for optimization of many industrial technologies related to surface modification, cleaning of plasmachemical devices, separation of macroparticles by size, synthesis of nanoparticles, and creation on nanostructured coatings [2–4]. Information on the dynamics of macroparticles in gas-discharge plasmas makes it possible to develop new methods of plasma diagnostics using dust grains as microprobes [5–7]. Recent studies have shown that dust grains can preserve their residual charge for several seconds after the discharge is switched off [8–12]. This phenomenon was observed both under microgravity [8] and laboratory conditions [9–11] and opens up new possibilities for dusty plasma applications. Negatively charged dust grains in the afterglow of an RF discharge were observed in the experiments performed aboard the International Space Station [8]. In [9], diffusion of dust grains with sizes of 8–50 nm in the afterglow of an RF discharge was studied and the fraction of charged grains after switching-off of the RF power was determined. In [10], the residual charge of submicrometer-sized polymer dust grains in an RF discharge in argon was measured. The grain charge was determined by analyzing the dynamics of grain motion in a low-frequency electric field after the RF discharge was switched off. In this case, the gravity force was balanced by the thermophoretic force. It was found that, after the RF power was switched off, the grains conserved their charge for several tens of seconds, the average charge varied from -3e to -5e, depending on the gas pressure in the discharge chamber. In [11], where the residual grain charge in the afterglow of an RF discharge was studied in more detail, it was found that, along with negatively charged grains, there were also positively charged grains and a model was proposed that explained the emergence of positively charged grains in the post-discharge phase. Here, we present results from experimental studies of the dust grain dynamics in the afterglow of an RF discharge in air. In contrast to [10], polydisperse dust grains were injected into the discharge plasma from the outside. The experiments were performed in a discharge chamber with cooled electrodes. This made it possible to eliminate the influence of the thermophoretic force on dust grains. The dust grain dynamics after the RF power was switched off was controlled by applying an external DC electric field. THE EXPERIMENTAL SETUP AND RESULTS Figure 1 shows a schematic diagram of the setup for investigation of dusty plasma in an RF capacitively coupled discharge. The discharge was excited at a frequency of 5.28 MHz between two 120-mm-diameter cooled copper electrodes separated by a distance of L = 21 mm. The plasma-forming gas was atmospheric air at a pressure of 100 Pa. The specific input power deposited in the discharge was W = 0.5 W/cm. Under the experimental conditions, the 15 International Symposium on Flow Visualization June 25-28, 2012, Minsk, Belarus ISFV15 – Minsk / Belarus – 2012 electron density and the electron and ion temperatures were ne ∼10 cm, Te ∼2 eV, and Ti ∼0.03 eV, respectively. Polydisperse Al2O3 grains with effective dimensions of rp ~ 0.1–20 μm were used as dust particles.