Session ET1: Negative Ion Complex, and Dust Particle Containing Plasmas

Plasma separation: physical separation at the molecular level

Separation techniques are usually divided in two categories depending on the nature of the discriminating property: chemical or physical. Further to this difference, physical and chemical techniques differ in that chemical separation typically occurs at the molecular level, while physical separation techniques commonly operate at the macroscopic scale. Separation based on physical properties can in principle be realized at the molecular or even atomic scale by ionizing the mixture. This is in essence plasma based separation. Due to this fundamental difference, plasma based separation stands out from other separation techniques, and features unique properties. In particular, plasma separation allows separating different elements or chemical compounds based on physical properties. This could prove extremely valuable to separate macroscopically homogeneous mixtures made of substances of similar chemical formulation. Yet, the realization of plasma separation techniques’ full potential requires identifying and controlling basic mechanisms in complex plasmas which exhibit suitable separation properties. In this paper, we uncover the potential of plasma separation for various applications, and identify the key physics mechanisms upon which hinges the development of these techniques.   

Effect of the gas temperature and pressure on the nucleation time of particles in low pressure Ar-C$_2$H$_2$ rf plasmas

Particle formation in low pressure plasmas is a 3-step process. The first one corresponds to the nucleation and growth of nano-crystallites by ion-molecular reactions, the agglomeration phase to form large particles, and the growth by radical deposition on the particle surface. The nucleation phase was demonstrated to be sensitive to gas temperature and pressure. In this work, time of nucleation phase of particles formation in low pressure cold rf C$_2$H$_2$/Ar plasmas studied by varying gas temperature from 265 K to 375 K, gas pressure from 0.4 mbar to 0.8 mbar and rf power from 6 W to 20 W. The ratio of C$_2$H$_2$/Ar is fixed to 2/98 in terms of pressure. Several previous works reported that particle formation takes a few sec at room temperature in C2H2 plasmas and the time is much shorter than 0.1 s in SiH$_4$ plasmas. Time evolution of self-bias voltage was mainly used to determine nucleation time. The self-bias voltage was modified by phase transition between the steps from nucleation to coagulation. The experimental results showed that the nucleation time increased with gas temperature, decreased with gas pressure and discharge power. At constant gas pressure of 0.4 mbar and discharge power of 6 W, for example, the nucleation time increased from 5 sec to 30 sec with increas   

Dynamic of microparticles in vacuum breakdown.

Numerous applications, such as X-ray tubes or particle accelerators, use vacuum as of high voltage isolator. Their performance is limited by the risk of unpredictable breakdown events between electrodes. Moreover, the breakdown usually leads to the formation of arc discharges, which can damage the electrodes. This research aims to give a better description of the origin of the vacuum breakdown. One assumption considers the effect induced by the transport of micro-particles (MP) in the inter-electrodes gap. After their release from hot spots at the anode, MP are exposed to an intense field \textasciitilde 1-5 MV/m, and are bombarded by electrons released from the cathode micro-tips. These electrons can change the MP charge and can lead to partially/completely vaporization of the MP. The model OFEN (Orsay Field Emission Nanoparticles) developed at LPGP describes the MP transport in the inter-electrodes gap and the interactions (heating and modification of the MP charge) between electrons and the MP. The results clearly show four regimes of MP trajectories obtained for different emission currents, MP sizes and inter-electrode distances and the effect of the MP crash on the cathode.   

Charging of particles on a surface

This contribution focusses on the seemingly easy problem of the charging of micrometer sized particles on a substrate in a plasma. This seems trivial, because much is known about both the charging of surfaces near a plasma and of particles in the plasma bulk. The problem, however, becomes much more complicated when the particle is on the substrate surface. The charging currents to the particle are then highly altered by the substrate plasma sheath. Currently there is no consensus in literature about the resulting particle charge. We shall present both experimental measurements and numerical simulations of the charge on these particles. The experimental results are acquired by measuring the particle acceleration in an external electric field. For the simulations we have used our specially developed model. We shall compare these results to other estimates found in literature.   

Numerical Analysis on Size Dependency of Particulate Matter Behavior in Electric Collector

Recently the adverse effect of particulate matter having aerodynamic diameters (AD) of less than 2.5$\mu $m (PM2.5) on the human body has been attracted a lot of attention and it is expected to increase concerns about the effect of smaller particles, namely particulate matter having AD of less than 0.1$\mu $m (PM0.1). For air purifiers it is important to have the ability of efficiently collecting these particulate matters. In this work attention is paid to a plasma electric collector which utilizes Coulomb's force and is suitable to collect relatively small particles. Numerical simulations of plasma characteristics including two charging mechanism and tracking of charged particles are used to evaluate charging and collecting properties of the particles. Charge number attached on each particle decreased with decreasing AD and the number attached on each 0.1$\mu $m particle is one-by-hundredth to that of 2.5$\mu $m particle. However, the results of simulation on tracking of charged particles show that the collecting efficiency of 0.1$\mu $m particles is higher than that of 2.5$\mu $m particles. This is expected due to the fact that the dependencies of external forces (Coulomb's, gravitational and aerodynamic drag) on the AD differ from each other. Detail results will be shown during presentation.