Session VR4: Dusty Plasma

Live

Atmospheric pressure low-temperature plasmas (AP-LTPs) are often utilized for aerosol synthesis, treatment, and removal systems. In these systems, dust particles become highly negatively charged in the plasma. However, little is known about how process parameters affect dust particle charge states after the spatial plasma afterglow. In this work, monodisperse aerosol streams were passed through a radiofrequency AP-LTP and dust particle electrical mobility distributions were measured using a scanning mobility particle sizer. Dust particle size, dust particle material, and flow velocity were varied. Increasing flow velocity made dust particles more positively charged after they had passed through the spatial afterglow, which is the opposite of what one might expect given that particles are highly negatively charged in the plasma. To describe this phenomenon in more detail, a model was constructed. After the diffusive loss mechanism of electrons and positive ions switches from ambipolar to free diffusion in the spatial afterglow, the ion density briefly becomes much higher than the electron density. Since the charging timescale for a particle is small, particles neutralize and can even become net positively charged in this ion-rich zone.   

Live

Dusty plasmas have attracted a great deal of attention due to its applications in industry, technology and other areas of modern science. A different kinds of wave modes formed in dusty plasmas have been studied to understand the underlying physics in laboratory, space and astrophysical environments. Among nonlinear structures, rogue waves are found to occur in different environments. The effect of polarization force and other plasma parameters have been studied on rogue waves in dusty plasmas comprising of hot electrons and non-Maxwellian ions. Multiple scale perturbation technique is employed to derive non-linear Schrodinger equation. Its rational solutions are determined. The critical wave number threshold where the modulational instability sets in is determined. The variation of critical wave number is analysed under the influence of various physical parameters. The combined effects of plasma parameters significantly influence the amplitude and formation of rogue waves. We have also studied the super rogue waves and rogue wave triplets This study may be helpful in understanding the formation of nonlinear structures in space/astrophysical environments.   

Live

We propose a new method of estimating the mean diameter and dispersion of clusters of particles, formed in a cooling gas, after the nucleation stage. The gas could be for example created by plasma arc or dielectric barrier discharge. The cluster formation is described by Friedlander's model [S.K. Friedlander, Ann. N.Y. Acad. Sci. 354 (1983)]. In the case of an uniform growth of supercritical particles, Friedlander's model can be simplified and if the cooling rate is typically smaller than $10^6 K/s$, primary constituents consumption by nucleation can be neglected with respect to their deposition on clusters. The nucleation rate can then be approximated by an exponential function. Using this approximation we derive analytical formulas for both mean diameter and its dispersion after the nucleation stage, as a function of cooling time and collision time between gas particles. These formulas can be used to predict diameter and dispersion variation with the initial particle concentration and cooling rate. It is also possible to use them as an input to the coagulation stage, without the need to compute complex cluster generation during the nucleation burst. We compared our results with a nodal code (NGDE) and got excellent agreement.   

Live

Low pressure plasmas are appealing for growing high quality nanoparticles (NPs) with customizable properties including size and chemical composition. Due to the highly non-equilibrium plasma environment, NPs may be produced with compositions difficult to achieve through traditional methods, for example core-shell and hyper-doped. The flux of reactive species to the NPs can be tuned based on the plasma operating conditions. However, the reactivity of species which grow NPs can be functions of the surface composition which in turn depends on the plasma conditions. In this work, results of a computational investigation of growing silicon containing NPs in laboratory plasmas sustained in Ar/SiH$_{\mathrm{4}}$ at pressures of a few Torr will be discussed. A 3D kinetic model for dust particle trajectories and growth, the Dust Transport Simulator (DTS), was coupled with a multi-fluid plasma simulator, the Hybrid Plasma Equipment Model (HPEM), for this work. Kinetic algorithms have been added to the DTS to track the composition of NPs and account for changing effects of surface reactivity on growth rates. Reactive sticking coefficients for small silane radicals (e.g., SiH$_{\mathrm{x}})$ with larger NPs were obtained from Molecular Dynamics simulations. Results for growth rate and composition of Si NPs as a function of process parameters -- power, flow rate, gas mixture -- will be discussed, as well as the potential for growing more complex NPs.   

Live

The charging and forces on dust particles near the edge of the plasma sheath are of interest for applications in the mitigation of contamination issues in semiconductor processing. In this work, the ion collection rate, force, and energy on negatively charged particles in an intermediate collisional regime are calculated using ion trajectory models accounting for a linear external field in the plasma sheath, ion inertia, and finite ion mobility. We show that in this system, dimensionless collision rates, forces, and incoming energies are dependent on the dimensionless external field strength (normalized by particle charge induced field) and an ion Stokes number, defining the ratio of ion inertia to gas resistance to motion. Results show that ion Stokes number has a drastic effect on collision rates, with a reduced effect on collection force and energy transfer. Interestingly at Stokes numbers above unity, ions adopt orbiting trajectories around particles with finite numbers of rotation before colliding or leaving the domain. Such orbits are observed over narrow impact parameter ranges. While individual ions can contribute negative momentum transfer to particles, in all cases, we find that the collection force is positive in the direction of initial ion flow.   

On Demand

High-precision nanofabrication based on plasma processing has been one of the main technology drivers of modern information society[1]. Development of highly sensitive diagnostic methods in process plasmas is imperative for understanding and controlling interactions between the materials and plasma. A diagnostic method using few dust particles in plasma is a possible solution of this problem. We succeed in measuring profile of same electric field intensity in Ar plasma using laser tweezers. Ar plasmas were generated between a powered ring-electrode by applying rf voltage. PMMA particles of 10 µm in diameter were injected into the plasmas. Some particles were suspended at plasma/sheath boundary by the balance among gravity, ion drag, and electrostatic forces. To trap the particle, a laser was irradiated to it. Because the trapped particle was negatively charged, it can be a high sensitive probe of force of qE. When we move the particle horizontally by laser, the height position of the particle changes due to force balance. Assuming the charge amounts is constant, we deduce information of a profile of same electric field intensity with high resolution of a few µm. Moreover, we compare these experimental results with simulation ones. We will discuss details at the conference. [1] M. Soejima et al., Proc. IEEE-Nano (2016) 671.