Session QI2: Basic Plasma Physics II

Anomalous dust transport in strongly coupled 2D complex plasmas

Stochastic motion of dust grains in strongly coupled complex plasmas is affected not only by the neutral gas, but also by the interaction between the grains and between grains and the surrounding plasma. Under certain conditions these interactions create strongly ordered states with some disordered motion of the dust grains through the lattice. This motion can be tracked, and some of the statistical properties of the motion are known. However, the dynamical basis of these statistics is poorly understood. A straightforward technique is to produce the probability density functions (PDFs) of the particle displacements as a function of the time-lag. For relatively immobile states the PDF forms a stretched Gaussian whose variance expands faster with time than a normal diffusion process (superdiffusion) until the displacement approaches the mean interparticle distance $d$. There are good reasons to believe that this superdiffusion is associated with long-range memory effects in the particle dynamics. For greater times the PDF develops local humps at integer multiples of $d$, but the overall distribution is Gaussian and the diffusion is normal, i.e. memory is lost when particles escape from their cage in the lattice. In more mobile states superdiffusion on short time scales is still observed, but the humps due to caging are lost. On even longer time scales new humps develop on the PDFs due to trapping of particles in large vortices. At these longer time scales the transport is superdiffusive, and the memory effects are due to viscoelastic vortex flows covering a wide spectrum of temporal and spatial scales. \newline S. Ratynskaia, C. Knapek, K. Rypdal, S. Khrapak, G.E.Morfill, Phys. Plasmas 12, 022302 (2004) \newline S. Ratynskaia, K. Rypdal, C. Knapek, S. Khrapak, A. Milovanov, J. J. Rasmussen, G. E. Morfill, Phys. Rev. Let. 96 , 105010 (2006)   

Diffusion and super-diffusion in strongly-coupled dusty plasmas

Strongly-coupled plasma physics is easily studied using laboratory dusty plasmas. Micron-size polymer spheres are introduced into a low-temperature plasma, where they gain a large charge of typically $-8000 e$. They represent a third plasma species, in addition to electrons and ions. Ambipolar electric fields of a gas-discharge plasma confine the microspheres, and frictional drag on neutral gas cools them to about $300~{\rm{K}}$. Due to their inter-particle potential energy that is large compared to $k_BT$, the microspheres arrange in a crystalline lattice. Using laser-light scattering and video cameras, we can track the motion of all the microspheres. To heat the lattice and melt it, we use random kicks applied to the microspheres by moving laser beam. We characterize random motion as diffusive or super-diffusive by using three diagnostic methods: mean-squared displacement (MSD) vs. time, velocity autocorrelation function, and probability distribution function. Doing this, we find indications of superdiffusion, where random particle motion results in larger displacements than for normal (Fickian) diffusion. Molecular-dynamics simulations show similar results.   

Electrostatic Collective Modes in a Pair Fullerene-Ion Plasma

Pair plasmas consisting of electrons and positrons with an equal mass have experimentally been generated. However, the investigation of basic properties and collective modes in the positron-electron plasmas is very difficult because the annihilation time is short compared with the plasma period and the plasma density is low. Therefore our attention is concentrated on the stable generation of a pair-ion plasma consisting of positive and negative ions with an equal mass and the collective-mode investigation, where fullerenes are used as an ion source. When a hollow electron beam with 100 eV is injected into a fullerene vapor under a uniform magnetic field, positive ions are produced by electron impact ionization and electrons with low energy ($<$ 10 eV) are simultaneously produced. Negative ions are produced by electron attachment of these low-energy electrons. The electrons and the ions are radially separated by a magnetic-filtering effect. Only the positive and negative ions are expected to exist in the midmost of the hollow plasma, passing through an annular hole toward the downstream. The electron-free pair-ion plasma generation is attained here. Longitudinal-electrostatic modes propagating along the magnetic field lines are excited by a cylindrical/grid exciter, dispersion relations of which are investigated in detail. There appear three modes: an ion acoustic wave (IAW) and an ion plasma wave, both of which are predicted in the two-fluid theory, and an unprecedented intermediate-frequency wave which behaves like a backward wave. Furthermore, IAW is divided into two branches around the ion cyclotron frequency and another new backward wave appears, which joins the two branches together. The properties of the modes and the phase lag between the positive- and negative-ion density fluctuations will be discussed. The collaborator is W. Oohara.   

Ultrafast Dynamics of Strongly Coupled Plasmas

Structural and dynamical properties of complex systems, including molecules, liquids, solids, and plasmas, can be understood via the properties of the multidimensional potential energy function $U$. In general, local minima in $U$, and fluctuations between them, determine the properties of the systems. Recently, {\it rapid switching} of the potential energy function, and observations of the subsequent nonequilibrium evolution, has emerged as an interesting and important extension to the equilibrium case. Equally ubiquitous, this case occurs whenever matter absorbs radiation into the electronic component, thereby modifying the effective ion-ion interaction in the potential energy function $U$. A well known example is the nonthermal melting of solid surfaces in which laser excitation weakens the ionic bonds and leads to melting {\it without direct energy flow} into the ions via radiation or collisions. Our understanding of such nonequilibrium dynamics would benefit from dilute experiments, with their longer timescales, and the ability to rapidly switch from extremely strong to extremely weak interactions, or vice vers. Such experiments are, in fact, possible with a strongly coupled plasma, and experiments [Chen et al., {\it PRL} {\bf 93}, 265003 (2004)] have begun to be carried out. Such plasmas are created from a neutral gas, in which $U=0$, which is rapidly photoionized to a plasma with strong Coulomb interactions. Beyond confirmations of earlier predictions [Murillo, {\it PRL} {\bf 87}, 115003 (2001)], the experiments reveal a complicated relaxation processes in which oscillations in the kinetic energy appear. Here, I will discuss the physics of such systems in general with a focus on recent theoretical results that quantify the ultrafast dynamics [Murillo, {\it PRL}, {\bf 96}, 165001 (2006)] of plasmas experiencing an impulsive change in $U$. In particular, I will show that kinetic energy oscillations seen in the experiments are associated with formations of pair correlations in a strongly coupled plasma.   

Kinetic effects in Hall thruster discharge

The purpose of the talk is to describe recent advances in nonlocal electron kinetics in low-pressure plasmas. The talk will briefly review the invited papers of ``Nonlocal, Collisionless Electron Transport in Plasmas'' workshop, which are published in the special issue of IEEE Transactions on Plasma Science \textbf{34}, N3 (2006). As an example of importance of taking into account kinetic effects, the Hall thruster will be discussed. Recent analytical studies and particle-in-cell simulations suggested that the electron velocity distribution function in a Hall thruster plasma is non-Maxwellian and anisotropic. The electron average kinetic energy in the direction parallel to walls is several times larger than the electron average kinetic energy in direction normal to the walls. Electrons are stratified into several groups depending on their origin (e.g., plasma discharge or thruster channel walls) and confinement (e.g., lost on the walls or trapped in the plasma). Practical analytical formulas are derived for wall fluxes, secondary electron fluxes, plasma parameters and conductivity. The calculations based on analytical formulas agree well with the results of numerical simulations. The self-consistent analysis demonstrates that elastic electron scattering on collisions with atoms and ions plays a key role in formation of the electron velocity distribution function and plasma-wall interaction. The fluxes of electrons from the plasma bulk are shown to be proportional to the rate of scattering to loss cone, thus collision frequency determines the wall potential and secondary electron fluxes. Secondary electron emission from the walls is shown to enhance the electron conductivity across the magnetic field, while having almost no effect on insulating properties of the near-wall sheaths. Such a self-consistent decoupling between secondary electron emission effects on electron energy losses and electron crossed-field transport is currently not captured by the existing fluid and hybrid models of the Hall thrusters.   

Nonlocal effects in a bounded low-temperature plasma with fast electrons

Effects associated with nonlocality of the electron energy distribution function (EEDF) in a bounded, low-temperature plasma containing fast electrons, can lead to a significant increase in the near-wall potential drop, leading to self-trapping of electrons in the plasma volume, even if the density of this fast group is only a small fraction ($\sim $0.001{\%}) of the total electron density. If self-trapping occurs, the fast electrons can substantially, increase the rate of stepwise excitation, supply additional heating to slow electrons and reduce their rate of diffusion cooling. Altering the source terms of these fast electrons will, therefore, alter the near-wall sheath and, through modification of the EEDF, a number of plasma parameters. This could have technical applications. Self-trapping of fast electrons is especially important in an afterglow plasma, which is a key phase of any pulse-modulated discharge. In the afterglow, the electron temperature is less than a few tenths of an eV, and the fast electrons will have energies typically greater than an eV. It will be shown that in the afterglow plasma of noble gases, fast electrons, arising from Penning ionization of metastable atoms, can lead to the above condition and significantly change the plasma and sheath properties. Similar effects can be important in technologically relevant electronegative gas plasmas, where fast electrons can arise due to electron detachment in collisions of negative ions with atomic species. Following a brief overview of nonlocality, both experimental and modeling results will be presented to illustrate these effects.