Different types of photometric observations in the 1930s (Trumpler 1930; Stebbins et al 1934; 1939) clearly showed that the dark "holes" in the Milky Way, observed by William Herschel almost 150 years earlier, were in fact re gions of heavy obscuration by cosmic dust. Continuing observations since then have established that dust is an almost ubiquitous component of the cosmic environment. Remote sensing of dust in the interstellar, circumstellar, inter planetary, circumplanetary, and cometary environments has, more recently been complemented by in-situ detections of the last three. Furthermore, the inference of the existence of very small grains (so-called VSGs with dimensions of 10-100 A) in the interstellar medium (Puget & Leger 1989) as well as their in-situ detection in the environment of comet PlHalley (Sagdeev et al 1989) reinforces the reasonable expectation that the transition from gas to large dust particles in the cosmic environment is a continuous one through macromolecules, clusters, and VSGs. These dust grains are invariably immersed in ambient plasma and radiative environments. They must therefore be necessarily electrically charged and consequently coupled to the plasma through electric and magnetic fields, with the coupling becoming stronger as the grain size decreases. While any plasma containing such charged dust grains is often loosely referred to as a dusty plasma, there are different regimes characterized by the relative magnitudes of three characteristic length scales, namely the dust grain size a, the plasma Debye length AD, and the average intergrain distance d(� n;; 1/3 , wherend is the dust number density). In general cosmic plasma en vironments that are contaminated by dust can be characterized by either of two conditions 1. a « AD < d or 2. a « d < AD. In the fi rst case the dust may

The transport of dust particles in tokamak fusion devices is studied using computer simulations with the dust transport code, DUSTT. Recent developments in modelling with the DUSTT code are reported. The improved model of dust dynamics in edge plasmas takes into account several additional effects, including thermionic and secondary electron emission which affects dust charging and heating, dust grain size effect on thermal radiation, and the presence of impurities in the plasma. It is shown that thermionic emission leads to enhanced dust heating by the plasma that boosts destruction of dust particles. The zone structure of tokamak plasmas is introduced for a qualitative analysis of dust survivability conditions. It is shown that a dust particle can experience net deposition in relatively cold carbon-contaminated plasma regions. Trajectories of sample dust particles in the DIII-D tokamak are simulated and analysed using the zone plasma description. Statistical averaging over an ensemble of particle trajectories is used to obtain spatial distributions of dust characteristics in the edge plasma of tokamaks. It is shown that transport of dust in tokamaks can significantly enhance penetration of carbon impurities towards the core plasma.

A combined theoretical and molecular dynamics (MD) simulation study of the collective modes and their dispersion in a two-dimensional Yukawa system in the strongly coupled liquid state is presented. The theoretical analysis relies upon the quasilocalized charge approximation; the MD simulation generates static pair correlation functions and dynamical current-current correlation spectra.

No abstract

By virtue of being generally immersed in a plasma environment, cosmic dust is necessarily electrically charged. The fact that secondary emission plays an important role in determining the equilibrium grain potential has long been recognized, but the fact that the grain size plays a crucial role in this equilibrium potential, when secondary emission is important, has not been widely appreciated. Using both conducting and insulating spherical grains of various sizes and also both Maxwellian and generalized Lorentzian plasmas (which are believed to represent certain space plasmas), we have made a detailed study of this problem. In general, we find that the secondary emission yield δ increases with decreasing size and becomes very large for grains whose dimensions are comparable to the primary electron penetration depth, such as in the case of the very small grains observed at comet Halley and inferred in the interstellar medium. Moreover, we observe that δ is larger for insulators and equilibrium potentials are generally more positive when the plasma has a broad non‐Maxwellian tail. Interestingly, we find that for thermal energies that are expected in several cosmic regions, grains of different sizes can have opposite charge, the smaller ones being positive while the larger ones are negative. This may have important consequences for grain accretion in polydisperse dusty space plasmas.