Combining a particle-particle, particle-cluster and cluster-cluster agglomeration model with an aggregate charging model, the coagulation and charging of dust particles in various plasma environments relevant for proto-planetary disks have been investigated. The results show that charged aggregates tend to grow by adding small particles and clusters to larger particles and clusters, leading to greater sizes and masses as compared to neutral aggregates, for the same number of monomers in the aggregate. In addition, aggregates coagulating in a Lorentzian plasma (containing a larger fraction of high-energy plasma particles) are more massive and larger than aggregates coagulating in a Maxwellian plasma, for the same plasma densities and characteristic temperature. Comparisons of the grain structure, utilizing the compactness factor, φ σ , demonstrate that a Lorentzian plasma environment results in fluffier aggregates, with small φ σ , which exhibit a narrow compactness factor distribution. Neutral aggregates are more compact, with larger φ σ , and exhibit a larger variation in fluffiness. Measurement of the compactness factor of large populations of aggregates is shown to provide information on the disk parameters that were present during aggregation.Subject headings: accretion disk -dust -planets and satellites: formation -plasmasprotoplanetary disks The formation of planetsAt the time of writing more than 500 exoplanets have been observed with more than 400 of these confirmed, and more planets are being detected and confirmed on a weekly basis 1 . Even though these discoveries show that the process of planet formation is in itself a general one, they have also shown that our Solar System is everything but the perfect example of the average planetary system, Pluto, or no Pluto. Partly due to the inherent bias of the available observational techniques, many of the earliest discovered systems involved large gaseous planets orbiting close to the parent star and planets on very eccentric orbits, much in contrast with our Solar System (Ollivier et al. 2009). Since many early planet formation theories were based on the Solar System (and in many cases these were then tested against our Solar System), these observations make clear that our knowledge of planet formation is incomplete.The environment in which planet formation takes place is generally accepted to be a proto-planetary disk (PPD), a disk of gas and small (nanometer to millimeter sized) dust particulates accreting matter onto the central young stellar object (YSO), a famous 1 1http://www.exoplanets.org, http://www.exoplanet.eu

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Experiments are performed in which dust particles are levitated at varying heights above the powered electrode in a RF plasma discharge by changing the discharge power. The trajectories of particles dropped from the top of the discharge chamber are used to reconstruct the vertical electric force acting on the particles. The resulting data, together with the results from a selfconsistent fluid model, are used to determine the lower levitation limit for dust particles in the discharge and the approximate height above the lower electrode where quasineutrality is attained, locating the sheath edge. These results are then compared with current sheath models. It is also shown that particles levitated within a few electron Debye lengths of the sheath edge are located outside the linearly increasing portion of the electric field.

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The charging of dust grains in astrophysical environments has been investigated with the assumption these grains are homogeneous spheres. However, there is evidence which suggests many grains in astrophysical environments are irregularly-shaped aggregates. Recent studies have shown that aggregates acquire higher charge-to-mass ratios due to their complex structures, which in turn may alter their subsequent dynamics and evolution. In this paper, the charging of aggregates is examined including secondary electron emission and photoemission in addition to primary plasma currents. The results show that the equilibrium charge on aggregates can differ markedly from spherical grains with the same mass, but that the charge can be estimated for a given environment based on structural characteristics of the grain. The "small particle effect" due to secondary electron emission is also important for determining the charge of micron-sized aggregates consisting of nano-sized particles.