The charging of an isolated dust grain in a collisionless plasma is well described by the orbital motion limited (OML) model. For laboratory plasmas, the dust particles are negatively charged. Once the charge density represented by dust particles becomes comparable to the ion density, the amount of free electrons is dramatically reduced. The increase of the total dust charge is described by the parameter P = Aa(μm)Nd/n∞ [1], with A =1μm(4πε0kBT_e/e~2) and a being the dust particle radius. The parameter P can also be written as P=4πλ~2_(De)an_d. In the limit P 1, any dust grain can be considered as an isolated particle (Fig. lá). The dust charge depletion effect can be understood by a model proposed in Ref. [2]. When P ≈ 1, the cloud potential Φ_C(II) deviates from the ambient plasma potential Φ_p(I) (Fig. lb). The dust becomes so densely packed that the potential in the neighborhood of the dust particles can no longer approach Φ_p. When dust density n_d increases even further, the potential barrier between the dust particles vanishes and 4_c(II) approaches the floating potential Φ~p_f(II) of the dust particles (Fig. lc) and the cloud can now be considered as a superparticle.
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