Small three-dimensional strongly coupled charged particles in a spherical confinement potential arrange themselves in a nested shell structure. By means of experiments, computer simulations, and theoretical analysis, the sensitivity of their structural properties to the type of interparticle forces is explored. While the normalized shell radii are found to be independent of shielding, the shell occupation numbers are sensitive to screening and are quantitatively explained by an isotropic Yukawa model.

The ground state of an externally confined one-component Yukawa plasma is derived analytically. In particular, the radial density profile is computed. The results agree very well with computer simulations of three-dimensional spherical Coulomb crystals. We conclude in presenting an exact equation for the density distribution for a confinement potential of arbitrary geometry.

Strong correlation effects in classical and quantum plasmas are discussed. In particular, Coulomb ͑Wigner͒ crystallization phenomena are reviewed focusing on one-component non-neutral plasmas in traps and on macroscopic two-component neutral plasmas. The conditions for crystal formation in terms of critical values of the coupling parameters and the distance fluctuations and the phase diagram of Coulomb crystals are discussed.

Small three-dimensional charged-dust clusters, so-called Yukawa balls, are analyzed with regard to their construction principle. For that purpose, in an experimental approach, different (metastable) configurations of clusters with fixed particle number (N<100) have been generated under identical plasma and trapping conditions. Metastable states are frequently observed. In combination with molecular dynamics simulations, it is shown that particle interaction with screening strongly affects the appearance probabilities of metastable configurations. Small clusters show different average density distributions with screened interaction compared to pure Coulomb, although having the same ground state configurations.

In small confined systems predictions for the melting point strongly depend on the choice of quantity and on the way it is computed, even yielding divergent and ambiguous results. We present a very simple quantity that allows us to control these problems-the variance of the block averaged interparticle distance fluctuations.