"How Spherical Plasma Crystals Form"


Three-dimensional plasma crystals consisting of charged particles arranged on concentric spherical shells have been observed in a variety of systems, including laser-cooled neutral plasmas, ions in traps or dusty plasmas. Whereas in the former case it was observed that shell formation starts in the center of the plasma cloud, in the latter two systems shells always emerge at the outer plasma boundary. The present paper explains the origin of these differences. It presents a full time-dependent analysis of the crystallization process for the example of a confined dusty plasma and studies the influence of the particle interaction (Coulomb or Yukawa) and of the trapping potential. The authors demonstrate that 1) in a harmonic confinement shell formation always begins at the boundary and 2) screening of the Coulomb interaction leads to a more rapid formation of inner shells. The shell formation dynamics is shown in the figure where the interaction changes from Coulomb (e.g. trapped ions, top figure) to weak and strong screening (e.g. dusty plasmas, middle and lower part of the figure). Finally the crystallization direction was found to be governed by the shape of the confinement potential. It is shown how this knowledge can be used to control the crystallization direction in a dusty plasma.