Controlled experiments on self-organization of ordered structures in a pure electron plasma

We review our recent studies on structure formation in pure electron plasmas confined in magneto-electric traps. Two-dimensional dynamics of strongly magnetized plasmas is studied in terms of a two-dimensional vortex composed of high-density clumps and a low-density distribution in the background (BG). By controlling the initial distributions we examine in detail the fundamental processes where the clumps form highly ordered geometrical configurations (vortex crystals). Experimental studies of the formation of three-dimensional density structures have been initiated with a newly built strong-magnetic-field (2.2 T) trap operatable with various potential profiles including a square-well potential (Malmberg trap) and harmonic potential (Mohri trap). In the Mohri trap, Penning-type distributions are observed to form as a result of interaction of string-shaped distributions of electrons. The density distributions observed in the relaxation process are quite different from those observed in a square-well potential most probably due to three-dimensional deformations and merger of the string distributions. The equilibrium distributions generally consist of sets of spheroidal distributions, a core distribution plus halo distributions. Observationally the halo distributions appear to be associated with the amount of excessive angular momentum provided through the injection of off-axial electron strings. Like the BG vorticity distribution in the two-dimensional merger of clumps, the halo may act as the absorber of momentum in the course of the merging process of strings to create the core distribution. In the Penning-like distributions we have also observed the accumulation of negative ions replacing the electrons in time scales much longer than thermal relaxation.