A vortex crystal is a quasi-stationary, symmetric array of intense vortices (clumps). The dynamics of mutually interacting point vortices has been the subject of theoretical and simulational studies of 2D turbulence. In a non-neutral plasma, the experiment by Fine [1] and a subsequent simulational study by Schecter et al.[2] were the first to reveal the important role of the interaction between the clumps and the low level vorticity filling the space around the clumps in the vortical relaxation processes toward the crystal structures.

The subject of this work is to experimentally examine the contribution of a low-level of background vorticity (BGV) to the relaxation of the clumps' dynamics toward the formation of a crystal structure [3]. To simplify the problem and quantify the degree of order, we focus on the dynamics of three clumps which are the minimum to form a unit cell in 2D crystal structures.

We examine the time-evolution of the triangle pattern of three clumps initially placed either in vacuum or in different levels of BGV distribution (BGVD). Quantitative analyses are made in terms of the degree of symmetrization. The settling time to the quasi-stationary state is shorter for the higher levels of BGVD. The settling process quantified by the symmetry parameter (defined as divided area of triangle by square of its periphery) is related to the reduction of random velocities of the clumps. We quantify the time evolution of vortex configuration as dispersion of the velocities of the clumps.

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