Structure of Debye clusters in a biharmonic potential well

We experimentally investigate the arrangement of nearly identical charged dust particles confined in a two-dimensional biharmonic potential well. As the well shape changes from elliptical (anisotropic) to circular (isotropic), we find that clusters with $n=6$ and 8 particles change from an elliptical configuration where all particles lie on the edge to a configuration with one central particle surrounded by the remaining $n-1$ particles. The mechanism for this transition is identified as a transverse instability in the finite two-chain configuration.