  The stable structures and melting properties of ion clouds in isotropic
octupole traps are investigated using a combination of semi-analytical and
numerical models, with a particular emphasis at finite size scaling effects.
Small-size clouds are found to be hollow and arranged in shells corresponding
approximately to the solutions of the Thomson problem. The shell structure is
lost in clusters containing more than a few thousands of ions, the inner parts
of the cloud becoming soft and amorphous. While melting is triggered in the
core shells, the melting temperature unexpectedly follows the rule expected for
three-dimensional dense particles, with a depression scaling linearly with the
inverse radius.
