Ion Crystal Metamorphoses in a Paul trap

The standard second-order pseudo-oscillator potential used in many analytical investigations of the properties of ions stored in a Paul trap has serious limitations. In this paper we show that ion-crystal configurations exhibited by 2, 3, and 4 simultaneously stored ions in a Paul trap are not predicted by the standard pseudo-oscillator potential, but are all captured qualitatively and quantitatively by an extended pseudopotential derived in this paper. The power of our extended pseudopotential extends in particular to the prediction of the border lines between different crystal configurations (morphologies) in the Paul trap's $a$, $q$ stability diagram. In the three- and four-ion cases, several of the ion-crystal structures predicted by our improved pseudopotential have never been observed experimentally before. We present them here as a challenge for experiments.