# Crystallization of ion clouds in octupole traps: structural transitions, core melting, and scaling laws

## TL;DR

This paper investigates the structural and melting properties of ion clouds in octupole traps, revealing size-dependent shell structures and a linear scaling law for melting temperature depression.

## Contribution

It introduces a combined semi-analytical and numerical approach to study finite size effects and melting behavior in ion clouds within octupole traps.

## Key findings

- Small ion clouds form hollow shells resembling Thomson problem solutions.
- Large ion clouds lose shell structure and become amorphous in the core.
- Melting temperature depression scales linearly with inverse radius.

## Abstract

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.