Structural phase transitions in multipole traps

TL;DR

This paper investigates structural phase transitions in laser-cooled ions within multipole traps, revealing how ion configurations influence thermal transfer and confinement properties through molecular dynamics simulations.

Contribution

It introduces a detailed analysis of ion structural transitions in multipole traps and proposes an analytic model for ion bunching behavior.

Findings

Single-ring configuration inhibits thermal transfer, enabling Doppler-limited cooling.

Transition from double to single ring observed and characterized.

Analytic expression developed for ion bunching phenomena.

Abstract

A small number of laser-cooled ions trapped in a linear radiofrequency multipole trap forms a hollow tube structure. We have studied, by means of molecular dynamics simulations, the structural transition from a double ring to a single ring of ions. We show that the single-ring configuration has the advantage to inhibit the thermal transfer from the rf-excited radial components of the motion to the axial component, allowing to reach the Doppler limit temperature along the direction of the trap axis. Once cooled in this particular configuration, the ions experience an angular dependency of the confinement if the local adiabaticity parameter exceeds the empirical limit. Bunching of the ion structures can then be observed and an analytic expression is proposed to take into account for this behaviour.