Two-frequency operation of a Paul trap to optimise confinement of two species of ions

TL;DR

This paper introduces a two-frequency operation mode for Paul traps that enables simultaneous tight confinement of ions with vastly different charge-to-mass ratios, enhancing trapping efficiency for diverse ion species.

Contribution

The authors derive stability conditions for two-frequency Paul traps and demonstrate improved confinement of mixed ion species through theoretical analysis and experimental validation.

Findings

Two-frequency operation allows stable trapping of ions with large charge-to-mass differences.

The method reduces sheath formation around weakly confined species.

Experimental results confirm reduced parametric heating with dual-frequency trapping.

Abstract

We describe the operation of an electrodynamic ion trap in which the electric quadrupole field oscillates at two frequencies. This mode of operation allows simultaneous tight confinement of ions with extremely different charge-to-mass ratios, e.g., singly ionised atomic ions together with multiply charged nanoparticles. We derive the stability conditions for two-frequency operation from asymptotic properties of the solutions of the Mathieu equation and give a general treatment of the effect of damping on parametric resonances. Two-frequency operation is effective when the two species' mass ratios and charge ratios are sufficiently large, and further when the frequencies required to optimally trap each species are widely separated. This system resembles two coincident Paul traps, each operating close to a frequency optimized for one of the species, such that both species are tightly confined. This method of operation provides an advantage over single-frequency Paul traps, in which the more weakly confined species forms a sheath around a central core of tightly confined ions. We verify these ideas using numerical simulations and by measuring the parametric heating induced in experiments by the additional driving frequency.