An analytical approach to symmetry breaking in multipole RF-traps

TL;DR

This paper develops an analytical model for RF potentials in multipole traps with electrode misalignments, enabling improved compensation of defects and enhanced trap stability through a novel voltage tuning approach.

Contribution

It introduces a comprehensive analytical equation for asymmetric octupole RF-traps, facilitating defect characterization and correction without detailed electrode position data.

Findings

Analytical model accurately predicts potential with up to 4% defect size.

Proposed voltage tuning method effectively compensates for geometrical imperfections.

Model validated by comparison with numerical simulations.

Abstract

Radio-frequency linear multipole traps have been shown to be very sensitive to mis-positioning of their electrodes, which results in a symmetry breaking and leads to extra local minima in the trapping potential \cite{pedregosa17} disturbing the operation of the trap. In this work, we analytically describe the RF-potential of a realistic octupole trap by including lower order terms to the well-established equation for a perfectly symmetric octupole trap. We describe the geometry by a combination of identified defects, characterised by simple analytical expressions. A complete equation is proposed for a trap with any electrode deviation relying on a combination of the simple cases where the defects are taken individually. Our approach is validated by comparison between analytical and numerical results for defect sizes up to 4\% of the trap radius. As described in \cite{pedregosa18}, an independent fine-tuning of the amplitude of the RF voltage applied on each electrode can be used to mitigate the geometrical defects of a realistic trap. In a different way than in \cite{pedregosa18}, the knowledge of an analytical equation for the potential allows to design the set of RF-voltages required for this compensation, based on the experimental measurement of the ion position in the trap, without information concerning the exact position of each electrode, and with a small number of iterations. The requirements, performances and limitations of this protocol are discussed via comparison of numerical simulations and analytical results.