Universal non-monotonic structure in the saturation curves of MOT-loaded Na$^+$ ions stored in an ion-neutral hybrid trap: Prediction and observation

TL;DR

This paper predicts and observes a universal non-monotonic behavior in the steady-state ion number in rf traps as a function of loading rate, revealing four dynamical regions with specific scaling laws confirmed experimentally.

Contribution

The study introduces a universal prediction of non-monotonic saturation curves in rf traps, supported by analytical, numerical, and experimental evidence for MOT-loaded Na$^+$ ions.

Findings

N_s(λ) exhibits four dynamical regions with distinct behaviors.

Maximum and minimum in N_s(λ) occur at intermediate loading rates.

N_s(λ) scales as λ^{2/3} at high loading rates.

Abstract

We predict that the steady-state ion number $N_s$ for radio-frequency (rf) traps, loaded at a rate of $\lambda$ particles per unit time, shows universal non-monotonic behavior as a function of loading rate $\lambda$. The shape of $N_s(\lambda)$, characterized by four dynamical regions, is universal in the sense that it is predicted to manifest itself in all rf traps independently of the details of their construction. For $\lambda\ll$ 1 particles / rf cycle (Region I), as expected, $N_s(\lambda)$ increases monotonically with $\lambda$. However, contrary to intuition, at intermediate $\lambda \sim 1$ particles / rf cycle (Region II), $N_s(\lambda)$ reaches a maximum, followed by a minimum of $N_s(\lambda)$ (Region III). For $\lambda\gg 1$ particles / rf cycle (Region IV), $N_s(\lambda)$ again rises monotonically. In Region IV numerical simulations, analytical calculations, and experiments show $N_s(\lambda)\sim \lambda^{2/3}$. We confirm this prediction experimentally with MOT-loaded Na$^+$ ions stored in a hybrid ion-neutral trap.