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

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.