Finite element modeling of Rb-129Xe spin-exchange optical pumping and optimized Rb source distribution

Rubidium (Rb) vapor density ($\left[\text{Rb}\right]$) is a key parameter in xenon-129 polarization ($P_{\text{Xe}}$) build up in spin-exchange optical pumping. In practice, $\left[\text{Rb}\right]$ within the cell often falls below saturation levels and is spatially heterogeneous leading to system underperformance. In this study, finite element modeling was performed to investigate the role of Rb source distribution in heterogeneous in-cell $\left[\text{Rb}\right]$, and to optimize a Rb presaturator to achieve homogeneous $\left[\text{Rb}\right]$ and reduce the flow rate dependence of $\left[\text{Rb}\right]$. Lower than expected $P_{\text{Xe}}$ in previous iterations of our polarizer can be attributed to sub-saturation $\left[\text{Rb}\right]$ due to the small surface area of the Rb source in the main cell body and the absence of upstream Rb vapor presaturation, leading to lower than desired $P_{\text{Xe}}$. We found that increasing the surface area of the Rb source in the main cell body does not effectively reduce $\left[\text{Rb}\right]$ heterogeneity. Instead, achieving a more uniform distribution of $\left[\text{Rb}\right]$ necessitates the use of a sufficiently long presaturator at a given gas flow rate, increasing $P_{\text{Xe}}$. We also report discrepancy between modeled and experimentally measured laser absorption, highlighting limitations of the existing optical pumping model and suggesting directions for future model revisions and the investigation of currently unexplored areas.