Inter-ion coulomb interactions in a magneto-optical trap ion source

Abstract

We have investigated the role played by inter-ion Coulomb interactions in a magneto-optical trap ion source (MOTIS). Using a Monte Carlo simulation accounting for all pair-wise ion-ion Coulomb interactions in the source, we have calculated the broadening of the transverse spatial and velocity distributions as well as the increase in emittance over a range of beam currents and extraction electric fields. Using a 7Li MOTIS, we have experimentally studied the broadening of the spatial distribution as a function of total beam current and extraction electric field by measuring the fraction of the beam current that passes through a 20 μm diameter aperture. The Monte Carlo simulations agree well with the experimental results, indicating that such simulations capture the essential physics of the source. Our results show that while Coulomb interactions can cause a significant increase in emittance in some situations, it is possible to keep the effects to an acceptable level by suitable choice of extraction field and beam current. These considerations are essential to the process of optimizing MOTIS geometry and operating conditions for use in high-resolution focused ion beam applications.