Axial and radial space-charge effects in radiofrequency gas-filled low-pressure quadrupole ion guides

This paper presents theoretical and experimental study results of the space-charge (SC) effects in gas-filled radiofrequency ion guides, typically operating at -1 Pa gas pressures. A self-consistent Coulomb interaction simulation allows modeling the ion self-propulsion by the own SC, optionally supplemented by an external axial field. The simulations are accelerated with an improved collision model, combining the hard-sphere and polarization interaction models. Simulated ion beam parameters are consistent with the experimental measurements performed with an orthogonal accelerating time-of-flight mass spectrometer. The experimental and theoretical studies consistently demonstrate notable SC effects in ion guides, even at moderate ion currents in the sub-nA range. The effects become softer when using a weak -0.02 V/mm axial field. The efficiency of the ion guides improves with their miniaturization. The simulations and experiments demonstrate stronger beam compression for multiple-charged ions, even in the presence of SC effects, thus providing shorter turnaround times in orthogonal accelerators. The SC effects depend on the ion beam composition in the ion guides; the presence of light or multicharged ions strongly affects the ion motion of heavier species with a lower charge.