Simulation of grid morphology’s effect on ion optics and the local electric field

Abstract

Electric field ion optics are employed by many scientific instruments for investigating ions, e.g., using time-of-flight mass spectrometers. A common design feature of such instruments is the grounded grid that provides boundaries between regions that need to have different electric fields. In order to save computer memory, these grids are often modeled by indefinitely thin conducting sheets. This approximation does not include the effects of the grid morphology on the electric field. This paper investigates these grid morphology effects on both the electric field and the trajectories of ions passing through the grids using finite element analysis. The simulations in this paper indicate that a significant amount of the electric potential will leak through a grid’s empty space. The leakage of this field through the grid slows an ion down relative to the speed that it would be assumed to have based on the indefinitely thin sheet model. The ions are then eventually accelerated back to the energy that they would have if the grid were a thin sheet. However, this deceleration and acceleration result in the lengthening of the ion time of flight, independent of the size of the drift region. The deflection of the ions passing through the grid increases with the ion’s proximity to the grid struts, the size of the acceleration region, and the shape of the grid cell. This deflection also results in a small but potentially significant loss of focus and changes in the path length.