Simulations of shaping and reducing effects of image charge attraction on ion transmission through untilted mica capillaries

We simulated transmission of 7-keV Ne⁷⁺ ions through untilted mica capillaries to study the influence of the image charge attraction. In simulations, the repulsive interaction of incident ions with the charges accumulated on the capillary surface was also considered. The capillaries used have various regular polygonal cross sections. Our results clearly show that the image attraction greatly reduces the ion transmission. Moreover, the shaping effect, already reported previously, is also reproduced. For a regular polygonal shaped capillary the transmitted angular distribution remains regular polygonal, but its orientation with respect to the capillary differs by some angle. An analytic model calculation was conducted to describe the reduction of the image attraction on the transmitted ion fraction through the untilted capillary, agreeing well with the simulated results. The reducing effect of the image attraction on ion transmission was found to be strong for the combination of low-energy ions and capillaries with large aspect ratios (length/diameter).