Evolution of the electric potential of an insulator under charged particle impact

Insulating glass capillaries have been shown to lead to ion transmission without any change in either the ion charge state or in the ion kinetic energy. This surprising process has been attributed to a self-organized distribution of charge patches creating the necessary guiding electric potential on the capillary walls. By the use of our original electrometer, it has been possible to measure and monitor simultaneously and in a nondestructive way the electric potential and the transmitted beam intensity during the charging up by an ${\mathrm{Ar}}^{+}$ ion beam. We show that glass microcapillaries can reach potentials higher than 500 V, even in the case of singly charged ions, opening the possibility of high transmission rates and providing a renewed sight into ion beam transport by tapered capillaries. The setup, also suitable for the determination of leakage currents governing the capillary potential dynamics, allowed one to evidence that secondary electrons may strongly affect the rise of the capillary potential and consequently avoid Coulomb blocking of the beam transmission across insulating capillaries.

Figure 1

Electrode setup of conical-shaped borosilicate capillaries with deflection plates. The 3-mm insulator gap of $\sim {10}^{15}$ $\mathrm{\Omega }$ separates the grounded entrance from the floating electrodes.

Figure 2

Equivalent electrical circuit of the electrode setup. The resistance $R$ stands for the resistance separating the floating electrodes of capacity $C$ from the ground. The capacity $C^{\prime }$ and impedance $R^{\prime }$ of the glass-metal interface at the outer capillary surface have been added for completeness, but are irrelevant for this study. They merely control the drop of the potential at the interface.

Figure 3

Monitoring of the potential of the capillary during the charging with a constant beam intensity as indicated in the legend. The characteristic time constants have been extracted from the fit of the experimental data with Eq. (3).

Figure 4

Asymptotic potential reached during the charge-up (black points) as a function of the injected beam intensity. The low intensity part of the curve is well fitted by expression (5). Above the threshold $U_c=245$ V, the curve increases linearly as predicted by Eq. (7). The uncertainties on the potentials are those given by the fit in Fig. 3. An absolute uncertainty of 0.1 pA is estimated on the currents.