Guiding of slow ${\text{Ne}}^{7+}$ ions through nanocapillaries in insulating polyethylene terephthalate: Incident current dependence

The transmission of highly charged ions through nanocapillaries in insulating polyethylene terephthalate (PET) polymers was investigated. In experiments at laboratories in RIKEN (Japan) and HMI (Germany) different detection methods were applied to study the ion current dependence in a wide range covering two orders of magnitude. At HMI an electrostatic ion spectrometer was used and at RIKEN a two-dimensional position sensitive detector was implemented. New PET samples with parallel capillaries and low density were manufactured. For tilted capillaries, the ions are guided along the capillary axis, since the majority of ions are deflected in a charge patch created in the capillary entrance. The results provide insights into the mechanisms of capillary guiding. The fraction of transmitted ions was found to be nearly independent on the incident ion current indicating a sudden increase in the discharge current depleting the entrance charge patch. The experimental results were well-reproduced by model calculations based on a nonlinear (exponential) expression for the discharge current.

Figure 1

Capillary guiding of highly charged ions in a PET capillary. In the entrance region the main charge patch is created that deflects the ions to the capillary exit. The exit region is affected by a potential of depth $U_g$ in which the ions can gain perpendicular velocity. The deflection angle $\alpha$ is obtained from $\text{tan}\alpha =v_s∕v_p$ where $v_s$ and $v_p$ are the perpendicular and longitudinal velocities, respectively.

Figure 2

Scanning electron microscopy plot containing capillaries with a diameter of 200 nm and a density of $4\times {10}^6{\text{cm}}^{-2}$. The foil surface is evaporated with a gold layer of 20 nm thickness. The graph was obtained by means of scanning electron microscopy.

Figure 3

Time evolution of emission profiles of transmitted ${\text{Ne}}^{7+}$ ions for the tilt angles $\psi =0°$ (left column) and $\psi =5°$ (right column). The energy of the incident ions is 3 keV and the current 0.1 nA. The parameter $Q_d$ denotes the charge deposited at the front of the capillary foil. The solid lines are Gaussian fit curves.

Figure 4

Time evolution of emission profiles of transmitted ${\text{Ne}}^{7+}$ ions for the tilt angle of $\psi =2.8°$. The graphs in the left column show two-dimensional contour plots and in the graphs in the right column the corresponding cuts at $\theta =0$ are plotted. The energy of the incident ions is 3.5 keV and the current 50 pA. The parameter $Q_d$ denotes the charge deposited at the front of the capillary foil. The solid lines in the right-hand graphs are Gaussian fit curves.

Figure 5

Total yield $Y$ (left column) and transmitted ion fraction $f$ (right column) of 3.5 keV ${\text{Ne}}^{7+}$ ions for the tilt angle of $\psi =2.8°$. The ions are incident with a current of $I_p=5$ and 50 pA as indicated. The rise time ${\tau }_c$ and corresponding charge $Q_c={\tau }_c{I}_p$ are given in each graph.

Figure 6

Total yield $Y$ (left column) and transmitted ion fraction $f$ (right column) of 3 keV ${\text{Ne}}^{7+}$ ions for the tilt angle of $\psi =5°$. The ions are incident with a current of $I_p=0.1\text{nA}$ and 1 nA as indicated. The rise time ${\tau }_c$ and corresponding charge $Q_c={\tau }_c{I}_p$ are given in each graph.

Figure 7

Discharge current $J_d=c_d{J}_{in}$ as a function of the equilibrium charge $Q_{\infty }$ deposited in the capillary entrance region. The data points represent $Q_{\infty }^e$ estimated from experimental results. The RIKEN and HMI data are given as open squares and closed points, respectively. The solid curves follow from the quasiexponential function given by Eq. (7). The dashed curve represents results from a linear model (see text).

Figure 8

Ion transmission $f_{\infty }$ at equilibrium as a function of the incident ${\text{Ne}}^{7+}$ current. The open squares are data measured at RIKEN and the closed circles are results from HMI. The solid and dashed curves represent results from calculations using a nonlinear and linear model, respectively (see text). Note that the data from RIKEN are increased by a factor of 1.25 for graphical reasons.