## 6 Discussions
### Collimator as Electron Screen  
When studying the ion beam of EP devices such as gridded ion engines [73] or FEPs [75] with a Faraday cup the collimator is used to redirect secondary electrons back to the collector while preventing thermal ions from entering the cup. Those EP devices operate with ion energies from several hundreds to thousands of electron-volts. For plasma diagnostic standardization we shall start to compare FC architectures and measurement methods. Hall thrusters deliver high current densities with ion energy relativity low compared to other EP technologies. Therefore, using electric field to enhance the ion collection efficiency of a FC while working with a HT can be problematic. Voltages applied to a Faraday cup are of the same order of magnitude than HT discharge voltages which can greatly disturb the thruster behavior and properties. We used SIMION to assess the plasma-probe interaction. SIMION is an ion optics simulation program that models ion optics problems with 2D symmetrical and/or 3D asymmetrical electrostatic and/or magnetic potential arrays. In our study, singly-charged Xe ions with different kinetic energy are flown through a 2-D FC. Additionally, we used the software to draw the field lines inside and outside the cup to evaluate possible perturbation with the ambient plasma near the FC entrance. Two FC configurations were used, and outcomes are shown in Figure 21. The Faraday cup located on the right corresponds to X.X.P while the left one refers to X.X.E. The top of the figure shows potential lines inside FC with the collimator (R) grounded and the collector (C) biased to \(-\)50 V. At the bottom \(-\)75 V is applied to the collimator (R) and the collector (C) remains at \(-\)50 V. Field lines from \(-\)5 V to \(-\)75 V are plotted. For a grounded collimator no major differences are observed between the two configurations. However, when \(-\)75 V is applied to the collimator the X.X.P design shields better field lines in the plasma in the probe vicinity. Figure 20: Evolution of the current utilization (**top**), propellant utilization (**middle**) and divergence angle (**bottom**) computed from current density distributions obtained with different inlet apertures. Hall thruster is operated at 200 V (blue), 250 V (red) and 0.66 A. The current density acquired by G.05.E and Al.05.P enable the computation of \(\eta_{b}\), \(\alpha\) and the divergence angle, see Figure 22. There both FC collectors are biased to \(-50\) V while the collimator (R) is grounded. Although there is no impact on the determination of the divergence angle, we observe that \(\eta_{b}\) and \(\alpha\) increases with the Al.05.P configuration. Both parameters are around 10% higher when the Faraday cup housing front is used to collimate the ion flux. With this design the collector is consequently further away from the point of collimation. Therefore, primary and secondary electrons from the probe entrance region are lost before reaching the collector. The latter is better shielded from external perturbations. Furthermore, SIMION was used to simulate the behavior of different monoenergetic ion beams through the cup, see Figures 23 and 24. The Faraday cup configuration studied is X.05.P. No material can be selected with the software. Two scenarios were considered. In the first case, the simulation displayed in Figure 23 includes 200 singly-charged xenon ions with energy of 5 eV (red), 10 eV (black), 100 eV (green) and 250 eV (blue). Ions have a velocity vector collinear to the Faraday cup axis. In the simulation a 10\({}^{\circ}\) divergence angle was applied to maximize the number of ions entering the cup. The collector, represented by the central cup, is biased to \(-50\) V. Twice the potential of the collector is applied to the collimator, which is only an electron screen now. Section 5.4 shows with such a FC  
Figure 21: Field lines computed with SIMION. Configurations X.X.E (left) and X.X.P (right) with the collimator grounded (top) and at \(-75\) V (bottom).