## 7 Conclusions  
In this paper, challenges in designing and using the collimator of a Faraday cup have been investigated in the case of Hall thruster plume through both experiments and computer simulations. Results allow us to propose general guidelines to optimize the architecture of a Faraday cup collimator. First, the front of the probe should be made with material that can sustain local heating and that exhibits a low sputtering yield (Sections 5.2 and 6.1). Secondly, the distance between the point of collimation and the collector should be greater than the plasma sheath (Section 6.3). Moreover, in the case where the far-field plasma region of the ISCT200 is studied with FC architecture presented in this paper, the hypothesis of a point source is valid if the probe aperture diameter is \(\geq\)5 mm (Sections 5.3 and 6.2). Further studies where the probe to thruster distance varies and the HT mean diameter (D\({}_{HT}\)) changes could bring valuable information to find a limit and relation between HT dimension, Faraday cup aperture diameter (d\({}_{a}\)) and probe-thruster distance (I). In our case, we recommend for I \(\sim\) 6 D\({}_{HT}\) to use a FC with D\({}_{HT}\) \(<\) 10d\({}_{a}\). Additionally, the collector must be biased negatively, the exact value must be determined with I-V characterization since it depends on the thruster plume properties, to attract and capture primary ions of the thruster plume (Section 5.1). Finally, the use of an additional electrode, placed between the point of collimation and the collector, to enhance SEE recollection is not necessary (Sections 5.4 and 6.3). Eventually, to reduce measurement perturbations due to plasma-probe interaction (CEX, SEE), particular attention must be paid to the optimization of the geometry and material properties of the cup and collector. Based on results obtained in this study, we can propose an optimal Faraday cup front architecture to study the plume of a low-power Hall thruster at a distance greater than four times the HT channel outer dimension. The configuration X.X.P provides more reliable results and shield better the collector against perturbation induced by ambient plasma in the cup vicinity. Therefore, the FC should be equipped with a collimator used as electron screen and should use the front of its housing to collimate the ion flux through the collector cup. Additionally, this study did not assess the collector materials and dimensions. Nevertheless, based on our experiments a long cup with a metallic foam placed at its rear side seems to be adequate.  
Investigation, H.V.; Supervision, M.S.; Validation, M.S.; Writing--original draft, H.V. and M.S.; Writing--review and editing, H.V. All authors have read and agreed to the published version of the manuscript.  
Figure 26: Ion simulations computed with the software SIMION. Each configuration displays the possible trajectory of ion rebounds from the bottom of the FC. The collector is at \(-\)50 V. The collimator is either grounded (top) or biased to \(-\)100 V (bottom). Ion energies are 10 eV (black), 50 eV (red), 75 eV (green) and 100 eV (blue).  
**Funding:** V.H. benefits from an ENPULISION grant. Part of this research is financially supported by the ESA, ESTEC under grant PO-5401003120, by the Region Centre-Val de Loire through the PEPSO-2 program and by the European Union's Horizon 2020 research and innovation program under grant agreement 730135 CHEOPS.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Plume data obtained in the frame of this study are available upon request. Contact the corresponding author. Data about the ISCT200 Hall thruster design are confidential.  
**Acknowledgments:** The authors would like to thank [PERSON] for his support on theoretical subject regarding plasma diagnostics design and behavior. Additionally, we would like to thank [PERSON] for his help and expertise on Hall Thrusters.  
**Conflicts of Interest:** The authors declare no conflict of interest.