## 3 Faraday Cup  
### Architecture  
Measurements of the ion current density in the plasma plume or ion beam of electric propulsion devices are of great relevance as the flux of ejected ions determines the thruster properties such as the thrust level, the specific impulse, the propellant use and it plays a key role in overall thruster performance. Moreover, an accurate and comprehensive knowledge about ion current density is critical for the validation of plume modelling and numerical simulations [42; 43], for thruster acceptance tests, for the study of facility effects [44; 45] and for understanding the interactions between plasma plume and spacecraft elements [46; 47; 48; 49]. The ion current density can be measured using an electrostatic probe that basically consists of a conducting electrode, termed the collector, polarized to a high negative voltage with respect to the local floating potential to repel electrons and capture ions. There are various configurations of probes from a simple metal disk to architectures with collimators, filters and guard rings with various materials [16]. A FC is a special kind of electrostatic planar probe. It is basically an isolated conductive cup dedicated to the detection of charged particles in a low-pressure or vacuum environment [16]. When a Faraday cup operates as an ion collector, which means the cup is negatively biased with respect to the floating potential, the ion current in the probe direction can be accurately measured. Contrary to other electrostatic probes, edge effects due to plasma sheath formation are negligible with a FC owing to the closed geometry. In this study, FCs with collimator diameters of 10, 7, 5, 3 and 1 mm have been built. A large orifice collimator minimizes the impact of the neutral gas inside the cup. As a FC is collimated, the collection solid angle is below \(2\pi\), contrary to a planar probe, therefore a fraction of the thermal ion current is not captured [16; 17]. Nevertheless, a collimator has a key role to define the velocity vector of ions entering the device. Moreover, the collimator also limits the ion flux, which avoids saturation of the measurement signal when the FC is placed in the center of the ion beam, especially for high-power EP devices. A schematic view of the FC is shown in Figure 4. The FC assembly is composed of 5 different parts. The collimator (3) is used to screen the electrons and define the solid angle of collection, see Table 3. In this work the FC is bombarded by high energy xenon ions. Graphite and molybdenum have thus been employed. The orifice dimensions are assessed in Section 5.3. A trade-off must be considered between increasing velocity filtering and minimizing pressure inside the cup. Indeed, if the FC is not evacuated a large pressure inside the cup enhances charge-exchange collision events, which may disturb the ion flux. The collimator is electrically isolated from the FC body and cup enabling independent potential variation. Figure 4: Scheme showing the 5 main parts of a Faraday Cup. A metal foam ion collector (1) forms the back of the cup (\(1+2\)). The collector is mechanically attached to the cup without welding. The cup is a stainless steel hollow cylinder. The diameter of the collector is 12 mm. The aluminum foam was provided by Exxentis. The type used in this study is an open-cell AlSi7 Mg foam Nr. 4. Each pore has a diameter between 0.4 and 1 mm and are connected to each other via channels with diameters from 0.15 to 0.2 mm. The whole volume porosity is 60 \(\pm\) 5%. The complex structure of a foam should reduce secondary electron emission and prevent ion rebounds [50; 51; 52; 53; 54]. The active length of the stainless steel cup is 50 mm (\(l_{cup}\)). The total volume of the cup is therefore 5.65 cm\({}^{3}\). The length should be longer than space-charge sheath thickness inside the probe to ensure the measurement outcomes are insensitive to the local plasma properties.