## 6 Discussions  
### FC Material  
Experiments showed the influence of the material on the measured ion current. The software SRIM (Stopping and Range of Ions in Matter) was used to find the sputtering yield of Al, C (=graphite) and Mo with Xe as the projectile. SRIM includes a group of programs which calculate the stopping and range of ions into matter. They use a quantum mechanical treatment of ion-atom collisions where projectile atoms are considered to be \"ion\" and target atoms as \"atoms\" [64]. Simulations that included 10000 singly-charged xenon ions were run to compute the yield and heat losses. The latter corresponds to the energy transferred by projectiles to the target materials and subsequently converted into heat. Figures 18 and 19 show results for carbon (i.e., graphite) and molybdenum under singly-charged Xe ion bombardment. Simulation outcomes give an idea on how the target behaves under Xe ions bombardment. Figure 17: Current density angular profile without exposure to the plasma beam (Al05.P configuration) for different collimator potentials. The thruster fires at 0.66 A, 200 V (**a**) and 250 V (**b**). Profiles are displayed with a linear (**top**) and logarithmic (**bottom**) scale. The software computes the sputtering yield based on the binding surface energy (\(E_{surf}\)) specific to each target. When this value is not known the heat of sublimation is used instead [64]. Moreover, \(E_{surf}\) strongly depends on the material cleanliness and roughness. These properties will change over time as the target gets bombarded. It can lead to quick change of the sputtering yield. For instance, Figure 18 shows the sputtering yield as a function of the target atom's energy which reach the surface of the material. Singly-charged xenon ions are used as projectile and targets are made of carbon (top) and molybdenum (bottom). The vertical blue line, which defines the average surface binding energy, marked 7.4 eV and 6.8 eV for the carbon and molybdenum targets respectively. The arrow, to the left of this line, with the legend \"not sputtered\" implies the number of atoms which reached the surface with more than 7.4 eV (C) or 6.8 eV (Mo) is 0.005 and 0.18, respectively. These values correspond to the sputtering yield and are listed in Table 4. The vertical blue line will shift towards the left when the material surface gets damaged. The filled area shows how much effect small changes of the surface roughness (\(\equiv E_{surf}\)) will make on the final sputtering yield. It is seen that the two materials experience a fast degradation of the yield when the material surface is damaged. However, the maximum sputtering yield for carbon remains small (0.32 atoms/ion) compared to Mo yield that is 10 times higher. Molybdenum targets release 3 to 1 ratio sputtered atoms per incoming ion which is not negligible. This is coherent with a simple comparison based on the atomic number between target and projectile. Xenon has an atomic number close to molybdenum, 54 and 42 respectively, while it is 6 for carbon. Since C is much lighter than Xe, the singly-charged xenon ions need numerous collision events to start sputtering carbon materials. Figure 18: Carbon (**top**) and molybdenum (**bottom**) sputtering yields and threshold value for 250 eV Xe ions computed with the SRIM software. In the case of carbon, shortly after the vertical blue line, the yield becomes very small and sputtered particle only have energies from 7.4 eV to 12 eV. Molybdenum sees its sputter energy distribution to be more important. The atoms can reach energies from 6.8 eV up to 20 eV. From these two plots we understand that molybdenum will tend to sputter more atoms with an energy distribution broader than carbon. Any material subject to bombardment undergoes a heating process. Its impact can be estimated with the software SRIM as pictured in Figure 19. It shows the energy loss by the projectile for carbon and molybdenum. The red curve labelled \"IONS\" is the direct energy transferred from the ion to the target electrons.