## 5 Impact of the Collimator Characteristics
### Screened Collimator  
In this experiment the probe is equipped with an aluminum housing (Al.05.P) with a front aperture of 5 mm. A 10 mm diameter graphite collimator is placed behind and is electrically isolated. The collector remains a foam. The first study is made with both housing and collimator grounded. The second study is performed with a polarized collimator while the housing is kept grounded. In both studies a voltage sweep is applied to the collector. Measurement outcomes are compared with the ones obtained with FC G.05.E configuration. The aperture diameter is similar for the two designs. The beam profiles of the first study are displayed in Figure 14a,b (G.05.E and Al.05.P). At both discharge voltages Al.05.P measures the highest ion current density. On the thruster axis, the signal acquired by configuration X.X.P is 15 to 20% higher than X.X.E. The difference in current density is observed all over the beam profile. Actually, the current density decreases when the potential is more negative on the collimator. When the collimator is biased to \(-75\) V, \(j_{i}\) is near 5% less than the one obtained with a grounded collimator. Figure 13: Current density angular distribution measured with different collimator diameters (G.01.E, G.07.E, G.05.E, G.03.E). 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. Numbers refer to the different zones. In the second part of the experiment four different potentials (\(-15\) V, \(-25\) V, \(-50\) V and \(-75\) V) have been applied to the collimator in the FC configuration Al.05.P. I-V curves at \(\theta\) = 0\({}^{\circ}\) are plotted in Figure 15. One can notice all I-V curves are relatively flat when the FC potential is below \(-100\) V. As mentioned in previous studies [73] SEE is characterized by a drop of ion current when the potential applied to the collector is higher than the electrode placed at the entrance of the FC (housing, collimator). Such behavior is not observed in Figure 15, which confirms the choice of FC architecture to suppress SEE. Figure 16 shows I-V curve with a voltage sweep applied to the collimator while the collector is grounded, the current density measured on the collimator increases when the voltage applied is more negative. This trend is characteristic for I-V curves of Langmuir and non-guarded planar probes [16]. The collimator is a simple disk electrode with no specific design to reduce SEE. Moreover, sheath expansion effect is no longer negligible. This results in an artificial ion current increase as displayed in Figure 16.  
Figure 14: Current density angular profile with (G.05.E) and without (Al.05.P) exposure to the plasma beam. 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. Figures 15 and 16 suggest that applying a negative voltage to the collimator, prevents a part of the ion current to reach the collector. To better understand changes observed in the measured ion current density, simulations have been carried out with the SIMION software [74]. The outputs of numerical simulations are discussed in Section 6.3. Figure 17a,b show the beam profile obtained with FC Al.05.P with the collector potential fixed at \(-50\) V and the collimator grounded (blue) or biased at \(-75\) V (red) with the thruster firing at \(200\) V and \(250\) V, respectively. Only zones 1 and 4 introduced in Section 5.3 with Figure 13a,b are plotted. These zones are the most influenced by the probe voltage configuration. Indeed, in zone 1 there is a non-negligible drop of the current density measured by the FC configuration with \(-75\) V applied to the collimator. In contrast, in zone 4 the current density measured with V\({}_{r}\) at \(-75\) V overtakes the grounded collimator configuration. The lower signal measured by the \(-75\) V configuration in zone 1 is caused by larger ion energy distribution, see Section 5.3.