## 4 Conclusions  
The experimental diagnostics of the uncompensated ion beam revealed the strong influence of the probe diagnostics on the value of the plasma potential. The performed analysis indicates that the measured increase can be attributed to violation of ion beam self-compensation. The probe holder introduces an additional sink of electrons, which has to be compensated for by the plasma to remain quasineutral. The electron depletion causes an increase in the plasma potential, which enhances the facility SEE by increasing the energy of the bombarding ions. The new equilibrium is established through elevated values of the plasma potential and an increase in electron flux from the facility walls. As soon as the probes are removed from the ion beam, the previous state is reestablished. The numerical simulations reproduce the experimentally observed findings and measured values. Calculated values of the electron density and plasma potential show good agreement with the measured values. Additionally, the insight of ion trajectories indicates that elevated values of the plasma potential first slow down the ions, but then, the overall ion energy remains almost unchanged. Figure 16: **Left—**comparison of downstream potential \(\phi(x)\). **Right—**comparison of ion velocity \(u_{\text{Xe}^{+}}(x)\). Disturbance of the neutralization process of the ion beam results in a significantly higher potential and slow-down of the ions downstream. The grey-shaded regions indicate the (axial) location of the probes. Figure 17: **Left—**comparison of electron number density at \(\Delta x=50\,\text{mm}\) predicted by PIC-DSMC with experimental measurements for the undisturbed and disturbed (i.e., experimental) cases. **Right—**EDF at different radial \(y-\)positions at \(\Delta x=50\,\text{mm}\) compared with experimental measurements of the electron temperature (denoted by the vertical lines). The markers on the \(x-\)axis indicate the electron temperature obtained from the PIC-DSMC simulation at the respective locations \(y=[30,\,40,\,60]\,\text{mm}\). These findings indicate that invasive diagnostics should be used with care when investigating ion beams in self-neutralization mode. The elevated values of plasma potential could falsify the lifetime assessment of thrusters due to increased values of the erosion rate. In order to prevent the unwanted effects, invasive diagnostics should be used during short periods of time compared with the total duration of the test campaign. As self-neutralization is used only with small-sized ion thrusters, these implications do not apply to larger systems, where explicit neutralization of the ion beam is usually performed. In general, any other plasma-based electric propulsion system should be free from the effects described in this work if explicit sources of electrons are present. The presented results show the significance of the SEE for ion thruster operation in a vacuum facility. The SEE is a _facility effect_, which, in general, depends on the size of the facility, distance the beam is allowed to travel, wall material, minimal operational pressure and other factors. So further research is necessary to quantify the impact of the different factors on the described findings. There are non-invasive alternatives for the assessment of some plasma parameters of interest. The electron density and temperature can be determined by optical emission spectroscopy when combined with the appropriate collisional-radiative model. Another option is the Thomson laser scattering technique, though the low electron densities make it difficult to apply. Almost no alternatives to probes exist for the determination of the plasma potential. It would be possible to deduce the plasma potential by integration of the electric field values, which can be determined optically from the Stark broadening of spectroscopic lines. But practically, this technique can only be applied to the case of extremely high electric fields.