## 1 Introduction  
Certain types of electric propulsion (EP) systems produce thrust by expelling charged particles of one polarity, presumably ions. The locally high positive potential produced by the ion beam results in radial spreading or \"blow-up\" of the beam downstream, which is expected due to repulsion of the positively charged ions [1]. In the environment of space, the usage of a neutralizer is required to ensure SCC in order to prevent the beam from electrostatic blow-up as well as electric current neutralization in order to avoid electrically charging the satellite. The neutralizer emits charges of opposite polarity (presumably electrons) and performs the required neutralizations. Ground testing in vacuum facilities of many EP systems such as gridded radio frequency ion thrusters (RITs) depends on the class of the thruster. While big N-class thrusters are usually operated together with neutralizers, smaller mN-class thrusters are often operated without neutralizers and rely on the self-neutralization effect of the ion beam due to interaction with the facility walls. Employing a neutralizer (usually a hollow cathode discharge) would introduce additional neutral gas flow comparable to that from the small-size thruster itself and would result in a increased production rate of charge-exchange ions (CEX ions) and thus affect the erosion rate of the thruster grids. It is believed that operation without a neutralizer does not bias the test results regarding the lifetime assessment. Current neutralization in the present study is performed by connecting the thruster-grounded electrode to the vacuum facility itself. However, intrusive diagnostics such as Langmuir probes introduce non-negligible effects in self-compensated ion beams. The present results of Langmuir probe measurements in the plasma plume downstream of the thruster, where a mN-class RIT was tested, indicate that the behavior of the beam is significantly altered by the measurement devices themselves. In particular, the position of the Langmuir probe (inside or outside the beam) is found to have substantial influence on the measured values of the plasma potential, which is an important quantity as it determines the impact energy of the CEX ions. The measured values of several hundreds of volts considerably deviates from the generally accepted plasma potential of a few tens of volts [2]. The recombination of the electrons and ions on the surface of the probe and probe holder may be the reason for the elevated values of the plasma potential. The few sources of electrons in self-compensated beams are ionization by electron or ion impact and secondary electron emission (SEE) from the facility walls. The volumetric processes, i.e., chemical reactions such as ionization of a neutral gas, are ineffective due to the low density of neutral gas downstream; thus, SEE should be the only source of electrons. By placing the probe inside the beam, it acts as an additional sink of electrons, and thus, the process of SCC is significantly affected. In order for the plasma to stay in a stationary condition, this sink has to be compensated for by an additional source of electrons. As the only relevant source of electrons is the SEE, its rate should be increased. This can be achieved by an increase in the energy of the CEX ions impacting a surface. This energy increase can be provided by elevating the values of the plasma potential that is experimentally observed. In order to investigate the reason for the observed unusual values of the plasma potential, movable Langmuir and emissive probe diagnostics were installed. The setup allows for radial scans of the beam at fixed positions downstream. The results indicate that as soon as the probe enters the ion beam, a jump in the potential on the order of several hundreds of volts takes place. Electric probes are widely used tools for diagnostics of plasmas. Depending on the conditions occurring in the plasma, different theories of probe operation are required [3].