## 1 Introduction
They are routinely applied for the diagnostics of plasmas produced by EP [4; 5] and due to direct electrical contact with the plasma (_invasive measurement_); they are the method of choice for the determination of electrical parameters such as the floating or plasma potential and electric fields. Measuring electron density \(n_{e}\) and temperature \(T_{e}\) is achieved mostly through Langmuir probes. While the measurement of the floating potential \(\phi_{float}\) can be performed by this type of measurement technique with high accuracy, difficulties are encountered when determining the plasma potential \(\phi_{plasma}\)[6; 7]. For this reason, emissive probes are usually employed since they allow for precise determination of \(\phi_{plasma}\)[7; 8; 9]. Another drawback of invasive measurement is the disturbance of the plasma itself. In the theory of classic Langmuir probe measurement, it is assumed that the disturbance is confined to the sheath region, which has a size of several Debye lengths \(\lambda_{D}\) around the probe. It was demonstrated by [10] that, depending on the plasma parameters, the disturbance may have a length scale of the typical relaxation length of the electron energy distribution function. Specifically for EP, a study was carried out by [11] to quantify these probe disturbances for Hall thrusters. Both types of probes were successfully employed by [12; 13; 14] to analyze the neutralized plasma plume produced by RIT thrusters. However, due to some uncertainties, careful analysis of ion densities \(n_{i}\)[14] and \(T_{e}\)[15] should be performed when using a Langmuir probe. Detailed information on important aspects of carrying out probe measurements in plumes of RIT thrusters can be found in [15] and references therein. The complex physical phenomena taking place in ion thruster plumes require sophisticated numerical approaches in order to analyze these both temporally and spatially. Widely used methods are particle-in-cell (PIC) simulations, with a cross-section-based collision database for modeling collision, CEX, or ionization events. The neutral gas is usually modeled with the widely used direct-simulation Monte Carlo (DSMC) method proposed by [16], but it is not coupled with PIC simulations. Simpler plasma simulations make use of assuming that, due to their low inertia, electrons may be modeled according to the Boltzmann relation. State-of-the-art simulations include fully kinetic models of electrons within the framework of coupled PIC-DSMC setups. DSMC of xenon in an electric thruster were reported by [17; 18], who published results of fully kinetic simulations of a plasma thruster. In terms of ion beam neutralization of ion thrusters, a detailed study was carried out by [19], where different neutralizer arrangements and modeling approaches were compared. To the best of the authors' knowledge, no research on probe measurements in _non-_ or _self-neutralized_ ion beams exists in the literature. In the present study, the applicability of probe diagnostics and the degree of its influence on the plasma and ion beams of the plume is investigated. A series of experiments are performed, where radial scans at different positions downstream are made. Moreover, partial neutralization of the beam is achieved by placing a thermionic cathode outside the beam. The value of the potential jump depends on the degree of partial compensation, while other plasma quantities such as the electron density \(n_{e}\) and energy \(E_{e}\) do not.