## 2 Methodology  
### Experimental and Diagnostic Approach  
The facility used for carrying out the experiments is a vacuum chamber (R2D2) of UniBwM that is specifically suited for simulating space conditions. A schematic of the test facility is outlined in Figure 1. R2D2 has a main chamber volume of 4.5 m\({}^{3}\) and consists of a test chamber and hatch, which are separated by a pneumatic high-vacuum-isolation gate valve. The test chamber is composed of two segments and has a diameter of about 1.9 m and a total length of about 4 m, while the hatch has dimensions of about 0.75 m in diameter and length. The pumping system of R2D2 has three stages. The first level consists of a rotary vane pump followed by a root pump that is used for the first pump down phase. The second level includes a rotary vane pump, a turbo-molecular pump (TMP) for rough pumping and four TMPs. The rotary vane pump provides a rough vacuum for the ac[TMP] and delivers the back rough vacuum pressure for the TMPs. After the first stage reaches the required pressure level, the second pumping stage is activated and then runs continuously during the whole test procedure. This stage reaches a pressure of \(1\times 10^{-4}\) Pa. Two cryo vacuum panels (powered by a helium compressor) complete the vacuum system (third level). The cryo stage reduces the pressure significantly down to a pressure of \(p=5\times 10^{-6}\) Pa without the thruster being in operation. Pressure measurement devices, labeled as vacuum gauge heads (VGHs) in Figure 1, are positioned at roughly half the total downstream distance from the thruster's exit plane. A mN-class ion engine (described in detail by [20]) acts as the ion beam source and is installed within the upper hatch. The ion exhaust plume is ejected downstream into the main volume towards a beam target located at the bottom of R2D2. During operation, pressure levels of about \(1\times 10^{-4}\) to \(2.5\times 10^{-4}\) Pa are maintained throughout the facility. RITs use electrodeless inductive discharge for the generation of plasma, and their design is based on the standard configuration of an inductively coupled plasma source. The thruster is able to operate at thrust levels between 50 uN and 2 mN while requiring 7 to 75 W of power, respectively. For spatially resolved probe diagnostics, two translation units are installed inside the vacuum facility, allowing 2D scans of the plasma plume in one plane. The unit for controlling the position in the radial direction has a translation range of 17 cm with positioning accuracy of 0.1 mm, while the unit for setting the axial orientation has a range of 8.5 cm \(\pm\) 0.5 mm. The probes are placed with a radial distance of \(l_{r}=10\) cm from the RIT3.5 centerline axis, and an axial measuring range of 5 to 90 mm is employed. The diagnostic measurement itself is carried out with two Langmuir and two emissive probes. The spatial resolution of measurement is performed with a spacing of 10 mm. The relative positions of the probes and the thruster in one chosen radial position is shown in Figure 2. The Langmuir probes are made of 0.1 mm thick tungsten wire with a length of approx \(l_{\mathrm{LM}}=6\) mm. The emissive probes are made of 0.025 mm thick tungsten wire formed as a half-loop attached to two copper wires. The copper wires are insulated by glass capillaries which are placed in the supporting ceramic tube. The electric connections are realized using a coaxial cable and are shown in Figure 3.  
Figure 1: Detailed layout of the R2D2 facility, showcasing the mN-class RIT test setup for plasma plume and self-neutralization studies. The schematic highlights the attached pumps and positioning of the Langmuir and emissive probes used for carrying out the measurements downstream of the RIT’s exit plane. The facility walls act as the primary source of the electrons responsible for self-neutralization of the ion beam. They originate through secondary electron emission (SEE) elicited by keV ion bombardment.