## 4 Experimental Setup and HPT Test Plan  
The proposed HPT, based on a compact MEICP reactor and an MN acceleration scheme, is designed as a modular breadboard platform (Figure 7), allowing for the validation of the influence of key operational parameters. In this study, a fully electrodeless EP system was designed and manufactured, where the transition from the H-mode (ICP) to the W-mode (HWP) was analyzed using OES. In regard to standard measurement techniques for particle density, energy distribution, and flux, the electric space propulsion community relies on intrusive measurement techniques such as Langmuir probes, Faraday probes, and Retarding Potential Analyzers (RPAs). However, when the inner diameter of the plasma reactor becomes comparable to the size of the probe, these intrusive measurements can disrupt the discharge conditions of the thruster, affecting its acceleration dynamics. In such cases, intrusive probe diagnostics become unsuitable, making non-intrusive methods like OES highly advantageous. The area of particular interest was along the longitudinal axis of the breadboard model, specifically near the downstream region of the magnetic nozzle, where the magnetic field gradients undergo a directional change (electron diamagnetic thrust in the MN). Figure 6: (**a**) A schematic representation illustrating the conversion of azimuthal momentum into axial momentum within a magnetic nozzle, where B is the applied magnetic field and (**b**) the formation of the expanding magnetic field downstream of the MEICP reactor. The experimental HPT system (Figure 8) comprises the thruster unit mounted on the vacuum chamber along with an RF power supply system, propellant delivery system, vacuum infrastructure, and measurement lines. The experiments were conducted in a vacuum facility made of nonmagnetic stainless steel that was 508 mm in diameter and 400 mm in length, equipped with several flanges that provide electrical, gas, and optical feedthroughs. This vacuum infrastructure was designed to simulate the LEO environment, achieving an ultimate vacuum pressure of \(2\cdot 10^{-5}\) mbar. This level of vacuum was attained through the combined operation of a Pfeiffer mechanical pump (ACP 28-40) (Pfeiffer Vacuum GmbH, Asslar, Germany) and a Pfeiffer turbomolecular pump (HiPace 400) (Pfeiffer Vacuum GmbH, Asslar, Germany). Additionally, the chamber pressure was monitored using a TPG 361 Pfeiffer gauge. To regulate the argon flow rate within the range of 0-15 standard cubic centimeters per minute (sccm) during the discharge, a Bronkhorst mass flow controller (F-201 CV-500  
Figure 8: Helicon Plasma Thruster experimental system schematic. Figure 7: The Helicon Plasma Thruster Broadband Model during its commissioning procedure. AGD-33-V) (Bronkhorst High-Tech B.V., Ruurlo, The Netherlands) was employed. To ignite the HPT, an RFG600-13 RF power generator (Coaxial Power Systems, Lancashire, UK) was utilized in conjunction with an AMN600 impedance matching unit. The RF generator produces the RF wave responsible for creating the plasma and is designed to ensure optimal power transmission to the plasma by matching the impedance effectively. To match the impedance of the MEICP reactor with the 50 \(\Omega\) impedance of the RF generator, an automatic L-type impedance matching network along with an automatic matching network controller (AMNC) were utilized. This setup (Figure 9) also ensures that the reflected power remains below 15% of the forward power. The RF current supplied to the HWRH antenna was set at a fixed frequency of 13.56 MHz, with the RF power system operating externally in radio external control (REC) mode. EP systems operating in low-pressure plasmas require precise control of plasma parameters to optimize throttling capabilities. A promising strategy for controlling ion energies involves a pulsed RF power operation mode [57]. The strategic goal of the proposed experimental testing campaign was to investigate the discharge regimes (H-W mode) and density jumps in a HPT by employing OES.