## 6 Conclusions  
This study examines two operational modes of a HPT equipped with a compact MEICP reactor and a MN acceleration scheme. It demonstrates that, under pulsed power excitation (pulsed wave mode) at moderate RF power levels (below 500 W), the HPT undergoes a transition from inductive mode (H) to helicon wave-sustained discharge (W). This transition is characterized by a rapid increase in argon ion emissions (ArII). In W-mode, atomic line emissions are governed by neutral density and often reach saturation due to neutral depletion. In contrast, ion emissions, which are closely tied to electron density and temperature, show a substantial increase upon transitioning to the wave-heated mode (W). Conversely, in H-mode, atomic line emissions remain predominant.  
This scientific work highlights the transition between wave modes (H-W) in a 500 W-class HPT, specifically from the surface mode (involving helicon to TG-mode conversion) to the global heating mode dominated by helicon waves. The TG-mode plays a crucial role in plasma heating prior to the H-W transition. In W-mode, under moderate magnetic field conditions, such as the current case with a 400 G field, the plasma heating through mode conversion diminishes, leading to direct heating by the helicon mode, which becomes the primary mechanism for plasma heating. This transition results from the nonmonotonic relationship between power absorption and plasma density, influenced by reduced power absorption under the anti-resonance condition of the TG wave.  
The experimental characterization of a compact HPT-BM resulted in the development of a versatile MEICP source, facilitating rapid transitions between operating modes (H-W), thereby enhancing the thruster's throttleability. From optical emission spectroscopy (OES) data, a collisional radiative model (CRM) estimated the electron density and temperature of the argon plasma, yielding a specific impulse of 1100 s and a thrust of 3.2 mN for a propellant flow of 12 sccm.  
To improve the Technology Readiness Level (TRL) of the HPT based on the compact MEICP reactor and MN acceleration scheme, future work will focus on optimizing plasma ionization efficiency, refining RF power-plasma matching, and improving magnetic nozzle acceleration. Additionally, the development of advanced materials and the applications of both intrusive and non-intrusive plasma diagnostic setups will be essential for further enhancing the thruster's performance.  
The anticipated performance values of the HPT Breadboard Model have been highlighted. While the system is still in early development and not yet optimized, this testing campaign represents a critical step toward achieving competitive propulsion metrics.  
Conceptualization, A.-M.T.A. and D.E.C.; methodology, S.N.D., M.V.T. and A.C.; software, A.S.; validation, A.-M.T.A., M.V.T. and A.S.; formal analysis, D.E.C. and A.P.; investigation, S.N.D.; resources, A.-M.T.A.; data curation, A.P.; writing--original draft preparation, A.-M.T.A.; writing--review and editing (equal).; visualization, D.E.C.; supervision, D.E.C.; project administration, A.-M.T.A.; funding acquisition, A.-M.T.A. All authors have read and agreed to the published version of the manuscript.  
This work was funded by the ESA GSTP-De Risk Program under Contract No. 4000130736/20/NL/BJ/va., Assessments to Prepare and De-Risk Technology Developments/Helicon Plasma Thruster and by funds from the COMOTI Romanian Research and Development Institute for Gas Turbines.  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
**Acknowledgments:** The valuable support and consultation of [PERSON] on collisional radiative models and Advanced Space Propulsion concepts are greatly appreciated.