This thesis presents the design process of an innovative Retarding Potential Analyzer (RPA) for measurements analysis of a wide variety of plasma thrusters in air-breathing propulsion applications. This type of probe, widely used in the diagnostics of electric propulsion systems, allows for the extraction of the plasma energy distribution through direct current measurements. The proposed design employs four grids biased at different potentials: a floating grid to shield the external plasma and avoid influencing it, a repeller grid at a negative potential to repel the electrons, a bias grid whose potential is varied from $0$ to a maximum value to repel all ion species with energy below the potential barrier, and a suppressor grid at a negative potential to suppress any secondary electrons generated within the probe, followed by the collector plate. The design process was thoroughly supported by numerical validation, involving simulations that closely replicated the expected test conditions. These analysis were instrumental in generating current and energy distribution curves, providing a clear prediction of the anticipated test outcomes. By iteratively refining the design parameters based on the insights gained from these simulations, the optimization process ensured that the final RPA configuration performed effectively under a range of operating conditions. The results not only aligned well with theoretical expectations but also demonstrated consistency with similar findings reported in the literature, further validating the reliability and robustness of the design approach. The work concluded with the realization of the RPA, which features titanium grids and PEEK housing for the electrodes, manufactured by external suppliers. Future developments include the assembly of the various components, a critical step to ensure proper alignment of the apertures, and a vacuum chamber testing campaign to validate the design. Such test will be conducted both for atmospheric plasmas and more conventional propellants, thus expanding the operational envelope of the probe.
