A retarding potential analyzer (RPA) is being developed at the Swedish Institute of Space Physics to build expertise in designing plasma particle instruments measuring currents. This thesis presents the results of the project. First, a literature survey of RPAs was conducted to support the IRF’s working group in critical design choices. Subsequently, a 3D CAD model was designed by the mechanical engineering department. This model was used to perform ion optical simulations to investigate the behavior of the instrument. The simulations showed that potentials at the walls drastically affect the trajectories of charged particles in the instrument. Consequently, the instrument’s response diverges from simple analytical models. An effort was made to adapt these models to better describe the observed response. The instrument was also manufactured from the computeraided design (CAD) model by the institute’s own workshop. After fit-checking and thorough cleaning, the parts were assembled in a clean environment. Subsequently, the instrument was tested with an ion beam in the IRF’s vacuum chamber. The instrument exhibited an extremely low noise level and was successfully used to measure the ion beam. The measurements confirmed the instrument was performing as expected and allowed for an energy analysis of the ion beam. Furthermore, the high temporal resolution of the instrument enabled a closer inspection of fluctuations in the beam current. Overall, the project showed the IRF’s capability to rapidly design, manufacture, integrate, and test plasma instruments. It serves as the basis for future iterations of the instrument, optimizations, advanced simulations, and the development of an ion drift meter to complement the measurements by enabling directional observations.