The role of grid geometry and relative orientation in the performance of a retarding potential analyzer

In this study, a four-grid retarding potential analyzer (RPA) with drilled grid holes is investigated, focusing on correlations between grid orientations and resulting characteristics. The individual grids have a hexagonal hole pattern and can be mounted rotated relative to each other in multiples of 90°. An ion beam with a small divergence and a narrow energy distribution directed perpendicularly to the RPA grid system is used. We find that for certain grid configurations, particularly when grids are aligned, the characteristics deviate from the expectation of strictly monotonic behavior in plots of the collector current against the discriminator voltage. Specifically, aligning two of the inner grids leads to a positive slope and a distinct hump at voltages below the falling edge. When all three inner grids are aligned, the hump becomes significantly more pronounced, with the signal intensity nearly doubling. Several models are presented to reproduce and understand these observations. We find that grid holes can act as scattering centers, and a finite grid thickness mitigates the potential reduction that occurs inside the grid holes. Suggestions for the design of RPAs are derived based on the findings.