Compact retarding potential analyzers enabled by glass-ceramic vat polymerization for CubeSat and laboratory plasma diagnostics

We report the design, fabrication, and characterization of novel, digitally manufactured, compact retarding potential analyzers (RPAs), i.e., multi-electrode instruments that can be used as in-orbit mass spectrometers and as on-ground/in-orbit ion energy analyzers. Unlike most RPAs reported in the literature, our devices enforce active aperture alignment across the grid stack, maximizing ion transmission. The RPA electrode housing (the most critical component of the RPA, as it precisely positions the electrodes to maximize grid stack transmission while avoiding space charge effects and providing electrical insulation) is additively manufactured in Vitrolite® (a printable glass-ceramic) via vat polymerization, resulting in non-porous, high-temperature compatible, and high-vacuum compatible hardware. Metrology of as-printed (green) VitroliteⓇ parts shows an in-plane, per axis manufacturing accuracy of 60 µm (i.e., the in-plane length of the voxels) and an out-of-plane manufacturing accuracy of 100 µm (i.e., the height of the slices). VitroliteⓇ 3D-printed objects annealed at 900 °C show a ~5 % shrinkage, a slightly larger maximum tensile stress, a similar Young’s modulus, and a larger compression yield stress compared to green samples. Also, the assembly misalignment between the grids and a 3D-printed housing annealed at 900 °C is a twofold larger compared to using a green housing. Annealing 3D-printed VitroliteⓇ at temperatures above 900 °C causes significant distortion of the printed part due to material reflow, which is incompatible with a precision engineering application. Four different RPA designs were synthesized to probe the ionosphere (design with floating grid alignment at the aperture level) and laboratory plasmas (designs with floating grid aperture alignment at the cluster level). Simulations conducted in this study show that the RPA design that enforces floating grid aperture alignment at the aperture level attains the best performance, although RPA designs that implement floating grid aperture alignment at the cluster level are more resilient to grid misalignment. Experimental characterization of the RPAs using an ion source and a helicon plasma source is consistent with expected performance and the literature. Plasmas with a Debye length as small as 50 µm were successfully characterized using the reported sensors, matching the performance of state-of-the-art RPAs manufactured via semiconductor microfabrication.