Ion optics, or grids, for ion thrusters or ion sources are traditionally made by subtractive manufacturing methods such as milling/drilling or chemically etching holes from thin sheets of bulk material. This innovation uses additive manufacturing (AM), or 3D printing, to build the grids from powdered material.
Large-diameter optics are typically dished or domed to give a predictable movement when their temperature rises in use, to direct the ion beam in a desired direction, and to help them survive launch vibration. Tight tolerances must be met, and the dishing procedure can therefore be expensive and time-consuming. Dishing of NASA grids is typically performed by hydroforming flat sheets into the domed shape between shaped dies, either before or after the holes are etched into the sheets. There is a limit to the amount of dishing that can be obtained due to material stresses. Alternatively, multi-axis milling can be used to subtractively machine the grids from a bulk block.
By additively manufacturing the grids from the powder form, the grids can be made directly to the final (often nonplanar) shape, avoiding the need for hydroforming. Two AM methods are common and commercially available: direct metal laser sintering (DMLS) and electron beam melting (EBM). DMLS typically produces parts with finer features and better as-made surface quality; EBM build rates are faster, but as-made parts are rougher.
It is beneficial for the ion optics to have a high open area fraction to produce the highest current density beam in nearly all applications. Traditional grids use circular holes in a hexagonal aperture pattern, and there is a minimum material width for strength between hole centers that partially limits the open area fraction. Milled or drilled holes are traditionally circular and cylindrical in nature, while chemically etched holes typically exhibit a cusped shape where the midpoint of the hole is of smaller circular diameter than the entrance and exit to the hole. Alternative hole shapes are being investigated to dictate ion beam shape, reduce beam divergence, or increase open area. The creation of such shapes is enabled, or at least made more reasonable, using AM, particularly in optics for high-voltage operation that are larger dimensionally relative to the resolution of the AM machine.