(a) A schematic of the display-type three-grid RFA employing the improved grid arrangement. (b) A schematic of the partial spherical shell with radially directed cylindrical holes used for the second grid, i.e. the retarding grid. (c) A photograph of the fabricated partial spherical shell with a holed area corresponding to an acceptance angle of ±52°. (d) A zoomed-in photograph of the holed area on the inner side of the spherical shell.