Fig. 4
From: An in-flight plasma diagnostic package for spacecraft with electric propulsion
The retarding potential analyzer. (a) The instrument consists of a stack of four titanium grids and a collector plate. (b) The construction drawings show details of the geometry. Note the segmented collector with a central area (gray) and three outer ring segments (colored). The remaining surface of the collector plate (white) is also covered with a conductive layer and grounded. The individual segments are insulated from each other by a narrow groove