The figure shows the principle of a Retarding Field Analyzer.
A hole grid system of two grids (plasma grid and extraction grid) extracts particles from the plasma. The extraction grid can have a positive potential (measuring electron energy distribution) or a negative one (measuring ion energy distribution.
The collector measures the particle current in dependence on the retarding voltage. In order to find the energy distribution of the extracted particles, one has to differentiate the curve with respect to the retading voltage.