The two negatively biased grids are used for eliminating the influence of plasma electrons and secondary emission electrons. If you are working with low energy plasma source with thermocathode you can ground these grids. The positive retarding voltage V R increasing from zero to the value about discharge voltage V D is supplied to the middle grid and the collector current j S is recorded. The collector current j S is created only by ions with energy more than eV R . The ions with less energy will be retarded in the field with potential difference V R between the retarding and screening grids.
R shows the ion energy distribution.
It is not enough to measure the ion energy at one point of beam, for example, on it axis. The matter is that in the beam the drift of particles out of beam volume exists. This drift is a result of different energy, momentum and charge exchange interactions between the ions and gas molecules. The energy of ions in the secondary plasma is a few times less than in the beam plasma. The current probes are non selectable to the energy of ions so if you have got any ion current distribution by these probes, you can't be sure that the tails of distribution will work, for example, in the process of sputtering, like the center of beam.