The purpose of the PCD is to reduce the spacecraft potentials to a few volts, levels sufficiently low to allow
plasma particle measurements down to very low energies. For this purpose, the PDC emits beams of 6 keV indium ions to
compensate the positive charging that would otherwise exist under most circumstances. The emission principle is field
evaporation and ionzation of the liquid metal covering a needle by applying a high voltage. Beam currents are variable
and can either be set to fixed values, or are adjusted automatically based on the electron energy spectrum measured by
the 3DA instrument that is transmitted to PCD in real-time. PCD is identical to the ASPOC instrument developed for CLUSTER.