The complete solid-angle sphere is thus covered in one spin period of 1.5 s. 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 ionisation 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.
Optical fibers guide the emitted light to a photodiode operated in current mode.
A logarithmic amplifier converts this detector current to an analog output voltage. Energy discrimination is achieved by using three differently shielded channels. This way electrons above 0.26, 0.4, and 1.9 MeV are measured, and protons above 6.3, 9.5, and 35 MeV, with a time resolution of 64 s. A constant current is added periodically to calibrate the system.