The ASPOC (Active Spacecraft Potential Control) emitter instrument controls the electric potential of the spacecraft with respect to the ambient plasma by emitting a variable current of positive ions. In steady state, a spacecraft will charge to an equilibrium potential where all currents, namely photo-electron current caused by sunlight, plasma currents due to the environmental electrons and ions, secondary electron currents caused by the impact of primary electrons and ions, and the ion current generated by ASPOC compensate so that there is no net transfer of charge between the spacecraft and the environment. Without active control, spacecraft potentials along the Cluster orbit would range from a few volts positive in the solar wind and magnetosheath up to about 100 V in the magnetospheric lobes. Near the Cluster perigee and in the near-Earth plasmasheet, small negative potentials are possible. High floating potentials obscure the measurement of the core of the ion-distribution function measured by CIS and contaminate the low-energy portion of the electron spectra measured by PEACE by photo-electrons from the satellite surface, trapped by the positive satellite potential. The primary objective of ASPOC is the reduction of high positive spacecraft potentials to a constant value which is sufficiently low (a few volts) to reduce the above mentioned disturbances significantly.
At the heart of ASPOC are emitters of the liquid metal ion emitter type with indium as charge material. The emission principle is field evaporation and ionisation of the liquid metal covering a needle by applied high voltage. The melting of the indium from the cold state lasts about 15 minutes in a mode called ''startup'', after which high voltage is applied and successively increased until the field emission process ignites. The emission current is then electronically controlled, while the extraction voltage adjusts itself within 5-9.5 kV .
The emitters are arranged in two ''modules'' with four emitters each. The nominal maximum beam current is 50 µA, the nominal lifetime of a single emitter is 4000 hours. Emitters should be operated regularly, and therefore the emitter operation will be cycled at regular intervals of several hours. The emission current of an emitter may be increased to maximum current over a fraction of one minute as a precaution to remove any contamination from the emitter, thereby ensuring that the operating voltage remains within operational limits.
The instrument modes are defined by the method for the emission current control. There are two stand-alone modes involving the setting a current to a fixed value by time-tagged command.
The default active mode second stand-alone mode fixes the total current of the high voltage unit, including losses inside the lens system (mode ITOT). Experience has shown that the resulting emission of an almost constant ion current fulfils all requirements for spacecraft potential control in the magnetosphere and the solar wind even without on-board feedback from measurements of the spacecraft potential.
The second stand-alone mode active mode controls the ion emission current to a constant value (mode IION).
Two more elaborate operating modes, so-called feedback modes, involve the transmission of such measurements by EFW or PEACE via an inter-experiment link (modes FEFW and FPEA). As this feedback loop could not be tested on the ground, several contingency modes have been implemented in ASPOC to cope with a failure of one of the links. In this case ASPOC will generally return to a stand-alone mode or standby. The feasibility of the active spacecraft potential control loop will finally be determined during the in-orbit commissioning phase.
In standby mode (STDB) both the emitters and their heaters are turned off.
In order to reduce the time before emission starts, a ''hot standby'' mode (HOTS) keeps the indium in a liquid state. This mode can be used to interrupt the ion emission by command, without change of modes or emitters before and after the break. The re-ignition time is reduced to the time required to sweep the high voltage.
A ''test and commissioning'' mode (T&C) describes a method to sweep the total ion current in steps lasting 8 or 16 s, and with 2 or 4 µA current increment. This mode will be used regularly within the calibration periods to establish the current-voltage characteristics of the spacecraft.
Finally, the instrument features a technical mode (TECH) for low-level commanding.
The time resolution of the spacecraft potential data in the inter-experiment link is 4 s (EFW) or one spin period (PEACE). In feedback mode, the ion current is adjusted in the same intervals. In stand-alone mode the current remains constant, with corrections due to varying efficiency of the emitters taking place once per second.

This description has been obtained from Section 3.5 of the ''Users Guide to the Cluster Science Data System'', DS-MPA-TN-0015.