spase://ESA/NumericalData/Cluster-Rumba/ASPOC/PrimeParameter/PT4S

The ASPOC instrument is a single unit consisting of an electronics box and two cylindrical ion emitter modules. The emitters produce indium ions at approximately 6 KeV, in a current of less than 50 microamps. This is done by field evaporation of indium in the apex field of a needle. In the basic feedback mode of operation, a measurement of the spacecraft potential is supplied to the instrument from either the electric field experiment (EFW) or the electron analyzer (PEACE). This information is then used to adjust the emission current to reduce the spacecraft potential to some predetermined value. By default, priority is given to the EFW data, because of the higher resolution (0.034 V vs. ~1.4 V) and the more straightforward way in which the potential is derived. A calibration mode will measure the current voltage characteristics of the spacecraft, at the beginning of the mission and occasionally later to account for changes in the photoemission properties of the surface. This measurement is carried out by sweeping the ion emission current in incremental steps over some convenient range, allowing simultaneous measurements of the spacecraft potential. The length of each step is 2 to 4 spin periods. In addition to providing an improved environment for other experiments, ASPOC will permit scientific investigations of the photoelectric characteristics of the dependence of the spacecraft potential on plasma parameters, and of spacecraft charging in different plasma environments to be carried out in the so called active mode. For more details of the Cluster mission, the spacecraft, and its instruments, see the report 'Cluster: mission, payload and supporting activities,' March 1993, ESA SP 1159, and the included article 'Active Spacecraft Potential Control: an ion emitter experiment for Cluster,' by W. Riedler et al., from which this information was obtained.