Low Energy Electron Gun Power Control Unit
Near-Earth orbiting spacecraft are subject to a variety of physical environmental elements. Of these, natural space plasma and solar radiation produce spacecraft charging. Spacecraft charging consists of surface (external) and deep (internal) dielectric charging.
Natural space plasma is composed of electrons and positively charged atoms called ions. The plasma is generated by energy from solar radiation and high energy particles emitted by geomagnetic storms. The electrons produce a negative current and the ions produce a positive current. Positive photoelectron currents produced by solar radiation of spacecraft surfaces also add to the electrical fluxesi.
As spacecraft move through the atmosphere, they may be subjected to an unequal flux of electrons and ions. These unbalanced electrical fluxes induce charges on the surfaces and inside spacecraft. In the lower regions of the earth’s atmosphere the electrons and ions tend to be of a lower energy level than those at a higher altitude. Therefore, greater charges are generally induced at higher altitudes. Also, the higher energy particles tend to be the ones that penetrate deeply into the spacecraft and produce the dieletric (internal) charges.
Often, the entire surface of spacecraft is not crafted of the same material, resulting in uneven charge distribution on the surface and inside of spacecraft. This unequal charging may produce arcing which often causes equipment malfunction and failure, false instrument readings and poor sensor performance. Arcing may also cause physical damage to the spacecraft structure and contamination of the surface. Many cases of anomalies due to spacecraft charging have been recordedii.
Modern electronic systems employ low voltage and low current circuitry and are being designed in smaller and smaller packages. These new systems tend to be much more sensitive to space charging effects than the more robust systems employed in older equipment. Also, spacecraft are being deployed into orbits comparitevly higher than in past missions. The higher orbit particles pack a lot more punch, producing higher charging potentials and greater arcing possibilitiesiii.