Introduction to the Physics of Charging and Discharging
The fundamental physical concepts that account for space charging are described in this chapter. The appendices expand this description by means of equations and examples.
2.1 Physical Concepts
Spacecraft charging occurs when charged particles from the surrounding plasma and energetic particle environment stop on the spacecraft: either on the surface, on interior parts, in dielectrics, or in conductors. Other items affecting charging include biased solar arrays or plasma emitters. Charging can also occur when photoemission occurs; that is, solar photons cause surfaces to emit photoelectrons. Events after that determine whether or not the charging causes problems.
A plasma is a partially ionized gas in which some of the atoms and molecules that make up the gas have some or all of their electrons stripped off, leaving a mixture of ions and electrons that can develop a sheath that can extend over several Debye lengths. Except for LEO, where ionized oxygen (O+) is the most abundant species, the simplest ion, a proton (corresponding to ionized hydrogen, H+), is generally the most abundant ion in the environments considered here. The energy of the plasma, its electrons and ions, is often described in units of electron volts (eV). This is the kinetic energy that is given to the electron or ion if it is accelerated by an electric potential of that many volts. Whereas temperature (T) is generally used to describe ...