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### Charging phenomenon

A surface in contact with the space plasma is constantly hit by charged particles. This process leads to an electrostatic potential building which can reach sometimes many kV negative or a few volts positive. This effect can be troublesome. A possible, differential potential between two parts of a spacecraft can lead to electrostatic breakdown and subsequent discharges, which can disturb or affect the electronic payload. This effect has probably brought to the partial or total lost of satellites during the twenty last years (for example TELSTAR 401). For the scientific applications, the charging phenomenon can disturb the measurement of low energy particles.

There are several contributions to the charging current. Very generally the current, I, crossing a surface element on a body can be written,

 (1)

where :

                 is the current due to the incident or ambient electrons.
is the current due to the incident or ambient ions.
is the current of photoelectrons.
is the current of secondary electrons due to .
is the current of secondary ions due to .
is the current of backscattered electrons due to
$I_{Beam}(\phi)$ is the current due to all active source, as particulesbeams.
In a stationnary regime, this current is either null either completely conducted through the spacecraft structure. In general, in space, the two most important sources of current are the ambient plasma and the photoelectrons. However, in some situations (e.g. in eclipse and hot plasms) the ambient plasma electron current may be counter balanced by secondary electrons rather than by photo-electrons.

Next:Langmuir probe Up:General problem of plasma-body Previous:Space plasma environment

1999-10-10