C.4 Current collection and grounding to the plasma

For high potential surfaces such as solar array interconnects, the presence of dense plasma allows current to flow through the plasma, by collection of electrons and ions to positive and negative surfaces respectively.

This effect is particularly significant for LEO orbits where the plasma is dense and spacecraft surface potentials, e.g. on solar arrays, may be very high compared to the plasma temperature. The size of the current depends principally on the size of the sheath, which may be significantly larger than the surface itself. For example, the sheaths produced by positively charged solar array interconnects may merge (the snap-over effect) so that the effective collecting area is the whole array surface. Conversely, the presence of nearby strongly negatively charged areas may create a potential barrier preventing current collection to a small positive surface.

The higher mobility of electrons compared to ions means that current collection is far more efficient to positive surfaces and leads to these surfaces being driven towards plasma ground. This means that the way in which a solar array is connected to the spacecraft ground can affect the equilibrium potential of the whole spacecraft i.e.

• Negative end of the solar array connected to spacecraft ground – a small portion of the solar array remains slightly positive with respect to the plasma. The spacecraft ground floats negative with respect to the plasma at a substantial fraction of the array potential.

• Positive end of the solar array connected to spacecraft ground – the spacecraft ground remains very close to plasma potential. The solar array is negative.

The current that is effectively passing through the plasma from end of the solar array acts to the other acts as a drain on the power produced by the array. Usually this is a very low percentage of the total power but the percentage rises with solar array voltage for the same effective surface area.

In a similar way, to solar arrays, current collection occurs at both ends of a conducting space tether. In this case current collection is needed as it controls the effectiveness of a tether on tethered spacecraft. Whether a tether is being used to generate power or is being used as a motor by passing a current along it, effective collection of current from the plasma is required at both ends. For the positive end, passive electron collection is usually sufficient but for the negative end, an electron gun is often used.

3-D spacecraft-plasma interaction modelling codes such as SPIS (see D.1.1.4) can be used to analyse current collection effects.