C.9.4 Metallic discharge

C.9.4.1. Metallic discharge: grounded conductor

A criterion of metallic discharges is: discharge can occur if dielectric surface voltage are greater than 100 V positive relative to an adjacent exposed conductor. If a conductor is at the frame potential, this condition can only be attained when the spacecraft ground is negative with respect to space.

When the voltage of a conductor exceeds some hundred of volts, if the curvature radius is small enough, the electric field on the tip is sufficient to generate field emission (also called cold emission or Fowler-Nordheim emission). The electron is driven towards the relatively positive dielectric area with energy near the maximum secondary emission yield (Figure C-9). If the general configuration of the electric field provides trajectories to infinity, secondary electrons are blown off. For a yield larger than 1, the dielectric region is left more positively charged than before, which increases the electric field and field emission capability. So, the process avalanches only limited by the fusion of the tip, when heated by the increasing current density.

The discharge is interrupted when there is no more available charge on the conductor. In the case of a grounded conductor, the charge released is the total absolute spacecraft charge.

A typical current measured in the laboratory is in the 10 mA-1 A range. The discharge of a spacecraft frame can takes several microseconds.

C.9.4.2. Metallic discharge: floating conductor

The field configuration is identical to the grounded conductor case but the spacecraft potential may be zero. When a metallic component is let floating, differential secondary emission yields may render it negative with respect to surrounding dielectrics. If the differential voltage reaches the hazard threshold of about 100 V, arcing becomes possible (Figure C-9) with the same process as for grounded conductors.

This process is known as the "inverted gradient discharge" since the metal is negative with respect to dielectric in opposition with the "normal" gradient where the dielectric surface is negative with respect to the spacecraft frame.

Figure C-9: Schematic diagram of discharge at a triple point in the inverted voltage gradient configuration with potential contours indicated by colour scale.