NOTE 1 In general, charged particle currents from the environment are insufficient to neutralize the spacecraft and a neutralization system is provided with capacity to supply more current than the EP beam.
NOTE 2 See ECSS-E-ST-10-04 for worst case environments for different orbits.
NOTE 3 The objective of the requirement is to ensure that the beam current produced by the EP system is completely neutralized. Failure to ensure neutralization can lead to high absolute and differential spacecraft potentials, causing interference with other systems and possible spacecraft damage, severe loss of EP efficiency and contamination and erosion of spacecraft surfaces.
b. If neutral plasma thrusters have an integral cathode (effectively an integral neutralizer) which neutralizes the beam and is used as part of the ionization process, the neutralizer requirements in 11.2.1a shall be applied to these cathodes.
NOTE Hall effect thrusters are neutral plasma thrusters.
c. Thrusters shall not be operated on-board spacecraft without their associated neutralizers.
d. There shall be a ground path linking the neutralizer and the ion beam emitter so that there is a complete circuit including the flow of neutralizer electrons and thruster ions in space.
NOTE Unless there is a direct ground path linking the thruster and the neutralizer directly, so that the neutralization current does not pass through other on-board systems, there can be EMC issues.
e. If in low thrust systems e.g. FEEPs, in LEO, if it is demonstrated by an analysis which includes the worst case natural currents, that neutralization can be achieved through natural ionospheric currents, a neutralizer may then be omitted.
NOTE This case is expected to be highly unusual.
f. Except in the case specified in 11.2.1g, those neutralizers on spacecraft without active EP thrusters whose role is to control natural charging levels, shall be attached directly to spacecraft ground.
h. The current capacity of the neutralizers whose role is to control natural charging levels shall be higher than the largest natural charging current.
i. Neutralizers emitting particles of one polarity shall have active current control and measurement of spacecraft potential.
NOTE Measurements can also be useful for diagnosis of thruster operation.
j. Except in the case specified in 11.2.1k, neutralizers that emit charged particles of one polarity should not be used.
NOTE 1 An example is electron guns.
NOTE 2 The reason is that they can increase differential charging levels and can be rendered ineffective by potential barriers.
NOTE 3 These emitters use active control of the emitter current in the light of real-time potential measurements to maintain a fixed potential.
k. Provision 11.2.1j need not be satisfied in the following cases:
1. If it can be shown that differential surface charging remains below 100 V.
NOTE An example is when the surface is completely conductive or when the ambient plasma is very cold.
2. If scientific measurements can be contaminated in the environment induced by other types of neutralizers.
NOTE Examples are ambient plasma measurements.