  Spacecraft neutralisers are required as part of the ion propulsion system for
accurate station keeping in fundamental physics missions. This paper describes
the use of thin layers of insulating materials as coatings for the gated
silicon field emitter array structure used in a spacecraft neutraliser. These
thin coatings are postulated to reduce power consumption and reduce
overheating. The power consumption and lifetime of aluminium nitride and
amorphous hydrogenated diamond-like carbon coatings have been tested by
current-voltage and endurance tests. Diamond-like carbon coatings were
promising, performing better in endurance tests than uncoated samples, but
further work is required to characterise the coating's physical properties and
its effects on field emission. The thermal conductivity of the coating material
had little effect on measured sample lifetimes. Aluminium nitride had reduced
power consumption compared to diamond-like carbon coated and uncoated samples.
A thin (~5 nm) layer of aluminium nitride was found to be optimal, meeting
European Space Agency specifications for the neutraliser engineering model.
