Hollow cathode life time modelling

Abstract

Hollow Cathodes (HCs) are of primary importance in the field of electric space propulsion, being used as electron sources in ion and Hall-effect thrusters. Hence, their lifetime is a key factor in all these applications.

HCs have demonstrated the capability of providing up to 30,000 hours of operation, whereas no direct experimental data exist above this limit.

The importance of HC lifetime is a growing issue for deep space missions using
propulsive systems based on ion or Hall-effect thrusters that may require longer
lifetimes than those demonstrated up to now. To address these concerns about HCs and
to prove the suitability of an ion thrusters based solar electric propulsion subsystem for
future high-impulse missions (such as Bepi Colombo), a model able to predict the HC
lifetime is needed.

The model that has been developed in this thesis consists of three parts: a barium oxide
depletion model, a low work function surface coverage model and a plasma update
procedure to calculate the effects that a change in the insert surface work function will
produce on the cathode plasma.

The barium-oxide depletion model has been validated by comparing its results with
experimental measurements performed at QinetiQ and NASA, showing a good
quantitative agreement.

The low-work function surface coverage model is the first of its kind to include the
effect of ion bombardment. The plasma update procedure, even if semi-empirical, is
able to produce results that are in good agreement with the measurements.

Using these three models the lifetime of the NSTAR hollow cathode has been
simulated, yielding predictions that are in good agreement with the theoretical
expectations.

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