Pickup Ion Processes Associated with Spacecraft Thrusters Implications for Solar Probe Plus

Abstract

Chemical thrusters are widely used in spacecraft for attitude control and orbital manoeuvres. They produce a plume of neutral gas which produces ions via photoionisation and charge exchange. Measurements of local plasma properties will be aff ected by perturbations caused by the coupling between the newborn ions and the plasma. A model of neutral expansion has been used in conjunction with a fully three-dimensional hybrid code to study the evolution and ionisation over time of the neutral cloud produced by the ring of a mono-propellant hydrazine thruster as well as the interactions of the resulting ion cloud with the ambient solar wind. A parameter survey was performed for varying angles of injection and injection rates, particle kinetics were also investigated. Results are presented which show that the plasma in the region near to the spacecraft will be perturbed for an extended period of time with the formation of an interaction region around the spacecraft, a moderate amplitude density bow wave bounding the interaction region and evidence of an instability at the forefront of the interaction region which causes clumps of ions to be ejected from the main ion cloud quasi periodically and the ways in which these features are modifi ed by the degree of solar wind mass loading and the relative orientation of the magnetic fi eld to the angle of injection. This may a ffect Solar Probe Plus for a signifi cant duration as data taking and delicate sensory equipment may be required to cease operation until local fluctuations return to a more moderate level. The scale of the fluctuations seen are dependent upon the duration of the thruster ring and the speci fic geometry and therefore e ffects may vary in-situ.