Plasma diagnostic systems for Hall-effect plasma thrusters

A joint programme, involving research laboratories from CNRS (Centre Nationalde le Recherche Scientifique) and ONERA (Office National de RecherchesAérospatiales), was developed in France in connection with the French SpaceAgency (CNES) and industry (SNECMA) for the understanding of Hall-effectplasma thrusters. Different activities are pursued in parallel: anexperimental test of different laboratories' thrusters; the development ofdiagnostic techniques to characterize the plasma inside and outside thethrusters; and the development of simulation and modelling able to describecharacteristics and evaluate the thrusters' performances.This paper will be focused on diagnostics systems implemented in the PIVOINEfacility. Time- and space-resolved measurements of the ion beam energy,distribution electron density and concentration in the plume are performedwith a retarding potential analyser (RPA) and Langmuir probes mounted on a2.5 m movable drive. The thruster can be moved axially to allow a40×90 cm2 exploration of the plume. The investigation of theplasma inside the thruster is made by optical diagnostics. A CCD camera usedin fast imaging mode is set outside the tank. The 45° sight axis allowsan internal view of the thruster's channel. Furthermore, a spectroscopicanalysis is made by focusing the channel's light to a set of optical fibresconnected to an imaging spectrometer equipped with a CCD camera. A specificlaboratory thruster of 100 mm external diameter called SPT100-ML was studiedin more detail, this model being designed to allow the implementation ofoptical fibres and wall probes diagnostics inside the channel's thruster. Thestationary plasma thruster discharge is almost always characterized bylow-frequency instabilities of the order of 10 kHz where the discharge currentcan reach a very high instantaneous level. The variation of the discharge andion beam flux currents has been related to the spatiotemporal dynamic of theplasma inside the thruster's channel. The main features are explained by aone-dimensional (1D) hybrid model and a 1D particle-in-cell-Monte Carlomodel. A new thruster, working at a very low fluctuation level with a lowangular divergence ion beam, is now under investigation in connection withSNECMA.