Plasma diagnostic systems for Hall-effect plasma thrusters

A joint programme, involving research laboratories from CNRS (Centre National de le Recherche Scientifique) and ONERA (Office National de Recherches Aerospatiales), was developed in France in connection with the French Space Agency (CNES) and industry (SNECMA) for the understanding of Hall-effect plasma thrusters. Different activities are pursued in parallel: an experimental test of different laboratories' thrusters; the development of diagnostic techniques to characterize the plasma inside and outside the thrusters; and the development of simulation and modelling able to describe characteristics and evaluate the thrusters' performances. This paper will be focused on diagnostics systems implemented in the PIVOINE facility. Time- and space-resolved measurements of the ion beam energy, distribution electron density and concentration in the plume are performed with a retarding potential analyser (RPA) and Langmuir probes mounted on a 2.5 m movable drive. The thruster can be moved axially to allow a 40×90 cm2 exploration of the plume. The investigation of the plasma inside the thruster is made by optical diagnostics. A CCD camera used in fast imaging mode is set outside the tank. The 45° sight axis allows an internal view of the thruster's channel. Furthermore, a spectroscopic analysis is made by focusing the channel's light to a set of optical fibres connected to an imaging spectrometer equipped with a CCD camera. A specific laboratory thruster of 100 mm external diameter called SPT100-ML was studied in more detail, this model being designed to allow the implementation of optical fibres and wall probes diagnostics inside the channel's thruster. The stationary plasma thruster discharge is almost always characterized by low-frequency instabilities of the order of 10 kHz where the discharge current can reach a very high instantaneous level. The variation of the discharge and ion beam flux currents has been related to the spatiotemporal dynamic of the plasma inside the thruster's channel. The main features are explained by a one-dimensional (1D) hybrid model and a 1D particle-in-cell-Monte Carlo model. A new thruster, working at a very low fluctuation level with a low angular divergence ion beam, is now under investigation in connection with SNECMA.