Variable-specific-impulse electric propulsion systems can provide important advantages and cost reductions to any scale of planetary mission. The first part of this paper describes their potential benefits, with special emphasis on mission analysis aspects.The second part identifies the physical limitations of traditional electric propulsion technologies and presents some preliminary development results achieved within the frame of ongoing European activities.

This paper presents some aspects of the research developed in the frame of a coordinated program launched in France in 1996 and devoted to plasma thrusters for space technologies. Relevant results of physical studies have been selected from the literature with the addition of recent original results. The thrusters within the scope of this research are diagnostic equipped versions of industrial realizations, in a thrust level range of 0.1 N and electrical power 1.5 kW. The optical and electrical diagnostics concern studies of the thruster plasma and of the thruster plume. Transient phenomena in these two regions, related to discharge current fluctuations or oscillations on a typical time scale of 40 µs, have been space-time characterized. This has been achieved by developing a large panel of diagnostics including RFEA, Langmuir probes, OES, fast camera imaging and electron drift Hall current probe. They lead to a coherent representation of these phenomena , in rather good qualitative agreement with 1D modelling. But they emphasize also the importance of 2D effects. Insights obtained through combined LIF (on Xe+ ions) and OES diagnostics are also presented. They concern the ionization-acceleration region in the thruster plasma, where intrusive diagnostics are disturbing in nature, and open a new step for a significant improvement of the detailed understanding of these thrusters. Such improvements are required when looking at the final goal of a predicable modelling simulation able to help the design of optimized structures at various thrust levels, in spite of the important work devoted to these devices in the former USSR and by Russian teams in Moscow at the MIREA, MAI-RIAME and KOURCHATOV Institutes.

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 x 90 cm(2) 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 degrees 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.

Stationary Plasma Thrusters are under active development as alternative systems for satellites control and deep space programs. The basic physics involved in these crossed ExB Xenon discharges is a complex one, involving both pure plasma and plasma surface phenomena. The main one is the so called ionization instability associated with fluctuations or oscillations of the discharge current, at few tens kHz. The level of these (I-M/I-m) is really significant at the thrust optimized operating point. A detailed view on the dynamic behavior of the discharge channel and plasma jet has been acquired with a time resolution of 1 mus by using a specifically designed ion beam analyzer in the jet and a fiber optic array viewing the discharge channel. The data give new insights on space and time phenomena connected with the ionization dynamics in SPT thrusters, at least for two specific regimes hereafter called fluctations (I-M= 8A, I-m = 3A) and oscillations (I-M= 18 A, I-m = 2 A) They emphasize the requirement of 2D or 3D models and simulations for a better insight on the physics of such devices.

Low frequency oscillations in a stationary plasma thruster (SPT) have been studied using optical diagnostics and numerical models. The regime of low frequency, large amplitude current oscillations has been especially investigated. The current oscillations are associated with strong oscillations of the light emitted by the plasma in that region. A quasi-neutral hybrid model of the plasma can reproduce these plasma oscillations and suggests that they are due to an ionization instability in the exhaust region associated with the depletion of neutral atoms by electron impact ionization.