André Bouchoule

Industrial developments of scientific insights in dusty plasmas

In the last twelve years the research on dusty plasmas was at first required to understand the dust-induced problems in low-pressure plasma tools and surface processing technologies. Meanwhile, the research on physics and chemistry of dusty plasmas led to the discovery of a fascinating domain. The scientific knowledge acquired on dusty plasmas leads to new ideas for applications and the aim of this contribution is to present examples of such connections. An exhaustive review of the matter would be clearly impossible and only three aspects are emphasized. The first concerns plasmas where nanosized particles are grown and their use for deposition of thin films with new properties. The second concerns situations where a significant amount of dust is stored in a discharge, inducing high electron energies, with strong impact on plasma chemistries. Finally, the last one will concern the well-known confinement of dust particles by electrostatic sheaths and some examples of its actual or potential applications.

Transient phenomena in closed electron drift plasma thrusters : insights obtained in a french cooperative program

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.

Spontaneous oscillations in a Hall thruster

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.

Experimental Insights Into High-Frequency Instabilities and Related Anomalous Electron Transport in Hall Thrusters

The properties of high-frequency (HF) instabilities (f > MHz) in Hall thrusters are reviewed on the basis of a large experimental data set previously obtained with the use of antennas, probes, and magnetic coils. The phenomenological representations of their physical sources are discussed. The development of these HF instabilities results in anomalous electron transport, and the corresponding transport coefficients are evaluated from these experimental data.

Ion extraction from a helicon source and PIC simulation

The design of an ion extraction system often requires the help of computer codes. This article describes an experimental setup based on a helicon source equipped with a three-grid extraction system and that is used as an ion source. A numerical approach based on a particles in cells (PIC) method is introduced. The results obtained with the ion source for different gases are compared with the simulation and are used in order to validate the computer code. The numerical code is the first component of an ion source design package that deals with grid erosion and ion beam current distribution on the target.

Laser injection of ultra-short electron bursts for the diagnosis of Hall thruster plasma

The present developments of Hall thrusters for satellite control and space mission technologies represent a new step towards their routine use in place of conventional thermal thrusters. In spite of their long R and D history, the complex physics of the E ? B discharge at work in these structures has prevented, up to now, the availability of predictive simulations. The electron transport in the accelerating layers of these thrusters is one of the remaining challenges in this direction. From the experimental point of view, any diagnostics of electron transport and electric field in this critical layer would be welcome for comparison with code predictions. Appropriate diagnostics are difficult, due to the very aggressive local plasma conditions. This paper presents the first step in the development of a new tool for characterization of the plasma electric field in the very near exhaust thruster plume and comparison with simulation code predictions.The main idea is to use very short bursts of electrons, probing local electron dynamics in this critical plume area. Such bursts can be obtained through photoelectric emission induced by a UV pulsed laser beam on a convenient target. A specific study, devoted to the characterization of the electron burst emission, is presented in the first section of the paper; the implementation and testing of the injection of electrons in the critical layer of Hall thruster plasma is described in the second section. The design and testing of a fast and sensitive system for characterizing the transport of injected bursts will be the next step of this program. It requires a preliminary evaluation of electron trajectories which was achieved by using simulation code. Simulation data are presented in the last section of the paper, with the full diagnostic design to be tested in the near future, when runs will be available in the renewed PIVOINE facility. The same electron burst injection could also be a valuable input in the present discussion on the physics of the 5?10?MHz instability observed in almost all Hall thrusters.

High-frequency instabilities and low-frequency dynamics in Hall thruster plasma

Investigation of the transients of two time scales was carried out in the SPT-100ML Hall thruster. Detailed experimental studies of the high-frequency instabilities (1-10MHz) were carried out with six antennas installed at various longitudinal and azimuthal positions into the external ceramic wall of the accelerating channel. A complex structure of the signal was observed, varying from the periodic one with clearly defined peaks and a period representing the time of single turn of the electrons rotating at the drift velocity in the crossed electric and magnetic fields, to the structure with much shorter correlation times. Correlation between these instabilities and low-frequency (10-30 kHz) transients in thruster current was studied. Features of HF signals have been investigated for different operation modes of the thruster: discharge voltage Ud=200-450V, anode mass flow rate

Low Frequency Dynamical Behavior Of The Plasma At The Exit Plan Of A Hall Effect Thruster

A large research programme on Hall thrusters has been initiated in France in 1996. This programme includes modelling, as well as theoretical and experimental approaches. These researches are coordinated in the frame of a national Research Group. Recent results obtained at the laboratoire d’Aerothermique taking part to the activities of this group are presented. A short description of electric propulsion and especially the behaviour of a Hall Effect Thruster are presented in the first part of this paper. In the second part, experimental time resolved measurements performed by means of electrostatic probe are presented to show the dynamical behaviour of electron characteristics and compared with the recorded discharge current and potential. These results describe to ion and electron properties in the plasma flow in the surroundings of the thruster channel exhaust.

Extraterrestrial plasma crystals

Some 10 years ago scientific interest in dusty plasmas was motivated mostly by the needs of the microelectronics industry. In 1994, however, researchers at three laboratories almost simultaneously found that these low-pressure gas discharges can organize themselves into crystal-like structures. This opened the way to spectacular developments in controlled crystal growth, such as the synthesis of thin films for solar-cell applications.

TIME RESOLVED DIAGNOSTIC OF ION BEAM ENERGY DISTRIBUTION FUNCTION FOR PLASMA THRUSTERS

In order to test time resolved diagnostics, a controlled energy fluctuation source has been designed. This source is able to give beams of fluctuating energy in different time controlled regimes. Two operating modes have been tested. The first one is a square wave modulation of the ion beam energy. The second one is a triangular shaped energy variation in time. This source has been used to qualify a time resolved Retarding Field Energy Analyzer. Measuring the energy distribution functions of the ion beam, the RFEA is able to restitute both energy shapes at a 30-kHz frequency thanks to an appropriate data acquisition and treatment system.