Enrico Chesta

A characterization of plasma fluctuations within a Hall discharge

Experimental results are presented for studies of low (2-20 kHz) and intermediate-frequency (20-100 kHz) oscillations in crossed-field closed electron-drift Hall discharges. Conditional sampling using two electrostatic probes is used to identify and extract properties of coherent structures associated with the propagation of azimuthal and longitudinal instabilities within the discharge channel. The azimuthal component phase velocities are determined for a wide range of wave frequencies and over characteristic regimes of operation of these devices. A variety of propagation modes are observed and analyzed, including the appearance of an induced mode due to the presence of the probes themselves. This later result is believed to be the first direct evidence of how fluctuations can be influenced in these Hall discharges using relatively simple actuation methods.

Stability of a magnetized Hall plasma discharge

Using recent experimental data on the time-averaged, spatially varying plasma properties within a Hall discharge plasma, we present in this article, a theoretical study of the response of this plasma to small (linear) perturbations in its properties. As a starting point for this analysis, we assume a two-dimensional fluid description that includes a simplified equation for the electron energy, and constrain the azimuthal wave vector such that we excite only the dominant (m=1) azimuthal modes. The growth rate and frequencies of predominantly axial and azimuthally propagating plasma disturbances are obtained by numerical solution of the resulting eigenvalue problem under a quasiuniform plasma condition, along the entire discharge channel. The results identify the persistence of a low frequency instability that is associated with the ionization process, concentrated largely in the vicinity of the exit plane, where the magnetic field is at its maximum value, consistent with experimental observations for the r...

Modeling of FEEP Plume Effects on MICROSCOPE Spacecraft

Several aspects of plume effects of field-effect electric propulsion (FEEP) were studied. We first estimated the contamination by cesium deposit due to charge exchange of fast ions and slow neutrals and to direct neutral impingement. Levels are rather low, with local maximums of a few tens or hundreds of angstroms per year, and not much more than 1-10 Aring farther from thruster nozzles. Neutralization of the ions emitted by FEEPs was also addressed, both concerning FEEP ion space-charge compensation and spacecraft net current, i.e., the floating potential issue. With the presence of a cathode-grounded neutralizer, the spacecraft was shown to float somewhat negatively with little dependence on the ambient environment.

Neutralization for Micro Propulsion - Experiments and SPIS Simulations

This paper presents experimental and numerical investigations conducted on a neutralizer devoted to spacecraft micro propulsion. This neutralizer is composed of an emitting heated cathode coupled with an extractor electrode. The electron current produced is used to diminish the negative charging of spacecrafts whose propulsion systems are based on electrostatic acceleration of ions or charged particles. Experiments are conducted in a space plasma chamber simulating the Low Earth Orbit environment. The influence of various parameters such as plasma density and electrode voltages is determined. A complementary approach consists in performing numerical simulations of the neutralizer interaction with plasma. Experimental and numerical results are used to determine the neutralizer behaviour in flight conditions. Finally, the floating potential of the drag free spacecraft MICROSCOPE is estimated using an analytical model combined with the experimental results.

Numerical Studies of Instabilities in a Magnetized Hall Discharge

An analysis is presented of low to moderate-frequency wave propagation in Hall thruster channels. Starting with extensive experimental data on the time-averaged, spatially varying plasma properties within a Hall discharge plasma, we subject this plasma to small (linear) perturbations in its properties. For this perturbation analysis, we assume a two-dimensional fluid description that includes a simplified equation for the electron energy. The azimuthal wavenumber is selected to be resonant with the channel circumference, and the growth rate and frequencies of propagating plasma disturbances are obtained by numerical solution of the resulting eigenvalue problem under a quasi-uniform plasma condition, along the entire discharge channel. The results predict the possible emergence of low (10-100 kHz), moderate (100 kHz-10 MHz), and high (>10 n MHz) frequency instabilities concentrated largely in the vicinity of the exit plane. The low frequency instability, which is commonly seen in experiments, is found to be associated with the ionization process. While it is intrinsic to the discharge as an m = 1 azimuthal disturbance, the analysis performed here shows that it is also expected to be excited as an m = 4 mode, perhaps due to the four-fold symmetry of the magnetic circuit, and an m = 12 mode, due to the separation of the probes, consistent with recent experimental observations.

Fluctuation-induced electron transport in closed-drift Hall discharges

Summary form only given. Hall discharges are presently under development for use in space propulsion applications. These discharges exhibit characteristic cross-field transport, which is enhanced by fluctuations in the electric field and plasma density. Our recent electrostatic probe and laser-induced fluorescence measurements in closed-drift coaxial geometry discharges indicate that the effective Hall parameter ranges from 1-20. This value is much less than that based on classical electron transport, and while it brackets the value typical of anomalous Bohm transport, it is found to vary significantly along the axial direction of the discharge, increasing to unusually low values near the anode. In this paper, we shall present a comprehensive analysis of a large collection of data, which allows the determination of the effective Hall parameter from measured steady-state discharge properties along the channel. The dispersion characteristics of these fluctuations, believed to be responsible for the unusually low Hall parameters, are studied with negatively-biased ion collection probes. The measured oscillatory behavior of these discharges are interpreted using a linear perturbation analysis of the governing fluid equations for a magnetized, cylindrical plasma in which there are axial gradients in the plasma properties.