M. Dudeck

Wall material effects in stationary plasma thrusters. II. Near-wall and in-wall conductivity

Simulations and experimental characterizations of a stationary plasma thruster are compared for four different wall materials to investigate near-wall conductivity (dielectric materials) and in-wall conductivity (conducting materials) in such a discharge. Using a one-dimensional transient fluid model that takes into account a possible electron temperature anisotropy, it is shown that electron-wall backscattering plays a crucial role by maintaining a relatively high electron temperature along the magnetic field lines which in turn drives large electron currents toward the walls. The large differences in discharge current observed experimentally for the dielectric materials are qualitatively recovered, confirming that near-wall conductivity results from the combined effects of secondary electron emission and electron backscattering. A clear correlation is found between the appearance of space charge saturation at the walls and a jump of the discharge current observed in experiments when varying the discharg...

Wall material effects in stationary plasma thrusters. I. Parametric studies of an SPT-100

The operation of a laboratory version of the flight-qualified SPT-100 stationary plasma thruster is compared for four different discharge chamber wall materials: a boron nitride–silica mixture (borosil), alumina, silicon carbide, and graphite. The discharge is found to be significantly affected by the nature of the walls: changes in operating regimes, up to 25% variations of the mean discharge current, and over 100% variations of the discharge current fluctuation amplitude are observed between materials. Thrust, however, is only moderately affected. Borosil is the only material tested that allows operating the thruster at a low mean current, low fluctuation level and high thrust efficiency regime. It is suggested that secondary electron emission under electron bombardment is the main cause of the observed differences in discharge operation, except for graphite, where the short-circuit current inside the walls is believed to play a major role. It is also suggested that the photoelectric effect, which has a...

Transit-time instability in Hall thrusters

Longitudinal waves characterized by a phase velocity of the order of the velocity of ions have been recurrently observed in Hall thruster experiments and simulations. The origin of this so-called ion transit-time instability is investigated with a simple one-dimensional fluid model of a Hall thruster discharge in which cold ions are accelerated between two electrodes within a quasineutral plasma. A short-wave asymptotics applied to linearized equations shows that plasma perturbations in such a device consist of quasineutral ion acoustic waves superimposed on a background standing wave generated by discharge current oscillations. Under adequate circumstances and, in particular, at high ionization levels, acoustic waves are amplified as they propagate, inducing strong perturbation of the ion density and velocity. Responding to the subsequent perturbation of the column resistivity, the discharge current generates a standing wave, the reflection of which sustains the generation of acoustic waves at the inlet ...

Spectral analysis of Hall-effect thruster plasma oscillations based on the empirical mode decomposition

Hall-effect thruster plasma oscillations recorded by means of probes located at the channel exit are analyzed using the empirical mode decomposition (EMD) method. This self-adaptive technique permits to decompose a nonstationary signal into a set of intrinsic modes, and acts as a very efficient filter allowing to separate contributions of different underlying physical mechanisms. Applying the Hilbert transform to the whole set of modes allows to identify peculiar events and to assign them a range of instantaneous frequency and power. In addition to 25kHz breathing-type oscillations which are unambiguously identified, the EMD approach confirms the existence of oscillations with instantaneous frequencies in the range of 100–500kHz typical for ion transit-time oscillations. Modeling of high-frequency modes (ν∼10MHz) resulting from EMD of measured wave forms supports the idea that high-frequency plasma oscillations originate from electron-density perturbations propagating azimuthally with the electron drift v...

Determination of the electron anomalous mobility through measurements of turbulent magnetic field in Hall thrusters

Measurements of the turbulent magnetic field in a Hall thruster have been carried out between 1kHz and 30MHz with the aim of understanding electron transport through the magnetic field. Small detecting coils at the exit of the accelerating channel and outside of the ionic plume were used to characterize various instabilities. The characteristic frequencies of the observed power spectral densities correspond to known classes of instabilities: low frequency (20–40kHz), transit time (100–500kHz), and high frequency (5–10MHz). A model of the localized electron currents through a magnetic barrier is proposed for the high-frequency instability, and is found to be in good quantitative agreement with the observations. Based on the measured high-frequency turbulent magnetic field, the turbulent electric field is estimated to be about 1V∕cm outside of the plume and ranges from 10to102V∕cm at the channel midradius at the exit of the thruster. The “anomalous” electron collision frequency, related to the high-frequenc...

Low-frequency electron dynamics in the near field of a Hall effect thruster

Time-resolved electrostatic probe measurements were performed in the near field of a SPT100-ML Hall effect thruster in order to investigate electron properties changes on a microsecond time scale. Such measurements allow one to monitor the electron temperature Te, the electron density ne, as well as the plasma potential Vp during a time period that corresponds to one cycle of a breathing-type plasma oscillation with f≈15–30kHz. Although Te(t) stays constant in time, ne(t) and Vp(t) oscillate with the discharge current waveform frequency. The observed time delay between ne and anode discharge current (Ida) waveforms, which is of approximately 7μs, is linked to the ion transit time from the ionization layer to the probed near-field region. The same time gap is measured between Vp(t) and Ida(t), however Vp(t) and ne(t) are in phase opposition. The electron density reaches its highest value at the very moment ions are ejected out of the thruster discharge chamber, which also corresponds to the instant the cat...

Simple model for the electronic relaxation during the expansion of a plasma generated in an Ar–O2 mixture

The local properties of a plasma free jet are calculated with a collisional-radiative model where electron density and temperature are included as parameters. The kinetic equations are written for all the electronic states of the atomic species Ar and O. In the first step, only excitation and de-excitation by electron collisions are taken into account, together with spontaneous radiative decay. This allows the problem to be treated as a linear system of equations represented by a matrix. In the second step, collisional processes with atoms and residual molecules are included. The number of adjustable parameters is limited to the normalization factor of the reaction rate constants for excitation by electrons, the degree of dissociation of oxygen at the nozzle exit, and to the relative number of singly charged ions for oxygen and argon along the axis. Electron temperature and density are measured experimentally, or obtained separately from another calculation. Then, the population density of any level can b...

Magnetohydrodynamic velocimetry of a low‐pressure plasma jet

The velocity of a steady, low‐pressure (≊0.1 Torr) plasma jet is measured by analyzing the local response of the medium to the action of an external magnetic field set up by two Helmholtz coils placed at the edge of the jet, yielding a field of 0.92 G/A. The induced electric field is measured by two floating‐potential electrostatic probes. The mean velocity of the electrons (between 1000 and 5000 mu2009s−1) is found from the generalized Ohm’s law.

IS NEAR-WALL CONDUCTIVITY A MISNOMER?

The near-wall conductivity theory outlined by Morozov in 1968 constitutes one of the most prominent outcome of the early investigations of Hall thrusters, which describes the cross-eld electron diffusion mechanism induced by electron-wall collisions. The present work generalizes the near-wall conductivity theory to the case of a non-zero sheath potential at the walls. The general solution is found to differ qualitatively from the no-sheath solution of the classical theory, and puts into question the hypothesis that the so-called near-wall currents are conned to the close vicinity of the walls.

Laser Sustained Plasma Free Jet and Energetic Atom Beam of Pure Argon or Oxygen Seeded Argon Mixture

As emphasized by Schram and co-workers above in this volume, the development of the plasma generation and expansion is relevant to many scientific subjects and a large variety of important applications. This article deals with the physics of the translational and electronic relaxations in plasma free jets, but the results are of potential interest also for the production of plasma flows, as used in ground test facilities for the simulation of spacecraft-atmosphere interactions. This is of importance to study the material degradation and surface modifications due to the very agressive environment in space experiments.