Nathan Benjamin Meezan

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.

A low-power, linear-geometry Hall plasma source with an open electron-drift

The operating characteristics of a linear geometry Hall plasma source scaled to operate in the 50 to 100 Watt power range are described. Two thruster acceleration channels are implemented-one of alumina and one of boron nitride. Differences in operation with the two channel materials are attributable to differences in the secondary electron emission properties. In either case, however, operation is achieved despite the lack of a closed electron current drift in the Hall direction, suggesting that there is an anomalous axial electron mobility, due to either plasma fluctuations or collisions with the channel wall. Strong low frequency oscillations in the discharge current, associated with the depletion of propellant within the discharge, are seen to appear and vary with changes in the applied magnetic field strength. The frequency of this oscillatory mode is higher than that seen in larger (and higher power) discharges, due to the decreased residence time of the propellant within the channel. Linear geometry Hall thrusters permit simpler magnetic circuit configurations and enable stacking of multiple thrusters to provide modular arrays.

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...

Coherent structures in crossed-field closed-drift Hall discharges

The structure of a magnetized, collisionless crossed-field coaxial Hall discharge is investigated using low-impedance, negatively biased Langmuir probes. We present images depicting frequency-position renderings and cross-correlation plots, which identify coherent and random fluctuations in plasma density. These fluctuations are found to depend on the discharge operating conditions, and are believed to greatly affect cross-field electron transport.

Electron density measurements for determining the anomalous electron mobility in a coaxial Hall discharge plasma

A comprehensive analysis of measurements supporting the presence of anomalous cross-field electron mobility in Hall plasma accelerators is presented. Nonintrusive laser-induced fluorescence measurements of neutral xenon and ionized xenon velocities, and various electrostatic probe diagnostic measurements are used to locally determine the effective electron Hall parameter inside the accelerator channel. These values are then compared to the classical (collision-driven) Hall parameters expected for a quiescent magnetized plasma. The results indicate that in the vicinity of the anode, where there are fewer plasma instabilities, the electron-transport mechanism is likely elastic collisions with the background neutral xenon. However, we find that in the vicinity of the discharge channel exit, where the magnetic field is the strongest and where there are intense fluctuations in the plasma properties, the inferred Hall parameter departs from the classical value, and is close to the Bohm value of (omega(ce)tau)(eff) approximately 16. These results are used to support a simple model for the Hall parameter that is based on the scalar addition of the electron collision frequencies (elastic collision induced plus fluctuation induced), as proposed by Boeuf and Garrigues [J. Appl. Phys. 84, 3541 (1998)]. The results also draw attention to the possible role of fluctuations in enhancing electron transport in regions where the electrons are highly magnetized.

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.

Characterization of plasma disturbances in a Hall discharge using a double probe

An electrostatic double probe is used to explore the properties of plasma fluctuations in a closed electron-drift Hall discharge. Two negatively biased low-impedance probes separated in the azimuthal direction within the acceleration channel are used to identify coherent structures related to propagating plasma density disturbances. The time-average ion saturation current from a single probe is found to be dependent on both the axial and azimuthal position. The latter is due to azimuthal variations in the magnetic field strength, as the ion saturation current is found to increase at locations coincident with the positioning of the magnetic solenoids. The identified plasma disturbances from a single probe are also found to be strongly dependent on axial position, and where on the V-I envelope the discharge is operated. The strongest disturbances are located near the exit plane, where the magnetic field is the strongest. An analysis of the phase between frequency-resolved coherent disturbances identified on both probes separated azimuthally provides a preliminary mapping of the wave dispersion. At the lowest discharge operating voltage, where the Hall discharge is operating in the ionization branch of the V-I characteristic, this map of the wave dispersion shows that these structures have an associated randomness in propagation direction and/or propagation velocity.