Emily C. Fossum

Development and Testing of a Prototype Bismuth Cathode for Hall Thrusters

Using bismuth in place of gases such as xenon for Hall thruster propellant could potentially offer both physical and economical gains. As research continues to develop Hall thrusters that are fueled with bismuth, it will become advantageous to maintain one propellant supply rather than multiple supplies for the anode and cathode. The recent development of a bismuth Hall thruster at Michigan Tech, operated using a xenon LaB6 cathode, provided a motive to explore the feasibility of developing an entire bismuth system. This paper provides a background on the development and operation of a bismuth vapor LaB6 cathode. Comparisons of operating parameters are provided for the cathode running on xenon and bismuth propellants along with a description of the mass flow technique used. Complications in determining and controlling the mass flow rate are presented as well.

An Electron Trap for Studying Cross-Field Mobility in Hall Thrusters

An electron-trapping apparatus was constructed to study electron dynamics in the defining fields of a Hall-effect thruster. An electron plasma is confined using vacuum electric and magnetic fields in the absence of neutral-plasma effects and dielectric walls, which are present in a typical Hall thruster. Crossfield electron mobility was investigated in response to changes in magnetic and electric field strengths and background neutral density, and has been found to be much larger than what classical theory predicts. Experimental mobility is on the same order as Bohm mobility; however, the agreement with Bohm mobility is suspected to be coincidental, and the elevated mobility observed is thought to be due to static electric field asymmetries or external electrostatic perturbations, which can be correlated with electrostatic disturbances found in a Hall thruster.

Characterization of Near Field Plasma Environment of a Hollow Cathode Assembly

Cathode keeper erosion has been identified as a possible limit to thruster lifetimes. Therefore, it is important to characterize a cathode discharge at various operating conditions in order to identify potential erosion mechanisms. In this investigation the near field plasma environment of a hollow cathode assembly was examined under several operating conditions in the Isp Lab’s Xenon Test Facility (XTF) at Michigan Tech. This examination focused on the region within 3 cm of the cathode keeper orifice. In this paper the results from six operating conditions are summarized giving values of plasma potential, electron temperature and number density. It was found that electron density did not vary significantly with changes in xenon flow rate and discharge current. It was also found that as flow rate increased, electron temperature decreased, resulting in a shorter Debye length and a flatter potential structure near the orifice; as discharge current increased, electron temperature also decreased.

Benchtop energetics progress

We have constructed an apparatus for investigating the reactive chemical dynamics of mgscale energetic materials samples. We seek to advance the understanding of the reaction kinetics of energetic materials, and of the chemical influences on energetic materials sensitivity. We employ direct laser irradiation, and indirect laser-driven shock, techniques to initiate thin-film explosive samples contained in a high-vacuum chamber. Expansion of the reacting flow into vacuum quenches the chemistry and preserves reaction intermediates for interrogation via time-of-flight mass spectrometry (TOFMS). By rastering the sample coupon through the fixed laser beam focus, we generate hundreds of repetitive energetic events in a few minutes. A detonation wave passing through an organic explosive, such as pentaerythritol tetranitrate (PETN, C5H8N4O12), is remarkably efficient in converting the solid explosive into final thermodynamically-stable gaseous products (e.g. N2, CO, CO2, H2O…). Termination of a detonation at an ex...

Design and Construction of an Electron Trap for Studying Cross- Field Mobility in Hall Thrusters

*† Excessive cross-field electron mobility in Hall thrusters has a negative effect on thruster efficiency and has been shown experimentally to be much larger than predicted by classical collisional transport theory. An electron trapping apparatus was constructed at Michigan Tech’s Isp Lab in order to study electron dynamics in the defining electric and magnetic fields of a Hall-effect thruster. This apparatus was designed to stably trap a non-neutral electron plasma in a confining volume in order to study these dynamics in a highly controlled environment. Electrons are confined using only electric and magnetic fields in the absence of ions and dielectric walls, which are present in a typical Hall thruster. Mobility studies on a low-density, non-neutral plasma provide several advantages over a typical Hall thuster’s quasi-neutral plasma, including a well-defined electric field and the ability to take internal electrostatic probe measurements in the “acceleration” region. Cross-field electron mobility was investigated in response to magnetic and electric field strengths and background neutral density. Experimental design considerations including loading mechanisms, trapping potential, magnetic field design, calibration, and diagnostic techniques are presented along with preliminary experimental results. In this investigation, measured cross-field mobility is much larger than classical theory predicts and appears to be consistent with Bohm-like mobility rather than classical mobility.

Mobility Studies of a Pure Electron Plasma in Hall Thruster Fields

An electron trapping apparatus was constructed in order to study electron dynamics in the defining electric and magnetic fields of a Hall-effect thruster. The approach presented here decouples the cross-field mobility from plasma effects by conducting measurements on a pure electron plasma in a highly controlled environment. Dielectric walls are removed completely eliminating all wall effects; thus, electrons are confined solely by a radial magnetic field and a crossed, independently-controlled, axial electric field that induces the closed-drift azimuthal Hall current. Electron trajectories and cross-field mobility were examined in response to electric and magnetic field strength and background neutral density. Without wall effects or neutral plasma effects mobility is presumed to follow the classical mobility model. In the present research, measurement techniques are investigated, and results are verified against the classical model. Preliminary findings suggest that that the apparatus and techniques used will be valid for mobility studies in more complex field environments.

Time-of-flight mass spectrometry of laser exploding foil initiated PETN samples

We report the results of time-of-flight mass spectrometry (TOFMS) measurements of the gaseous products of thin-film pentaerythritol tetranitrate [PETN, C(CH2NO3)4] samples reacting in vacuo. The PETN sample spots are produced by masked physical vapor deposition [A.S. Tappan, et al., AIP Conf. Proc. 1426, 677 (2012)] onto a first-surface aluminum mirror. A pulsed laser beam imaged through the soda lime glass mirror substrate converts the aluminum layer into a high-temperature high-pressure plasma which initiates chemical reactions in the overlying PETN sample. We had previously proposed [E.C. Fossum, et al., AIP Conf. Proc. 1426, 235 (2012)] to exploit differences in gaseous product chemical identities and molecular velocities to provide a chemically-based diagnostic for distinguishing between “detonation-like” and deflagration responses. Briefly: we expect in-vacuum detonations to produce hyperthermal (v∼10 km/s) thermodynamically-stable products such as N2, CO2, and H2O, and for deflagrations to produce ...

Effects of Neutral Density on Electron Temperature and Mobility in a Crossed-field Trap

An electron trapping apparatus was constructed to emulate the electric and magnetic fields found in a Hall-effect thruster in order to investigate cross-field electron mobility. Anomalous mobility was previously observed in this device that is orders of magnitude higher than classical. The focus of this manuscript is to investigate the effect of neutral density on the electron temperature and cross-field mobility in the electron trap. It was found that electron temperature decreases with increasing neutral density. When electron temperature is taken into account in the calculation of classical mobility, trends are observed in this device that resemble classical scaling with neutral density; however, the magnitude of the observed mobility is 100 to 1,000 times higher than classically predicted. On further investigation of the electron temperature, it is determined that in some cases the electron temperature is much higher than would be possible if collisions were responsible for transport, as inelastic collisions, which prevail at higher electron energies, would cause electron cooling that is not seen here. Furthermore, an examination of the probe I-V characteristic reveals that the electron distribution function is highly non-Maxwellian in these cases, supporting a collisionless anomalous mobility.