Casey C. Farnell

Sputtering Studies of Multi-Component Materials by Weight Loss and Cavity Ring-Down Spectroscopy (Postprint)

Abstract : We report sputtering studies of multi-component spacecraft materials. We employ two complementary diagnostic methods: weight loss measurements and cavity ring-down spectroscopy (CRDS). The weight loss measurements provide total sputter yields as a function of ion energy and incidence angle. We present sputter yields from weight loss measurements for xenon ion sputtering of molybdenum, quartz, boron nitride, and kapton. The CRDS provides species-specific sputtering data (number density and velocity) as well as information on the differential (angular) sputtering distributions. We present CRDS results for the sputtering of molybdenum (from a molybdenum sample), and demonstrate measurements of multi-component materials by measuring the sputtering of chromium, iron, and molybdenum from Inconel 718.

Comparison of hollow cathode discharge plasma configurations

Hollow cathodes used in plasma contactor and electric propulsion devices provide electrons for sustaining plasma discharges and enabling plasma bridge neutralization. Life tests show erosion on hollow cathodes exposed to the plasma environment produced in the region downstream of these devices. To explain the observed erosion, plasma flow field measurements are presented for hollow cathode generated plasmas using both directly immersed probes and remotely located plasma diagnostics. Measurements on two cathode discharge configurations are presented: (1) an open, no magnetic field configuration and (2) a setup simulating the discharge chamber environment of an ion thruster. In the open cathode configuration, large amplitude plasma potential oscillations, ranging from 20 to 85 V within a 34 V discharge, were observed using a fast response emissive probe. These oscillations were observed over a dc potential profile that included a well-defined potential hill structure. A remotely located electrostatic analyzer (ESA) was used to measure the energy of ions produced within the plasma, and energies were detected that met, and in some cases exceeded, the peak oscillatory plasma potentials detected by the emissive probe. In the ion thruster discharge chamber configuration, plasma potentials from the emissive probe again agreed with ion energies recorded by the remotely located ESA; however, much lower ion energies were detected compared with the open configuration. A simplified ion-transit model that uses temporal and spatial plasma property measurements is presented and used to predict far-field plasma streaming properties. Comparisons between the model and remote measurements are presented.

EXPERIMENTAL EVALUATION OF SUB-SCALE CBIO ION OPTICS SYSTEMS

An experimental study of a sub-scale carbon-based ion optics (CBIO) design proposed by the Jet Propulsion Laboratory (JPL) is described. Perveance and crossover beamlet current limits, above and below which accelerator grid direct impingement currents rise, are documented over wide throttling ranges. Test data reported for assemblies with 1, 7, 19, and 37 aperture pairs suggest that gridlets containing only a small fraction of the total number of apertures in a full-sized optics system can be used to investigate the behavior of larger systems. A study of the effects of grid spacing on beamlet current-limit data that was performed using different types of mica spacers is also presented. Comparisons to numerical simulations suggest that the mica spacer that contained organic binder material changed dimensions slightly during testing. The second type of iso-mica, which is intended for high temperature service, was much more stable and better suited to the hotter test conditions that occur during operation at high specific impulse. In addition, results are presented that examine the onset of the electron backstreaming condition that can develop at the end of life (EOL) in some mission scenarios. Backstreaming voltage limits were measured for grid spacings that simulate their transient behavior and for accel holes with progressively larger accel diameters that simulate the effects of accel barrel erosion over life.

Measurement of Ion Energy Distributions Produced within an NSTAR Discharge Chamber

*† Measurements are presented of ion energy distributions produced within a prototype NSTAR discharge chamber as a function of its operational parameters. The goal of this effort is to document the ion energy distribution characteristics and determine features that could cause rapid sputter erosion of discharge chamber components. Ion energy distributions were measured with a Comstock model AC-901 electrostatic analyzer (ESA) that was located outside of the discharge chamber and sighted through a slit that was cut in the side of the anode and along a diameter across the pseudo-screen grid. Important results are that, at some locations within the discharge chamber at high discharge current operating conditions, high energy ion groups (with drift energies of order 100 eV that are separate and distinct from groups at the discharge energy) can be produced that would cause rapid sputter erosion to most discharge chamber thruster components comprised of molybdenum, stainless steel, tantalum, and tungsten. Under nominal operating conditions, no significant high-energy groups (separate from the discharge ion signal) are detected. However, high energy tails on the discharge ion signal are detected that display effective temperatures of order 10 eV. An additional result is that the discharge ion signal appears to be Maxwellian and ion temperatures of 3 to 4 eV have been measured. In some situations, the discharge ion signal is much stronger than high energy ion groups present to the extent that the discharge ion group tail would cause more sputter erosion than the high energy group (at the location of the ESA).

Extracted Current, Bias Voltage, and Ion Production of Cathodic Hollow-Cathode-Driven Plasma Contactors

Plasma properties on extracted electron current–bias voltage characteristics are presented for three selected plasma contactor configurations: a hollow-cathode-only concept, a motive discharge chamber concept, and a passive discharge chamber concept. Measurements were used to demonstrate how one could achieve low impedance performance without being affected by space plasma properties or by consuming significant propellant mass and power. A one-dimensional model was applied to describe the plasma expansion process that occurs downstream of a cathodic contactor. The model matched well with experimental trends and indicated that the plasma ion production rate within and nearby the plasma contactor dominated the emission-bias behavior of the devices. High ion production rate at a given total mass flow resulted in high propellant utilization and low discharge loss. However, plasma potential measurements showed that an anode sheath limited the maximum propellant unitization to less than ∼75% and led to a foldba...

The plasma properties and electron emission characteristics of near-zero differential resistance of hollow cathode-based plasma contactors with a discharge chamber

The formation of electron emission-bias voltage (I-V) characteristics of near-zero differential resistance in the cathodic plasma contactor for bare electrodynamic tether applications, based on a hollow cathode embedded in a ring-cusp ionization stage, is studied. The existence of such an I-V regime is important to achieve low impedance performance without being affected by the space plasma properties for a cathodic plasma contactor. Experimental data on the plasma structure and properties downstream from the ionization stage are presented as functions of the xenon flow rate and the electron emission current. The electrons were emitted from the cathode to the cylindrical vacuum chamber wall (r = 0.9 m) under ≈10−5 Torr of vacuum pressure. The ring-cusp configuration selected for the plasma contactor created a 125-Gauss axial field near the cathode orifice, along with a large-volume 50-Gauss magnitude pocket in the stage. A baseline ion energy cost of ≈300 eV/ion was measured in the ionization stage when n...

Characterization of an Atomic Oxygen Plasma Source for Ground-Based Simulation of the LEO Neutral Environment

A magnetically filtered oxygen plasma source has been developed and characterized for the purpose of simulating the low Earth orbit environment. The plasma source can be operated at a variety of discharge currents and gas flow rates, of which the plasma parameters downstream of the source are dependent. The characteristics of the generated plasma were examined as a function of these operating parameters to optimize the production of O + ions with energy relevant to LEO applications, where the ram energy of the ions due to the motion of the satellite relative to the LEO plasma is high (e.g. 7800 m/s, which corresponds to approximately 5 eV of kinetic energy for O+ ions). The plasma downstream of the source consists of streaming ions with energy of approximately 5 eV and ion species fraction that is approximately 90% O + .

Ion Emissive Membranes for Propulsion Applications

Experiments show electrostatic thrusters with components such as the discharge chamber or acceleration channel, solenoid or permanent magnets, hollow cathode, and keeper can be replaced by a simple, propellant‐selective, solid‐state, ion‐conducting membrane (Wilbur et al., 2007; Wilbur, Wilson, and Williams, 2005). In addition, analyzes show these membranes can be shaped, structured, and assembled into integrated thruster systems that will operate at much greater thrust densities and thruster efficiencies than those for state‐of‐the‐art, Hall and ion thrusters (Wilbur, Farnell, and Williams, 2005). The implications of these findings are revolutionary and promise an electrostatic propulsion system much less massive, more reliable, and less costly than ion and Hall thruster systems as they can be fabricated readily using traditional ceramic manufacturing techniques. The status of the Emissive Membrane Ion Thruster (EMIT) concept is described and recent measurements are used to estimate the performance of a ...

Multi-Axis Plasma Profiling System for Characterization of Plasma Thruster Plumes

A three-degree of freedom, actuator-based system is described for positioning an ion current density probe within the near field region of plasma and ion thrusters. This system improves upon existing beam profiling systems which usually characterize plasma and ion beams by sweeping a Faraday probe through a single motion at a fixed axial (or radial) distance downstream of a device under test. Fully automated computer control is used for positioning the probe, data collection and analysis, and to interface via communication ports to power supply and vacuum chamber control systems. The three-axis probe actuator system is capable of obtaining improved ion beam data critical for calculating plume uniformity, beam symmetry, total energetic particle flux through a given plane, beam divergence, beam centroid, thrust vector direction, etc. The high resolution data collected with this tool are planned for use in validating numerical simulations of the plasma plume properties of ion and plasma thrusters. Experimental results are presented that demonstrate the capabilies of the beam profiling system on a Veeco Instruments 5-cm DC ion source. A total of 15 operating conditions were characterized by taking planar (x-y) scans of the ion beam at axial (z) distances ranging from 12.5 to 32.5 cm. Beam ion energies ranging from 200 to 1000 eV were evaluated at beam currents from 30 to 90 mA. The measurements were analyzed to calculate the effects of beam ion energy, beam current, and charge exchange and ion scattering on the beam shape and attenuation. The attenuation data were reduced to determine estimates of charge exchange cross section, and good agreement was observed with values available in the literature.

Non-invasive Hall current distribution measurement in a Hall effect thruster

A means is presented to determine the Hall current density distribution in a closed drift thruster by remotely measuring the magnetic field and solving the inverse problem for the current density. The magnetic field was measured by employing an array of eight tunneling magnetoresistive (TMR) sensors capable of milligauss sensitivity when placed in a high background field. The array was positioned just outside the thruster channel on a 1.5 kW Hall thruster equipped with a center-mounted hollow cathode. In the sensor array location, the static magnetic field is approximately 30 G, which is within the linear operating range of the TMR sensors. Furthermore, the induced field at this distance is approximately tens of milligauss, which is within the sensitivity range of the TMR sensors. Because of the nature of the inverse problem, the induced-field measurements do not provide the Hall current density by a simple inversion; however, a Tikhonov regularization of the induced field does provide the current density distributions. These distributions are shown as a function of time in contour plots. The measured ratios between the average Hall current and the average discharge current ranged from 6.1 to 7.3 over a range of operating conditions from 1.3 kW to 2.2 kW. The temporal inverse solution at 1.5 kW exhibited a breathing mode frequency of 24 kHz, which was in agreement with temporal measurements of the discharge current.