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Williams, John D.

Publications and source records attributed to Williams, John D..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2019

Characterization of a Fixed-Volume Release System for Initiating an Arc Discharge in a Heaterless Hollow Cathode

Heaterless hollow cathodes may provide improved reliability, simplicity, and portability when compared with traditional heater-equipped hollow cathodes, traits which are well suited for low-power Hall-effect thrusters that are currently being developed for small satellite propulsion. Despite the advantages, there are concerns that the ignition process in heaterless hollow cathodes may impose an excessive burden on the propellant feed and/or electrical systems of a small satellite. To address this concern, a fixed-volume release flow protocol, which can be used to temporarily increase the propellant mass flow rate, was developed, modeled, and experimentally evaluated. The new protocol allowed for a heaterless hollow cathode to be ignited reliably with a moderate bias voltage and a minimal electrical power requirement. Specifically, a xenon fed heaterless hollow cathode was ignited with a 375 V bias using 17.3 mg of propellant. Repeating the tests with krypton showed that ignition could be achieved in the same heaterless hollow cathode assembly with a 300 V bias using 13.1 mg of propellant. We judge that a fixed-volume release system could be implemented in a satellite feed system while introducing minimal additional complexity.

Heaterless↗
NASA NTRS2019

Characterization of a Fixed-Volume Release System for Initiating an Arc Discharge in a Heaterless Hollow Cathode

Heaterless hollow cathodes may provide improved reliability, simplicity, and portability when compared with traditional heater-equipped hollow cathodes, traits which are well suited for low-power Hall-effect thrusters that are currently being developed for small satellite propulsion. Despite the advantages, there are concerns that the ignition process in heaterless hollow cathodes may impose an excessive burden on the propellant feed and/or electrical systems of a small satellite. To address this concern, a fixed-volume release flow protocol, which can be used to temporarily increase the propellant mass flow rate, was developed, modeled, and experimentally evaluated. The new protocol allowed for a heaterless hollow cathode to be ignited reliably with a moderate bias voltage and a minimal electrical power requirement. Specifically, a xenon fed heaterless hollow cathode was ignited with a 375 V bias using 17.3 mg of propellant. Repeating the tests with krypton showed that ignition could be achieved in the same heaterless hollow cathode assembly with a 300 V bias using 13.1 mg of propellant. We judge that a fixed-volume release system could be implemented in a satellite feed system while introducing minimal additional complexity.

Heaterless↗
NASA NTRS2019

Characterization of Propellant Flow and Bias Required to Initiate an Arc Discharge in a Heaterless Hollow Cathode

Heaterless hollow cathodes provide an opportunity to reduce complexity and improve reliability in electric propulsion systems. While removal of the heater has little effect on steady-state operation of a hollow cathode, it has a considerable effect on the ignition process. To successfully integrate a heaterless hollow cathode into a spaceflight electric propulsion system, it will be necessary to establish definitive requirements for the propellant feed and electrical subsystems so that ignition of a plasma discharge can be achieved reliably. The aim of this research was to form a better understanding of these requirements by performing an investigation of the propellant flow and voltage conditions required for the ignition of a plasma arc discharge. This aim was achieved by performing discharge initiation experiments using both a specially designed experimental apparatus and a functional heaterless hollow cathode assembly. It was demonstrated that there is a distinct difference in the voltage required to initiate a plasma discharge between two common electric propulsion propellants, xenon and krypton, which suggests that the developmental testing of heaterless hollow cathodes needs to be performed with the appropriate propellant gas species. Heaterless hollow cathode ignition experiments showed that the keeper orifice diameter has a strong effect on the voltage required to ignite a plasma discharge at a given propellant mass flow rate, while the effect of keeper-cathode separation distance was only strong at flow rates below 25 sccm (Xe).

Ham, Ryan K.↗
NASA NTRS2019

High-Frequency Density Oscillations from a Plasma Source Used for Simulating Low-Earth Orbit Plasma Environment

We present data from ground-based, vacuum-chamber tests demonstrating the ability to modulate the output of a plasma source capable of producing a low-Earth orbit (LEO) type plasma. We obtained plasma oscillations up to 2.5 kHz impingent on stationary test equipment, which corresponds to meter-level ionospheric structures in LEO. This plasma source is, therefore, suitable for developing scientific instruments that measure the LEO plasma environment, in situ, with meter-level spatial resolution. Measurements were made using a fixed-bias collector and an electrometer sampling at 40 kHz. A mechanical aperture was established at the output of the plasma source via two concentric grids. The outer grid was free to rotate in the azimuthal direction with respect to the fixed inner grid. An identical, alternating hole pattern in the two grids resulted in a variable aperture that cycles through 90 open/close cycles per revolution. The frequency of the plasma oscillations is limited by the mechanism used to spin the grids and the bearing assembly on which the grids rotate. Higher frequencies are obtainable by upgrading the drive mechanism, allowing the possibility of centimeter-level spatial resolution.

McTernan, Jesse K.↗
NASA NTRS2018

Hall Current Plasma Source Having a Center-Mounted or a Surface-Mounted Cathode

A miniature Hall current plasma source apparatus having magnetic shielding of the walls from ionized plasma, an integrated discharge channel and gas distributor, an instant-start hollow cathode mounted to the plasma source, and an externally mounted keeper, is described. The apparatus offers advantages over other Hall current plasma sources having similar power levels, including: lower mass, longer lifetime, lower part count including fewer power supplies, and the ability to be continuously adjustable to lower average power levels using pulsed operation and adjustment of the pulse duty cycle. The Hall current plasma source can provide propulsion for small spacecraft that either do not have sufficient power to accommodate a propulsion system or do not have available volume to incorporate the larger propulsion systems currently available. The present low-power Hall current plasma source can be used to provide energetic ions to assist the deposition of thin films in plasma processing applications.

Martinez, Rafael A.↗
NASA NTRS2017

12CaO-7Al2O3 Electride Hollow Cathode

The use of the electride form of 12CaO-7Al2O3, or C12A7, as a low work function electron emitter in a hollow cathode discharge apparatus is described. No heater is required to initiate operation of the present cathode, as is necessary for traditional hollow cathode devices. Because C12A7 has a fully oxidized lattice structure, exposure to oxygen does not degrade the electride. The electride was surrounded by a graphite liner since it was found that the C12A7 electride converts to it's eutectic (CA+C3A) form when heated (through natural hollow cathode operation) in a metal tube.

Rand, Lauren P.↗
NASA NTRS2016

12Cao-7Al2o3 Electride Hollow Cathode

The use of the electride form of 12CaO-7Al.sub.2O.sub.3, or C12A7, as a low work function electron emitter in a hollow cathode discharge apparatus is described. No heater is required to initiate operation of the present cathode, as is necessary for traditional hollow cathode devices. Because C12A7 has a fully oxidized lattice structure, exposure to oxygen does not degrade the electride. The electride was surrounded by a graphite liner since it was found that the C12A7 electride converts to it's eutectic (CA+C3A) form when heated (through natural hollow cathode operation) in a metal tube.

Rand, Lauren P.↗
NASA NTRS2005

Characterization of an 8-cm Diameter Ion Source System

Results of tests characterizing an 8-cm diameter ion source are presented. The tests were conducted in three separate vacuum test facilities at the University of Alabama-Huntsville, Colorado State University, and L3 Communications' ETI division. Standard ion optics tests describing electron backstreaming and total-voltage-limited impingement current behavior as a function of beam current were used as guidelines for selecting operating conditions where more detailed ion beam measurements were performed. The ion beam was profiled using an in-vacuum actuating probe system to determine the total ion current density and the ion charge state distribution variation across the face of the ion source. Both current density and ExB probes were utilized. The ion current density data were used to obtain integrated beam current, beam flatness parameters, and general beam profile shapes. The ExB probe data were used to determine the ratio of doubly to singly charged ion current. The ion beam profile tests were performed at over six different operating points that spanned the expected operating range of the DAWN thrusters being developed at L3. The characterization tests described herein reveal that the 8-cm ion source is suitable for use in (a) validating plasma diagnostic equipment, (b) xenon ion sputtering and etching studies of spacecraft materials, (c) plasma physics research, and (d) the study of ion thruster optics at varying conditions.

Li, Zhongmin↗
NASA NTRS2004

Ground-Based Simulation of Low-Earth Orbit Plasma Conditions: Plasma Generation and Characterization

A 16-cm diameter plasma source operated on argon is described that is capable of producing a plasma environment that closely simulates the low Earth orbit (LEO) conditions experienced by satellites in the altitude range between 300 to 500 km. The plasma source uses a transverse-field magnetic filter, and has been successful in producing low electron temperature plasmas that contain streaming ion populations. Both of these characteristics are important because the plasma in LEO is relatively cold (e.g., Te approx. 0.1 eV) and the ram energy of the ions due to the motion of the satellite relative to the LEO plasma is high (e.g., 7,800 m/s which corresponds to approx. 5 eV for O+ ions). Plasma source operational conditions of flow rate and discharge power are presented that allow the electron temperature to be adjusted over a range from 0.14 to 0.4 eV. The expanding plasma flow field downstream of the source contains both low-energy, charge-exchange ions and streaming ions with energies that are adjustable over a range from 4 eV to 6 eV. At low flow rates and low facility pressures, the streaming ion component of the ion population comprises over 90% of the total plasma density. In the work described herein, a large area retarding potential analyzer was used to measure both electron and ion energy distribution functions in the low density, expanding plasma produced downstream of the plasma source. The benefits of using this type of plasma diagnostic tool in easily perturbed, low-density plasma are identified, and techniques are also discussed that can be used to perform real-time measurements of electron temperature. Finally, recommendations are made that may enable lower electron temperatures to be produced while simultaneously decreasing the plasma source flow rate below 1 to 2 sccm.

Williams, John D.↗
NASA NTRS2003

NEXT Ion Optics Simulation via ffx

Simulations of the erosion processes for two proposed sets of ion thruster grids for the NEXT project are presented. Structural failure and electron backstreaming due to accelerator grid erosion are discussed as two possible failure mechanisms of these grid sets. The TAG grid set was seen to outperform the NSTAR grid set both in terms of margin against electron backstreaming and accelerator grid mass loss for a variety of operating points. An investigation into the possibility of reducing the accelerator grid voltage magnitude for the TAG grid set showed improved propellant throughput capability.

Soulas, George↗
NASA NTRS2003

Differential Sputtering Behavior of Pyrolytic Graphite and Carbon-Carbon Composite Under Xenon Bombardment

A differential sputter yield measurement technique is described, which consists of a quartz crystal monitor that is swept at constant radial distance from a small target region where a high current density xenon ion beam is aimed. This apparatus has been used to characterize the sputtering behavior of various forms of carbon including polycrystalline graphite, pyrolytic graphite, and PVD-infiltrated and pyrolized carbon-carbon composites. Sputter yield data are presented for pyrolytic graphite and carbon-carbon composite over a range of xenon ion energies from 200 eV to 1 keV and angles of incidence from 0 deg (normal incidence) to 60 deg .

Williams, John D.↗
NASA NTRS2003

Numerical Simulation of Ion Thruster Optics

A three-dimensional simulation code (ffx) designed to analyze ion thruster optics is described. It is an extension of an earlier code and includes special features like the ability to model a wide range of grid geometries, cusp details, and mis-aligned aperture pairs to name a few. However, the principle reason for advancing the code was in the study of ion optics erosion. Ground based testing of ion thruster optics, essential to the understanding of the processes of grid erosion, can be time consuming and costly. Simulation codes that can accurately predict grid lifetimes and the physical mechanisms of grid erosion can be of great utility in the development of future ion thruster optics designed for more ambitious applications. Results of simulations are presented that describe wear profiles for several standard and nonstandard aperture geometries, such as those grid sets with square- or slotted-hole layout patterns. The goal of this paper will be to introduce the methods employed in the ffx code and to briefly demonstrate their use.

Rawlin, Vincent K.↗
NASA NTRS2003

Xenon Sputter Yield Measurements for Ion Thruster Materials

In this paper, we describe a technique that was used to measure total and differential sputter yields of materials important to high specific impulse ion thrusters. The heart of the technique is a quartz crystal monitor that is swept at constant radial distance from a small target region where a high current density xenon ion beam is aimed. Differential sputtering yields were generally measured over a full 180 deg arc in a plane that included the beam centerline and the normal vector to the target surface. Sputter yield results are presented for a xenon ion energy range from 0.5 to 10 keV and an angle of incidence range from 0 deg to 70 deg from the target surface normal direction for targets consisting of molybdenum, titanium, solid (Poco) graphite, and flexible graphite (grafoil). Total sputter yields are calculated using a simple integration procedure and comparisons are made to sputter yields obtained from the literature. In general, the agreement between the available data is good. As expected for heavy xenon ions, the differential and total sputter yields are found to be strong functions of angle of incidence. Significant under- and over-cosine behavior is observed at low- and high-ion energies, respectively. In addition, strong differences in differential yield behavior are observed between low-Z targets (C and Ti) and high-Z targets (Mo). Curve fits to the differential sputter yield data are provided. They should prove useful to analysts interested in predicting the erosion profiles of ion thruster components and determining where the erosion products re-deposit.

Williams, John D.↗
NASA NTRS1992

Plasma contactor research - 1991

A report describing the operating principles of hollow-cathode-based plasma contactors emitting or collecting electrons from an ambient plasma is summarized. Preliminary experiments conducted to determine the noise generated by these plasma contactors in the emission-current return line and in the plasma near it are described. These noise data are measured as current fluctuations in the return line and to the Langmuir probe and then analyzed using a fast Fourier transform technique. The spectral compositions of the data are characterized using power spectral density plots which are examined to identify possible noise source(s) and production mechanism(s). The precautions taken in the construction and calibration of the instrumentation to assure adequate frequency response are described. Experimental results show that line-current noise levels are typically 2 percent of the electron current being emitted or collected. However, noise levels increase to as much as 20 percent of the electron current at a few electron-collection operating conditions. The frequencies associated with most of the noise were harmonics of the 60 Hz input to system power supplies. Plasma noise had characteristics similar in magnitude and frequency to those for the return-line noise, but they contained additional features at frequencies considered to be related to ion-acoustic instabilities. Also discussed is a new probe positioning system built to facilitate future plasma-contractor research.

Buchholtz, Brett↗
NASA NTRS1991

An experimental investigation of hollow cathode-based plasma contactors

Experimental results are presented which describe operation of the plasma environment associated with a hollow cathod-based plasma contactor collecting electrons from or emitting them to an ambient, low density Maxwellian plasma. A one-dimensional, phenomenological model of the near-field electron collection process, which was formulated from experimental observations, is presented. It considers three regions, namely, a plasma cloud adjacent to the contactor, an ambient plasma from which electrons are collected, and a double layer region that develops between the contactor plasma cloud and the ambient plasma regions. Results of the electron emission experiments are also presented. An important observation is made using a retarding potential analyzer (RPA) which shows that high energy ions generally stream from a contactor along with the electrons being emitted. A mechanism for this phenomenon is presented and it involves a high rate of ionization induced between electrons and atoms flowing together from the hollow cathode orifice. This can result in the development of a region of high positive potential. Langmuir and RPA probe data suggest that both electrons and ions expand spherically from this hill region. In addition to experimental observations, a one-dimensional model which describes the electron emission process and predicts the phenomena just mentioned is presented and shown to agree qualitatively with these observations.

Williams, John D.↗
NASA NTRS1991

Plasma contactor research, 1990

Emissive and Langmuir probes were used to measure plasma potential profiles, plasma densities, electron energy distributions, and plasma noise levels near a hollow cathode-based plasma contactor emitting electrons. The effects of electron emission current (100 to 1500 mA) and contactor flowrate (2 to 10 sccm (Xenon)) on these data are examined. Retarding potential analyzer (RPA) measurements showing that high energy ions generally stream from a contactor along with the electrons being emitted are also presented, and a mechanism by which this occurs is postulated. This mechanism, which involves a high rate of ionization induced between electrons and atoms flowing together from the hollow cathode orifice, results in a region of high positive space charge and high positive potential. Langmuir and RPA probe data suggests that both electrons and ions expand spherically from this potential hill region. In addition to experimental observations, a simple one-dimensional model which describes the electron emission process and predicts the phenomena just mentioned is presented and is shown to agree qualitatively with these observations. Experimental results of the first stage of bilateral cooperation with the Italian Institute of Interplanetary Space Physics (IFSI CNR) are presented. Sharp, well-defined double layers were observed downstream of a contactor collecting electrons from an ambient plasma created in the IFSI Facility. The voltage drop across these double layers was observed to increase with the current drawn from the ambient plasma. This observation, which was not as clear in previous IFSI tests conducted at higher neutral pressures, is in agreement with previous experimental observations made at both Colorado State University and NASA Lewis Research Center. Greater double layer voltage drops, multiple double layers, and higher noise levels in the region near the double layers were also observed when a magnetic field was imposed and oriented perpendicular to the line joining the contactor and simulator.

Williams, John D.↗
NASA NTRS1990

Laboratory experiments on plasma contactors

Experimental results describing the operation of hollow cathode plasma contactors collecting and emitting electrons from and to an ambient plasma at current levels of the order of one ampere are presented. The voltage drops induced between a contactor and an ambient plasma are shown to be a few tens of volts at such current levels. The development of a double sheath and the production of substantial numbers of ions by electrons streaming across it are associated with the electron collection process. The development of a complex potential structure including a high potential hill just downstream of the cathode orifice is shown to characterize typical contactor emitting electrons.

Wilbur, Paul J.↗