The role plasma contactors play in effective electrodynamic tether operation is discussed. Hollow cathodes and hollow cathode-based plasma sources have been identified as leading candidates for the electrodynamic tether plasma contactor. Present experimental efforts to evaluate the suitability of these devices as plasma contactors, conducted concurrently at NASA Lewis Research Center and Colorado State University, are reviewed. These research programs include the definition of preliminary...
Topics: NASA Technical Reports Server (NTRS), CONTACTORS, ELECTRODYNAMICS, PLASMA CONTROL, ELECTRON...
Results of experiments conducted on hollow cathode-based plasma contractors are reported. Specific tests in which attempts were made to vary plasma conditions in the simulated ionospheric plasma are described. Experimental results showing the effects of contractor flowrate and ion collecting surface size on contactor performance and contactor plasma plume geometry are presented. In addition to this work, one-dimensional solutions to spherical and cylindircal space-charge limited double-sheath...
Topics: NASA Technical Reports Server (NTRS), EARTH IONOSPHERE, HOLLOW CATHODES, PLASMA PHYSICS, SPACE...
The electron collection and emission characteristics of a simple hollow cathode contactor, an extended anode hollow cathode contactor supplied by JSC, and a ring cusp magnetic field contactor are presented and the effects of discharge power and argon or xenon expellant flowrate on these characteristics are examined. All of the contactors are shown to exhibit good electron emission performance over a wide range of discharge power and expellant type and flowrate. Good electron performance is...
Topics: NASA Technical Reports Server (NTRS), CONTACTORS, ELECTRODYNAMICS, FUNCTIONAL DESIGN...
An experimental study was performed to determine the effects of hollow cathode position, anode position, and ring cusp magnetic field configuration and strength on discharge chamber performance. The results are presented in terms of comparative plasma ion energy cost, extracted ion fraction, and beam profile data. Such comparisons are used to demonstrate whether changes in performance are caused by changes in the loss rate of primary electrons to the anode or the loss rate of ions to discharge...
Topics: NASA Technical Reports Server (NTRS), ELECTRIC DISCHARGES, HOLLOW CATHODES, ION PROPULSION, ION...
A simple model describing the process of electron collection from a low pressure ambient plasma in the absence of magnetic field and contactor velocity effects is presented. Experimental measurments of the plasma surrounding the contactor are used to demonstrate that a double-sheath generally develops and separates the ambient plasma from a higher density, anode plasma located adjacent to the contactor. Agreement between the predictions of the model and experimental measurements obtained at the...
Topics: NASA Technical Reports Server (NTRS), CONTACTORS, ELECTRODYNAMICS, HOLLOW CATHODES, PLASMAS...
The potential usefulness of the constrained sheath optics concept as a means of controlling the divergence of low energy, high current density ion beams is examined numerically and experimentally. Numerical results demonstrate that some control of the divergence of typical ion beamlets can be achieved at perveance levels of interest by contouring the surface of the constrained sheath properly. Experimental results demonstrate that a sheath can be constrained by a wire mesh attached to the...
Topics: NASA Technical Reports Server (NTRS), CHARACTERISTICS, DIVERGENCE, ION BEAMS, MATHEMATICAL MODELS,...
Results are presented which show that hollow cathodes can be operated on ammonia but that sustained operation in the high pressures where arcjet thrusters operate (of the order of 1000 Torr) is difficult to achieve. The concept of using contoured, fine wire meshes attached across the screen grid apertures in an ion thruster to effect control of the ion beam divergence is introduced. The concept is compared to conventional (free sheath) ion extraction and is shown to be potentially attractive....
Topics: NASA Technical Reports Server (NTRS), ARC JET ENGINES, ELECTROSTATIC ENGINES, ION PROPULSION,...
A methodology that can be used to determine which of several physical constraints can limit ion thruster power and thrust, under various design and operating conditions, is presented. The methodology is exercised to demonstrate typical limitations imposed by grid system span-to-gap ratio, intragrid electric field, discharge chamber power per unit beam area, screen grid lifetime, and accelerator grid lifetime constraints. Limitations on power and thrust for a thruster defined by typical...
Topics: NASA Technical Reports Server (NTRS), ION PROPULSION, PARAMETER IDENTIFICATION, ROCKET ENGINE...
Results of an experimental study of the characteristics of ion thruster hollow cathodes operating at high discharge currents (up to 60 A) are presented in a companion report. This work shows that ions produced near the cathode orifice can acquire sufficient energy to induce the high sputter erosion rates on cathode potential surfaces that have been observed in ion thrusters. A mechanism by which these ions could be produced is also described. A second, brief study showing how a discharge...
Topics: NASA Technical Reports Server (NTRS), CATHODES, ELECTRIC PROPULSION, EROSION, HOLLOW CATHODES, ION...
An investigation conducted on the electrical interference induced by the operation of a hollow-cathode plasma contractor in a ground-based facility is described. The types of electrical interference, or noise, which are important to Space Station Freedom designers are classified as either conducted or radiated emissions. The procedures required to perform conducted and radiated emission measurements on a plasma contactor are examined. The experimental data obtained are typically examined in the...
Topics: NASA Technical Reports Server (NTRS), CONTACTORS, ELECTROMAGNETIC COMPATIBILITY, ELECTROMAGNETIC...
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...
Topics: NASA Technical Reports Server (NTRS), ION OPTICS, SIMULATION, ION ENGINES, EROSION, ION PROPULSION,...
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...
Topics: NASA Technical Reports Server (NTRS), CONTACTORS, ELECTRONS, HOLLOW CATHODES, PLASMA CURRENTS,...
A theory of the plasma contacting process is described and experimental results obtained using three different hollow cathode-based plasma contactors are presented. The existence of a sheath across which the bulk of the voltage drop associated with the contacting process occurs is demonstrated. Test results are shown to agree with a model of a spherical, space-charge-limited double sheath. The concept of ignited mode contactor operation is discussed, which is shown to enhance contactor...
Topics: NASA Technical Reports Server (NTRS), ARC JET ENGINES, CONTACTORS, ELECTRIC PROPULSION,...
An experimental study of impingement current collection on the accelerator grid of an ion thruster is presented. The equipment, instruments, and procedures being used to conduct the study are discussed. The contribution to this current due to charge-exchange ions produced close to the grid is determined using a volume-integration procedure and measured ion beam current design, computed neutral atom density and measured beam plasma potential data. This current, which is expected to be almost...
Topics: NASA Technical Reports Server (NTRS), BEAM CURRENTS, CHARGE TRANSFER, ELECTRIC PROPULSION,...
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...
Topics: NASA Technical Reports Server (NTRS), XENON, SPUTTERING, ION ENGINES, THRUST MEASUREMENT, HIGH...
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...
Topics: NASA Technical Reports Server (NTRS), CONTACTORS, ELECTROMAGNETIC NOISE, FOURIER TRANSFORMATION,...
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...
Topics: NASA Technical Reports Server (NTRS), COMPUTERIZED SIMULATION, ION OPTICS, THREE DIMENSIONAL...
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...
Topics: NASA Technical Reports Server (NTRS), CONTACTORS, ELECTRON EMISSION, ELECTRON ENERGY, ENERGY...
The lubricating lifetime of thin films of a perfluoropolyether (PFPE) based on hexafluoropropene oxide in the presence of ion implanted 440C stainless steel is presented. Stainless steel discs, either unimplanted or implanted with N2, C, Ti, Ti + N2, or Ti + C had a thin film of PFPE (60-400 A) applied to them reproducibly (+/- 20 percent) and uniformly (+/- 15 percent) using a device developed for this study. The lifetimes of these films were quantified by measuring the number of sliding-wear...
Topics: NASA Technical Reports Server (NTRS), BOUNDARY LUBRICATION, ION IMPLANTATION, POLYMERIC FILMS,...
