John D. Williams

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[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(More)
We present the development of a sputtering diagnostic based on a quartz crystal microbalance (QCM). The QCM system allows angularly resolved (differential) sputter yield measurements as a function of both polar angle and azimuthal angle. We discuss development of the QCM diagnostic and present sputtering data for molybdenum by xenon ions in the energy range(More)
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(More)
Experiments are described that use sub-scale ion thruster optics (gridlets) comprised of 6 to 37 aperture pairs. Test results are used to determine limitations of several different ion optics systems that are operated at high net accelerating voltages of interest to high specific impulse ion thruster systems. Experimental results are compared to numerical(More)
—Full bandgap 1,2 (3D) photonic crystal materials offer a means to precisely engineer the electromagnetic reflection, transmission, and emission properties of surfaces over wide angular and spectral ranges. However, very few 3D photonic crystals have been successfully demonstrated with areas larger than 1 cm 2. Large sheets of photonic bandgap (PBG)(More)
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(More)
Differential sputter yields are reported for Molybdenum, Tantalum, and Tungsten after exposure to Xenon, Krypton, and Argon ion bombardment at multiple angles of incidence (0-60°) and ion energies (150-1500 eV). Differential yields were measured by sweeping a Quartz Crystal Microbalance (QCM) in a semi-circular arc over a given target in the plane defined(More)
Analytical equations historically used to predict the onset of electron backstreaming in ion thrusters tend to underestimate significantly the accel grid voltage required to block electron backflow from the beam plasma because they neglect detailed beamlet focusing and space charge effects inside the grid apertures. We present corrected analytical equations(More)
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(More)
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(More)