John D. Williams

Learn More
Lipid membranes regulate the flow of nutrients and communication signaling between cells and protect the sub-cellular structures. Recent attempts to fabricate artificial systems using nanostructures that mimic the physiological properties of natural lipid bilayer membranes (LBM) fused with transmembrane proteins have helped demonstrate the importance of(More)
Nanopore-based analysis is currently an area of great interest in many disciplines with the potential for exceptionally versatile applications in medicine. This work presents a novel step towards fabrication of a single solid-state nanopore (SSSN) in a thin silicon membrane. Silicon nanopores are realized using multistep processes on both sides of n-type(More)
This article investigates a device made from a porous silicon structure supporting a lipid bilayer membrane (LBM)fused with Epithelial Sodium Channel protein. The electrochemically-fabricated porous silicon template had pore diameters in the range 0.2~2 µm. Membranes were composed of two synthetic phospholipids: 1,2-diphytanoyl-sn-glycero-3-phosphoserine(More)
[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)
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)
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)
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)