Neutral density map of Hall thruster plume expansion in a vacuum chamber

A neutral background pressure map of the large vacuum test facility (LVTF) is presented. The LVTF is mapped at cold anode flow rates of 5.25, 10.46, and 14.09 mg/s14.09mg∕s. In addition, neutral background pressure maps are created at hot anode (i.e., discharge on) flow rates of 5.25 and 10.46 mg/s10.46mg∕s for discharge voltages of 300 and 500 V500V, corresponding to P5 Hall thruster operating conditions ranging from 1.5 to 5.0 kW1.5to5.0kW. The chamber pressure is mapped at nominal xenon pumping speeds of 140 000 and 240 000 l/s240000l∕s. The pressure map is performed with a rake consisting of five calibrated Bayard–Alpert hot-cathode ionization gauges. The plume expansion appears to be independent of anode flow rate and facility background pressure. Analysis of axial pressure profiles on the LVTF’s centerline shows that the plume pressure decreases from a maximum at the thruster exit plane down to the facility background pressure at approximately 2 m2m downstream of the exit plane. Comparison of axial pressure profiles on the LVTF’s centerline shows that the neutral density is nearly the same for cold flow and hot flow. The study shows that a cold flow neutral density background map accurately characterizes the neutral density in an operating Hall thruster plume.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87890/2/053509_1.pd

Hall Thruster Cluster Operation with a Shared Cathode

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76373/1/AIAA-23688-572.pd

The Effects of Nude Faraday Probe Design and Vacuum Facility Backpressure on the Measured Ion Current Density Profile of Hall Thruster Plumes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76894/1/AIAA-2002-4253-511.pd

Computation of Neutral Gas Flow From a Hall Thruster Into a Vacuum Chamber

The finite pressure present in vacuum chamber testing of space propulsion systems such as Hall thrusters can have a number of undesirable effects. For example, the thrust generated by the thruster is higher and the plume divergence angle larger in ground tests in comparison to space operation. To try to quantify these effects, the direct simulation Monte Carlo method is applied to model a cold flow of xenon gas expanding from a Hall thruster into a vacuum chamber. The simulations are performed for the P5 Hall thruster operating in a large vacuum tank at the University of Michigan. Comparison of the simulation results is made with experimental measurements of pressure obtained with a series of ion gauges. The mass flow rate through the thruster and the total pumping speed of the vacuum chamber are varied. A key physical parameter in the simulations concerns the probability that a xenon atom incident on a cryogenic pumping panel actually sticks to the panel. For a reasonable range of values for the sticking coefficient, excellent agreement between simulation and experiment is obtained for several different conditions. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87929/2/541_1.pd

Pressure Map of a Facility as a Function of Flow Rate to Study Facility Effects

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76369/1/AIAA-2002-3815-510.pd

A trans10-18:1 enriched fraction from beef fed a barley grain-based diet induces lipogenic gene expression and reduces viability of HepG2 cells.

Beef fat is a natural source of trans (t) fatty acids, and is typically enriched with either t10-18:1 or t11-18:1. Little is known about the bioactivity of individual t-18:1 isomers, and the present study compared the effects of t9-18:1, cis (c)9-18:1 and trans (t)-18:1 fractions isolated from beef fat enriched with either t10-18:1 (HT10) or t11-18:1 (HT11). All 18:1 isomers resulted in reduced human liver (HepG2) cell viability relative to control. Both c9-18:1 and HT11were the least toxic, t9-18:1had dose response increased toxicity, and HT10 had the greatest toxicity (P<0.05). Incorporation of t18:1 isomers was 1.8-2.5 fold greater in triacylglycerol (TG) than phospholipids (PL), whereas Δ9 desaturation products were selectively incorporated into PL. Culturing HepG2 cells with t9-18:1 and HT10 increased (P<0.05) the Δ9 desaturation index (c9-16:1/16:0) compared to other fatty acid treatments. HT10 and t9-18:1 also increased expression of lipogenic genes (FAS, SCD1, HMGCR and SREBP2) compared to control (P<0.05), whereas c9-18:1 and HT11 did not affect the expression of these genes. Our results suggest effects of HT11 and c9-18:1 were similar to BSA control, whereas HT10 and t-9 18:1 (i.e. the predominant trans fatty acid isomer found in partially hydrogenated vegetable oils) were more cytotoxic and led to greater expression of lipogenic genes

Effect of Backpressure on Ion Current Density Measurements in Hall Thruster Plumes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76301/1/AIAA-7713-781.pd

Magnetically filtered Faraday probe for measuring the ion current density profile of a Hall thruster

The ability of a magnetically filtered Faraday probe (MFFP) to obtain the ion current density profile of a Hall thruster is investigated. The MFFP is designed to eliminate the collection of low-energy, charge-exchange (CEX) ions by using a variable magnetic field as an ion filter. In this study, a MFFP, Faraday probe with a reduced acceptance angle (BFP), and nude Faraday probe are used to measure the ion current density profile of a 5 kW5kW Hall thruster operating over the range of 300–500 V300–500V and 5–10 mg/s5–10mg∕s. The probes are evaluated on a xenon propellant Hall thruster in the University of Michigan Large Vacuum Test Facility at operating pressures within the range of 4.4×10−4 Pa4.4×10−4Pa Xe (3.3×10−6 Torr3.3×10−6Torr Xe) to 1.1×10−3 Pa1.1×10−3Pa Xe (8.4×10−6 Torr8.4×10−6Torr Xe) in order to study the ability of the Faraday probe designs to filter out CEX ions. Detailed examination of the results shows that the nude probe measures a greater ion current density profile than both the MFFP and BFP over the range of angular positions investigated for each operating condition. The differences between the current density profiles obtained by each probe are attributed to the ion filtering systems employed. Analysis of the results shows that the MFFP, operating at a +5 A+5A solenoid current, provides the best agreement with flight-test data and across operating pressures.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87898/2/013503_1.pd

Single Atom Dynamics in Chemical Reactions

Many heterogeneous chemical reactions involve gases catalyzed over solid surfaces at elevated temperatures and play a critical role in the production of energy, healthcare, pollution control, industrial products, and food. These catalytic reactions take place at the atomic level, with active structures forming under reaction conditions. A fundamental understanding of catalysis at the single atom resolution is therefore a major advance in a rational framework upon which future catalytic processes can be built. Visualization and analysis of gas-catalyst chemical reactions at the atomic level under controlled reaction conditions are key to understanding the catalyst structural evolution and atomic scale reaction mechanisms crucial to the performance and the development of improved catalysts and chemical processes. Increasingly, dynamic single atoms and atom clusters are believed to lead to enhanced catalyst performance, but despite considerable efforts, reaction mechanisms at the single atom level under reaction conditions of gas and temperature are not well understood. The development of the atomic lattice resolution environmental transmission electron microscope (ETEM) by the authors is widely used to visualize gas–solid catalyst reactions at this atomic level. It has recently been advanced to the environmental scanning TEM (ESTEM) with single atom resolution and full analytical capabilities. The ESTEM employs high-angle annular dark-field imaging where intensity is approximately proportional to the square of the atomic number (Z). In this Account, we highlight the ESTEM development also introduced by the authors for real time in situ studies to reliably discern metal atoms on lighter supports in gas and high temperature environments, evolving oxide/metal interfaces, and atomic level reaction mechanisms in heterogeneous catalysts more generally and informatively, with utilizing the wider body of literature. The highlights include platinum/carbon systems of interest in fuel cells to meet energy demands and reduce environmental pollution, in reduction/oxidation (redox) mechanisms of copper and nickel nanoparticles extensively employed in catalysis, electronics, and sensors, and in the activation of supported cobalt catalysts in Fischer–Tropsch (FT) synthesis to produce fuels. By following the dynamic reduction process at operating temperature, we investigate Pt atom migrations from irregular nanoparticles in a carbon supported platinum catalyst and the resulting faceting. We outline the factors that govern the mechanism involved, with the discovery of single atom interactions which indicate that a primary role of the nanoparticles is to act as reservoirs of low coordination atoms and clusters. This has important implications in supported nanoparticle catalysis and nanoparticle science. In copper and nickel systems, we track the oxidation front at the atomic level as it proceeds across a nanoparticle, by directly monitoring Z-contrast changes with time and temperature. Regeneration of deactivated catalysts is key to prolong catalyst life. We discuss and review analyses of dynamic redox cycles for the redispersion of nickel nanoparticles with single atom resolution. In the FT process, pretreatment of practical cobalt/silica catalysts reveals higher low-coordination Co0 active sites for CO adsorption. Collectively, the ESTEM findings generate structural insights into catalyst dynamics important in the development of efficient catalysts and processes

Association between maternal sleep practices and risk of late stillbirth: a case-control study

Objectives To determine whether snoring, sleep position, and other sleep practices in pregnant women are associated with risk of late stillbirth