2,632 research outputs found

    Baffle aperture design study of hollow cathode equipped ion thrusters

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    A simple theoretical model which can be used as an aid in the design of the baffle aperture region of a hollow cathode equipped ion thruster was developed. An analysis of the ion and electron currents in both the main and cathode discharge chambers is presented. From this analysis a model of current flow through the aperture, which is required as an input to the design model, was developed. This model was verified experimentally. The dominant force driving electrons through the aperture was the force due to the electrical potential gradient. The diffusion process was modeled according to the Bolm diffusion theory. A number of simplifications were made to limit the amount of detailed plasma information required as input to the model to facilitate the use of the model in thruster design. This simplified model gave remarkably consistant results with experimental results obtained with a given thruster geometry over substantial changes in operating conditions. The model was uncertain to about a factor of two for different thruster cathode region geometries. The design usefulness was limited by this factor of two uncertainty and by the accuracy to which the plasma parameters required as inputs to the model were specified

    Ion thruster performance model

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    A model of ion thruster performance is developed for high flux density, cusped magnetic field thruster designs. This model is formulated in terms of the average energy required to produce an ion in the discharge chamber plasma and the fraction of these ions that are extracted to form the beam. The direct loss of high energy (primary) electrons from the plasma to the anode is shown to have a major effect on thruster performance. The model provides simple algebraic equations enabling one to calculate the beam ion energy cost, the average discharge chamber plasma ion energy cost, the primary electron density, the primary-to-Maxwellian electron density ratio and the Maxwellian electron temperature. Experiments indicate that the model correctly predicts the variation in plasma ion energy cost for changes in propellant gas (Ar, Kr and Xe), grid transparency to neutral atoms, beam extraction area, discharge voltage, and discharge chamber wall temperature. The model and experiments indicate that thruster performance may be described in terms of only four thruster configuration dependent parameters and two operating parameters. The model also suggests that improved performance should be exhibited by thruster designs which extract a large fraction of the ions produced in the discharge chamber, which have good primary electron and neutral atom containment and which operate at high propellant flow rates

    Review of the Leaf Essential Oils of the Genus Backhousia Sens. Lat. and a Report on the Leaf Essential Oils of B. gundarara and B. tetraptera

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    A review of the leaf oils of the 13 species now recognised in the genus Backhousia is presented. This review carries on from, and incorporates data from, an earlier (1995) review of the then recognised eight species. The leaf oils of two new species of Backhousia, B. gundarara and B. tetraptera are reported for the first time. B. gundarara contains a mixture of mono-and sesquiterpenes, with α-pinene (14%) and spathulenol (11%) being the main members. In B. tetraptera, the principal component of the mainly terpenoid leaf oil is myrtenyl acetate (20–40%). The review also incorporates the two species of the genus Choricarpia, which have been subsumed into Backhousia, viz. B. leptopetala and B. subargentea. Due to its history in Backhousia, Syzygium anisatum, which has been transferred out of Backhousia, is included in the review for historical reasons

    Effect of an agri-environmental measure on nitrate leaching from a beef farming system in Ireland

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    peer-reviewedAgricultural nitrogen (N) management remains a key environmental challenge. Improving N management is a matter of urgency to reduce the serious ecological consequences of the reactive N. Nitrate (NO3−–N) leaching was measured under suckler beef production systems stocked at two intensities: (1) intensive, 210 kg organic N ha−1 with two cut silage harvests; and (2) rural environmental protection scheme (REPS), 170 kg organic N ha−1 with one cut silage harvest. Three replicate plots of each treatment were instrumented with ceramic cups (8 per plot), randomly placed within each plot at a depth of 1 m to collect soil solution for NO3−–N at 50 kPa suction to collecting vessels one week prior to sampling. Samples were taken on a total of 53 sampling dates over 3 winter drainage periods (2002/03, 2003/04 and 2004/05). Over the course of the experiment the mean annual soil solution NO3−–N concentration exceeded the MAC twice out of 15 means (5 treatments over 3 years). The REPS grazing and silage sub treatments had significantly lower mean annual soil solution total oxidized N (TON) concentrations than the respective intensive treatments in years 2 and 3. Annual total NO3−–N losses over the three years in intensive and REPS systems ranged from 55 to 71 and 15 to 20 kg N ha−1, respectively. Mean N surpluses in intensive and REPS systems were 210 and 95 kg ha−1, respectively with the corresponding mean N inputs of 272 and 124 kg N ha−1. The reduction in N inputs under the REPS system results in lower N leaching losses and contributed to a significant reduction in pressures on water quality

    High-Power Solar Electric Propulsion for Future NASA Missions

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    NASA has sought to utilize high-power solar electric propulsion as means of improving the affordability of in-space transportation for almost 50 years. Early efforts focused on 25 to 50 kilowatt systems that could be used with the Space Shuttle, while later efforts focused on systems nearly an order of magnitude higher power that could be used with heavy lift launch vehicles. These efforts never left the concept development phase in part because the technology required was not sufficiently mature. Since 2012 the NASA Space Technology Mission Directorate has had a coordinated plan to mature the requisite solar array and electric propulsion technology needed to implement a 30 to 50 kilowatt solar electric propulsion technology demonstration mission. Multiple solar electric propulsion technology demonstration mission concepts have been developed based on these maturing technologies with recent efforts focusing on an Asteroid Redirect Robotic Mission. If implemented, the Asteroid Redirect Vehicle will form the basis for a capability that can be cost-effectively evolved over time to provide solar electric propulsion transportation for a range of follow-on mission applications at power levels in excess of 100 kilowatts

    The effect of Fe atoms on the adsorption of a W atom on W(100) surface

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    We report a first-principles calculation that models the effect of iron (Fe) atoms on the adsorption of a tungsten (W) atom on W(100) surfaces. The adsorption of a W atom on a clean W(100) surface is compared with that of a W atom on a W(100) surface covered with a monolayer of Fe atoms. The total energy of the system is computed as the function of the height of the W adatom. Our result shows that the W atom first adsorbs on top of the Fe monolayer. Then the W atom can replace one of the Fe atoms through a path with a moderate energy barrier and reduce its energy further. This intermediate site makes the adsorption (and desorption) of W atoms a two-step process in the presence of Fe atoms and lowers the overall adsorption energy by nearly 2.4 eV. The Fe atoms also provide a surface for W atoms to adsorb facilitating the diffusion of W atoms. The combination of these two effects result in a much more efficient desorption and diffusion of W atoms in the presence of Fe atoms. Our result provides a fundamental mechanism that can explain the activated sintering of tungsten by Fe atoms.Comment: 9 pages, 2 figure

    Frequency and characteristics of disease flares in ankylosing spondylitis

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    Objective. To examine the characteristics and frequency of disease flares in a cohort of people with AS

    AN Ultrasonic Ray-Trance Code for Complex Geometry Applications

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    Many applications of ultrasonic testing (UT) are made difficult by the complex geometrical shape of the part to be inspected. A simple ray-trace analysis of the geometry may suffice to determine transducer positions and beam angles to provide adequate coverage of the region to be interrogated. In addition, however, the analyst must have estimates of backscattered amplitudes to determine if flaw signals are measurable. Thus, a combination of complex-geometry ray tracing with calculations of flaw-scattering amplitudes is required for the design and optimization of UT procedures

    Stress radiography Is a reliable method to quantify posterior cruciate ligament insufficiency: A systematic review

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    PURPOSE: To perform a systematic review of posterior tibial stress radiography techniques and radiographic measurement methods to compare their accuracy and efficacy to aid clinicians in quantifying posterior cruciate ligament laxity. METHODS: Electronic databases, including PubMed, MEDLINE, Embase.com 1947- , Ovid Medline 1946- , Scopus 1823- , Cochrane Central Register of Controlled Trials (CENTRAL), and Clinicaltrials.gov 1997- were queried in December 2020. The abstracts of articles were reviewed by 2 authors for published studies comparing posterior tibial stress radiography techniques, describing, and comparing radiographic measurement methods, and comparing stress radiographs with instrumented knee testing. RESULTS: The systematic review included 13 studies that satisfied the inclusion and exclusion criteria. There were 3 studies comparing stress radiography with instrumented knee devices, 6 studies comparing stress radiography techniques, and 5 studies evaluating the reliability of radiographic measurements. Stress radiography was more sensitive for detecting posterior tibial translation than KT-1000 and KT-2000 and was similar to the Rolimeter knee arthrometer. The majority of studies found TELOS stress radiography to be more sensitive than gravity or hamstring contraction stress views. Kneeling stress radiographs were found to be equivalent to TELOS in one study and superior in another. All reported methods of radiographic measurement for posterior tibial translation showed good-to-excellent intraobserver and interobserver reliability, and no single technique demonstrated clear superiority. CONCLUSIONS: The results of this systematic review indicate that posterior stress radiography with TELOS and kneeling stress radiography are the most reliable methods to evaluate posterior cruciate ligament laxity. Gravity stress and hamstring contraction can be used but may underestimate posterior tibial translation. Radiographic measurement methods are reliable and no single method is clearly superior. CLINICAL RELEVANCE: This information will allow clinicians to use various radiographic methods to objectively measure posterior tibial translation to formulate a treatment plan
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