1,720,988 research outputs found
Thermal analysis of skidding energetic materials
No full textDetailed is the investigation of the thermal behavior of high explosives (HE). Temperature is determined by measuring the thermal infrared radiation emitted PBX 9501 and PBX 9502 during skidding over a sapphire surface. Skidding is referred to as the act of impacting a surface with forces oblique to the surface. This skidding is performed on an apparatus that allows individual control over normal force, surface velocity, and presence of grit particles. The HE is attached to an I-beam where mechanical strain is measured and converted to force. Experiments are verified with a Phantom VEO710s high-speed visible camera, capable of recording up to 680,000 fps with a 7 GHz pixel clock. A FLIR X6901sc high-speed mid-wave infrared (MWIR) camera is used to measure the temperature of the skidding HE. PBX 9501 and PBX 9502 are semi-transparent in the MWIR regime, posing a difficult task in determining their temperatures from measured thermal radiation. Accurate temperature readings require knowledge of the absorption coefficients of the HE, and the temperature gradients produced from skidding. Experiments with the FLIR camera and a thermal radiator were performed to estimate the absorption coefficient of the HE. COMSOL simulations were implemented to estimate temperature gradients beneath the surface.Embargo status: This title is currently under embargo. Request a copy through the form linked to the left, or by emailing [email protected]. You can also contact the author directly
An experimental apparatus and diagnostics for the study of vacuum surface flashover
No full textUpper operational limits in pulsed power vacuum systems are typically determined by the maximum electrical stress the apparatus can handle. If this limit is exceeded, the first point in these systems to fail is usually the dielectric interface separating the anode and cathode. This failure is due to a conductive plasma channel developing along the surface of the insulating material, which effectively shorts both electrodes together, compromising the electrical integrity. While breakdown can initiate at the cathode, anode-initiated flashover is believed to be more dominant with 45-degree insulators, which is the geometry many modern pulsed power machines use.
To investigate and characterize anode-initiated vacuum surface flashover, an experimental apparatus has been engineered to produce a repeatable flashover location. A cross-linked polystyrene (Rexolite) sample in a 45-degree wedge geometry separates a hemispherical anode from a planar cathode by 0.6 cm. The testbed housing both electrodes and the dielectric sample is located within a vacuum chamber with a nominal pressure of 10^(-6) Torr. An 8-stage Marx generator pulses the anode with up to 240 kV, resulting in flashover along the surface of the Rexolite sample. The voltage and current are measured using a calibrated capacitive voltage divider (CVD) and current viewing resistor (CVR). Additional diagnostics include time-integrated and temporally resolved imaging using an intensified charge-coupled device (ICCD) camera. Observations have shown that early light emission does originate near the anode-vacuum-insulator junction due to mechanisms that agree with the Anderson model of anode-initiated flashover. Post-test imaging also revealed dendrite pattern surface damage which appears to propagate from anode to cathode. A very similar pattern of surface scarring was observed by Anderson and is believed to be unique to anode-initiated flashover.Embargo status: Restricted until 06/2172. To request the author grant access, click on the PDF link to the left
Modeling and assessment of lighting attachment probabilities to building structures
No full textAn investigation of the risks of a conventionally protected building being stuck by lightning was performed. A stochastic random-walk model was developed to identify attachment to buildings and an upward leader model was investigated to consider the electrical elongation of lightning rods. A scheme for estimating risk as a function of strike current was developed, and total strike rates were calculated for an example building of 50 m by 100 m. The incidence of a strike to the building which was not intercepted by lightning protection was estimated at less than once per thousand years, and conservatively estimated to experience a rate of 1 per 100 years. Non-preferential striking, which gives all elements an equal likelihood of being struck, produced a few interesting interactions between the protection structures and the ground which were not readily available in literature. Non-preferential striking treated the ground as a lighting protection structure which was often found to be superior to the dedicated structures due to increased proximity. Protection quality was found to be insensitive to variations in height, aspect ratio, and footprint area. Recessed lightning protection structures were found to be detrimental, but small recess is tolerable. Development of the upward leader was predicted to be limited, but the streamer zone ahead of the lighting rod was predicted to be much larger, offering a plausible mechanism for the large protection radius these devices are predicted to afford.Embargo status: This title is currently under embargo. Request a copy through the form linked to the left, or by emailing [email protected]. You can also contact the author directly
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Calculation of Electron Collision Cross Sections in Fluorinated Nitrile (C4F7N) and its Dissociation Fragments
No full textC4F7N has received considerable attention in recent years as an insulating gas due to its higher dielectric strength and low global warming potential relative to SF6. To evaluate the behavior of this novel gas during high-voltage gaseous discharges, plasma models require electron-neutral collision cross sections to calculate parameters such as transport coefficients. At the time of drafting this thesis, however, a complete, accurate set of electron-neutral cross sections for C4F7N had not been published. While there are various techniques used to calculate these cross sections, this report concerns the use of the R-matrix method solver Quantemol-EC. This software uses the sophisticated UKRmol+ codes to solve the electron-molecule scattering problem at low to moderate energies (0.01 - 20 eV) and produce elastic, vibrational, electronic excitation, and dissociative attachment cross sections. This report describes the calculation of a set of cross sections for C4F7N and its dissociation fragments using an R-matrix method. The results are subsequently utilized within a multi-term Boltzmann equation solver to generate reaction rates and transport coefficients which are compared to experimental data. The use of the R-matrix method, including scattering model parameters and its limitations, is discussed
Electro-explosive fuse optimization for FCG current sources
No full textAs high power microwave (HPM) systems become more prevalent in defense technology, the need arises to make compact systems capable of delivering energy with the same order of magnitude as a large scale system. Various Pulsed power systems have been designed with compact flux compression generators (FCGs) which have very large energy densities to provide the initial current amplification. HPM systems require large voltages, not large currents; therefore a power conditioning system capable of transforming large currents from the FCG to large voltages, to drive the HPM source, is required. This can be done with an FCG driving an inductive energy storage system (IESS) with an electro-explosive fuse (EEF) in the current path. The interruption of current by the fuse will cause a large voltage spike that is in the right regime for high power microwaves. Often the electro-explosive fuse can be the largest component in the system; therefore, the need for optimization arises to reduce the size of the fuse without affecting the voltage multiplication of the fuse. This thesis will examine various parameters that control the operation of the fuse to find the maximum voltage multiplication possible while reducing the overall fuse size. Optimization of the individual parameters of the fuse including quenching media, fuse material, fuse strand length, and fuse shape will lead to complete optimization of the fuse. The end result will be a compact optimized electro-explosive fuse to be implemented in an FCG driven high power microwave system
Compact, repetitive Marx generator and HPM generation with the Vircator
Get PDFThe electrical characteristics and design features of a low inductance, compact, 500 kV, 500 J, 10 Hz repetition rate, Marx generator for driving a high power microwave (HPM) source are described. This includes a relevant background discussion of Marx generators and HPM sources, with an emphasis on HPM generation with the virtual cathode oscillator (Vircator). The particular Compact Marx Generator design benefits from the large energy density of mica capacitors, 4 mica capacitors were utilized in parallel per stage, keeping the parasitic inductance per stage low. Including the spark-gap switches, a stage inductance of 55 nH was measured, which translates with 100 nF capacitance per stage to ~ 18.5 Ohm characteristic Marx impedance. Using solely inductors, ~ 1 mH each, as charging elements, instead of resistors, enabled charging the Marx within less than 100 ms with little charging losses. The pulse width of the Marx into a matched resistive load is about 200 ns with 50 ns rise-time. Repetitive HPM generation with the Marx directly driving a small Vircator has been verified. The Marx is fitted into a stainless steel tube with 30 cm diameter and a total length of 0.7 m. Marx operation at up to 21 kV charging voltage per stage, with repetition rates of up to 10 Hz in burst mode, primarily into resistive loads is discussed. A lumped circuit description of the Marx is also given, closely matching the experimental results. Preliminary design and testing of a low cost, all metal Vircator cathode are also discussed
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