Explosive Field Research Articles

Blast Response of Walls Retrofitted with Elastomer Coatings

Abstract : Recent tests at Tyndall Air Force Research Laboratory show that an elastomer-coated to the inside wall of a building can offer significant protection for occupants by keeping wall fragments together and shielding out blast debris. In this paper, we develop an equivalent single degree-of-freedom model that can be used to predict dynamic response of a polymer-retrofitted concrete brick wall subjected to a stand-off explosion. As an example, we consider the blast response of a l 0 ft-square, 8 in-thick concrete brick wall coated with a 0.083 in thick layer of polyurea. Explosive field tests with a uniformly-distributed pressure pulse of 40 psi peak value and 20 ms pulse duration indicate that the retrofitted wall reaches a maximum deflection of about 7 in. Our analytical model suggested that the blast response of this wall could be simulated assuming the concrete modulus was reduced to about 5% of its original value. The proposed single degree-of-freedom model, which is based on coupling of the bending/membrane resistance of the wall, compared very well with the ABAQUS results when the maximum deflections of the wall were between 1-2 times the wall thickness.

Aging in gaseous photodetectors

This paper describes the present status of aging studies in various photosensitive detectors. New experimental data are presented on aging of trimethylamine (TMA) and ethylferrocene (EF) photosensitive vapors as well as on CsI and SbCs photocathodes. A new explanation of the aging process for CsI photocathodes based on solid-state physics theory is given. Finally, based on our studies, a general conclusion is made that thin polymer deposits on a detector's cathode due to aging can provoke breakdowns through the Malter-type mechanism, or more precisely, via an explosive field emission mechanism.

Experimental and computational estimate of bipolar flow parameters in an explosive field emission cathode

Explosive field emission cathodes constitute an important class of cathodes for high power microwave tubes. These cathodes have the advantages of being light weight and the capability of operating cold. In the past, this class of cathodes suffered from large amounts of outgassing, nonuniform emission, and very high emittance. These effects tended to dominate the diode performance, masking effects due to the anode. However, a type of carbon cathode has enabled the role of the anode in the diode to be better determined. In this letter, we compare experimental results with simulated diode performance, allowing an estimate of the bounds on the secondary emission coefficient from the anode surface as well as the amount of neutral gas liberated from this surface. In general, secondary electrons and neutral atoms lead to plasma formation in high power microwave devices, which in turn deleteriously affect the tube performance. Hence, an estimate of such quantities for use in particle-in-cell codes can prove pivotal for accurate modeling of experimental devices.

Carbon velvet field-emission cathode

Explosive field emission cathodes comprise an important class of cathodes for high power microwave tubes, having the advantages of light weight as well as requiring no heater for electron emission. Generally, however, this class of cathodes suffers from large amounts of outgassing, nonuniform emission, and very high emittance. This article describes a new class of carbon velvet cathodes that have been coated with a cesium iodide (CsI) salt. We discuss two manifestations of the cathode. We review the lifetime and operation of the cathodes with two different pulse durations, as well as the outgassing from the cathodes during operation. Lifetimes in excess of 980 000 pulses have been obtained, with an outgassing rate of 3.5 atoms per electron. Finally, we discuss the uniformity and emittance of tufted carbon cathodes that have been coated with CsI salt. For comparison, we relate these results to those previously obtained from other cathodes in this class. The cathodes have an emittance of 2.5π mm rad, as compared to the theoretical value, based on computation, of 2.3π mm rad. These new cathodes differ greatly from cathodes such as polymer velvet and tufted carbon fiber cathodes in that no volatiles reside on the cathode and in that a unique coating technique allows the cathodes to function. This new class of cathodes offers a potential replacement for existing thermal cathodes, in that no heater is required for superior operation with low outgassing and long lifetime.

Effects of anode materials on the performance of explosive field emission diodes

Explosive field-emission cathodes have been the electron emitter of choice, and often necessity, for high-power microwave (HPM) tubes for many years. The materials typically used for these cathodes range from polymer and cotton velvets, to metals such as stainless steel, and to carbon materials such as bulk carbon and carbon fibers. With several notable exceptions, the issues of the anode and its composition have been largely ignored. Generally, the diode performance, such as current levels, impedance collapse, and out-gassing, have been attributed to the cathode alone rather than to the combination of the cathode and anode. In this paper, we investigate the affects of various anode materials on the performance of explosive field emission cathodes. We show that bipolar flow significantly and rapidly alter diode performance at lower voltage and energy densities than usually observed. We show also the effects of anode material choice on out-gassing, and diode conditioning. Experiments have shown that bipolar flow is a significant issue in diode performance for even short pulses. The theoretical aspects of the diodes are discussed, with a comparison of experiment to theory.

Cathode and anode plasmas in short-pulse explosive field emission cathodes

Explosive field emission cathodes have been a subject of research for a number of years. These cathodes offer high current densities and total current without requiring a heater for the production of electrons. Generally these cathodes consist of some structure with a series of tips or metal–dielectric regions in which a large electric field enhancement can occur. A cathode plasma is then formed from these discharge points that then supplies the electrons necessary for space charge limited emission. This article reports on a series of optical measurements in which the cathode and anode plasmas of explosive field emission cathodes are observed. Three types of cathodes are investigated. These types are a polymer velvet cathode, a metal–dielectric cathode, and a tufted carbon fiber cathode in which the fibers have been coated with a cesium iodide salt. Cesium iodide coated carbon fiber cathodes have shown a great deal of promise for various field emitter applications. From these high speed photos, the evolution of the plasmas on both cathode and anode can be qualitatively ascertained. Experimentally we find that not only does cathode plasma behavior depend on the type of cathode, but the anode plasma behavior does also. Further, we find that the best performing cathode shows the most rapid plasma formation on both anode and cathode, yet without a rapid plasma expansion across the anode–cathode gap.

Emission uniformity and emission area of explosive field emission cathodes

Explosive field emission cathodes have been used extensively in high power microwave tubes. These cathodes emit electrons without the use of cathode heaters. Recently, some theoretical and simulation work has been performed to gain further understanding of the physics of these cathodes. The purpose of this letter is to provide the experimental background and justification for the theoretical work. The general idea of how explosive field emission cathodes operate is that plasma is rapidly formed, which provides the sea of electrons for space charge limited flow. However, recent theoretical and experimental work suggests edge effects, rather than plasma formation across the entire emission area, can also provide the same effect. In this letter we review three types of cathodes which have been tested. We provide optical data on the cathode emission uniformity as well as the electrical data for the same devices. In particular, we find that a large percentage of the cathode can fail to take part in the emission process and yet the voltage and current can appear identical from the case in which the entire cathode contributes electrons to the emission process.

The Human Genome Project The Dominance of Economy on Science- Ethical and Social Implications

Genetics today have occupied among sciences the privileged role of physics and chemistry of the beginning of this century. This explosive scientific field influences crucially various disciplines, among them life sciences and informatics. Moreover, it imposes “de facto” dramatic changes to our individual and collective life style, thus influencing the whole framework of our civilisation. The intensive involvement of the global economy in the progress of the research and the dissemination of its applications, arises ethical issues to be arranged.The danger of reappearance of Eugenic ideologies and practices does threaten the contemporary society. Anthropology has to contribute more intensively in the cultivation of a humanistic and moral nurture, necessary for the new framework of principles, efficient for the technological society of the 21st century.

High‐frequency wave propagation in the uppermost mantle

Short‐period, three‐component recordings of the seismic wave field of peaceful nuclear explosions recorded on deep seismic sounding profiles (Quartz and Ruby‐I, collected in 1984 and 1988, respectively) in northern Russia are used to constrain the nature of the high‐frequency teleseismic Pn phase, which can be observed for receiver distances of >3000 km. We suggest that this phase is caused by velocity fluctuations in the upper mantle acting as scatterers. To test this hypothesis and to determine the properties of the upper mantle scatterers, the elastic reflectivity method (one‐dimensional isotropic models) was used to model the coda of the high‐frequency teleseismic Pn phase. Both the observed data and the synthetic record sections were analyzed by examining the coda decay rates of the teleseismic Pn phase at 5 Hz. Synthetic seismograms are computed for different models based on the global model IASP91 with an added zone of randomly distributed velocity fluctuations (lamellae) just below the crust‐mantle boundary. These models may be characterized by the thickness of the scattering layer L, the vertical heterogeneity correlation length a, and the heterogeneity standard deviation σ. The numerical simulation of the wave propagation in these models generated a high‐frequency teleseismic Pn phase with a well‐pronounced, long coda. A comparison of the modeling results with the observations shows clearly that a velocity model containing fluctuations in the upper mantle can easily explain the presence of a high‐frequency teleseismic Pn. Comparing the coda decay rates of synthetic seismograms with the observation, we tried to constrain the properties of the velocity fluctuations in the upper mantle, L, a, and σ. In the distance range of the uninterfered high‐frequency teleseismic Pn phase beyond 1300 km, the coda decay rates for our best fitting model are similar to the observed ones. This model has a 75 km thick zone of scatterers below the Moho, containing lamellae with an average thickness of 2 km and a RMS velocity perturbation of 5%. The modeling results show also that two other possible models, the whispering gallery phase along the crust‐mantle boundary and scattering in the lower crust, taken alone are not able to explain the coda properties of the high‐frequency teleseismic Pn phase.

Calculation of the intensity of air shock waves in the near range of the explosion

The results of the studies dealing with the air shock-wave parameters are analyzed and the parameters obtained in the authors’ studies are reported. Generalized empirical dependences are obtained to calculate the wave intensity in the near range of the explosion of open concentrated and linear charges of large extent which consist of various shell-free explosives in a broad range of initial densities. The explosion fields of a full torus-like charge and linear and partially torus-like charges of finite length are considered theoretically. Formulas are derived to calculate the intensity of air shock waves of these charges, which agree with experiment.

Features of the hydrodynamic pressure field of an electric explosion in an equilibrium gas-liquid medium

The hydrodynamic features of an electric explosion in a bubble gas-liquid mixture are studied in the equilibrium approximation of the medium for the case of fine gas bubbles when the initial size of the latter ranges from units to tens of micrometers, as is observed when actual liquids contain natural gas. In mathematically modeling the electric-explosion processes, the characteristics of the hydrodynamic field were calculated, taking into account the finite size of the plasma piston for which the quasi-wave equation with the nonlinear barotropic equation of state of the mixture was numerically integrated, using an explicit finite-difference scheme in an ellipsoidal coordinate system. It is established that the presence of gaseous inclusions manifests itself when the gas concentration is ɛ0≳10−4, whereas appreciable nonlinear effects appear when ɛ0≳5×10−3.

Spin-flip transition and Faraday effect in antiferromagnet KMnF 3 in megagauss magnetic field

Faraday effect in the antiferromagnet KMnF 3 has been investigated in pulse explosive fields up to 500 T at T=78 K. The laser wavelength 0.63 μm was used in the experiment. The magnetic field dependence of Faraday rotation in this antiferromagnet shows a unique feature of a lack of saturation effect in the fields up to 500 T whereas critical field of spin-flip transition is about 120 T. The theoretical analysis of microscopic nature of Faraday rotation, including the diamagnetic, magneto-dipole and paramagnetic mechanisms has been performed. The strong competition of these mechanisms is important to explain the extremely small value of the effect and its unusual magnetic field dependence.

Sickle cell trait in Ivory Coast athletic throw and jump champions, 1956-1995.

In order to assess the performance of subjects with sickle cell trait (SCT) during brief and explosive exercise involving mainly alactic anaerobic metabolism, the percentage of athletes with SCT was determined in Ivory Coast track and field throw and jump champions, both men and women, for the period 1956-1995. Thirty-four (27.8%) sickle cell trait carriers (SCTC) were identified among the 122 national champions that we were able to contact. These 34 SCTC had won 78 national titles (24.5%) and established 37 national records (43.5%), distributed among the throw and jump events. These percentages were significantly higher than the prevalence of SCT in the general Ivory Coast population (12.0%). The women's high jump and men's shot put events had the highest percentages of SCTC record holders (90.9% and 87.5%, respectively). Moreover, the two top national record holders and title winners, one man and one woman, were SCTC athletes, and their hemoglobin S percentage (HbS: 39.1% and 39.4%, respectively) and mean corpuscular volume (MCV) excluded an associated alpha-thalassemia. We conclude that the significantly higher percentage of SCTCs among Ivory Coast track and field champions, as compared to the percentage in the general population suggests that SCT may be a determinant factor for success in brief and explosive track and field events involving mainly alactic anaerobic metabolism.

Short‐period observation of the 520 km discontinuity in northern Eurasia

Short‐period three‐component recordings of the seismic wave field of peaceful nuclear explosions (PNEs) along several long‐range profiles in northern Eurasia are analyzed to study the structure of the upper mantle. We use the data to search for evidence of the disputed 520 km discontinuity in the mantle transition zone. Here we present short‐period evidence for the existence of this proposed mantle discontinuity. On three long‐range profiles in Siberia we observe in the distance range from 2400 to 2600 km a typical change of the apparent velocity of the first arrivals and the appearance of two characteristic travel time branches. We interpret these phenomena as being caused by the proposed upper mantle discontinuity. To determine the optimum one‐dimensional P wave velocity model, synthetic seismograms were calculated with the reflectivity method. By comparing them with the observed data, we estimate the P wave velocity contrast across the 520 km discontinuity to be 0.25±0.05 km/s, while the velocity gradient below the boundary is 0.001±0.0005 1/s. This result indicates that in the mantle transition zone both phase transitions and variations in chemical composition can occur.

Observation of high‐frequency teleseismic Pn on the long‐range Quartz profile across northern Eurasia

Short‐period, three‐component recordings of the seismic wave field of Peaceful Nuclear Explosions (PNEs) on long‐range profiles are used to determine the fine structure of the mantle lithosphere. By analyzing the frequency content of the recorded phases and applying different band‐pass filters to the data, it is possible to divide the wave field into two distinctly different constituents: low‐frequency body waves traveling along Fermat paths (first arrivals) and the high‐frequency teleseismic (or long‐range) Pn phase traveling with a group velocity of 8.1 km/s and accompanied by a long, incoherent coda. This high‐frequency teleseismic Pn phase is observable from about 750 km, where it separates from the faster first arrival, to the maximum recording distance of 3145 km. It is recorded from shots at different locations and appears to be almost unaffected by the major tectonic feature along the profile, the Urals. The frequency spectrum of this Pn phase contains more high‐frequency energy (up to 12 Hz) than first arrivals that penetrate deeper into the upper mantle. The teleseismic high‐frequency Pn arrival has a remarkable coda, which is incoherent between closely spaced stations. The coda duration is dependent on the component of motion, being shortest on the vertical and longest on the transverse component. We propose a velocity model that is characterized by a zone extending from the crust‐mantle boundary to a depth of about 100 km. This zone has randomly distributed, spatially anisotropic velocity fluctuations. We propose that these velocity heterogeneities are “stretched” in the horizontal direction. This zone forms a scattering waveguide that confines the high‐frequency teleseismic Pn. There are indications that below this Pn waveguide, either the scale of the velocity fluctuations or the Q factor changes. This is expressed in a separation of the teleseismic Pn phase from the phase diving deeper into the mantle lithosphere.

High energy material research and development in India

The manuscript presents the research and development (R&D) work carried out by Indian scientists in the various areas of high energy materials (HEM) broadly covering propellants, explosives, and pyrotechnics. While India is self-sufficient in the field of high energy solid propellant, and both science and technology have reached international levels, more exhaustive R&D work is needed in the field of Earth storable liquid and cryogenic propellants. In the field of commercial and military high explosives, slurry/emulsion, cyclotrimethylenetrinitramine/cyclotetramethylenetetranitramine-based compositions have been extensively studied, evaluated, and used. However, to meet futuristic requirements, more powerful (velocity of detonation greater than 9500 m/s, density greater than 2 g/cc) and thermally stable explosives (>350°C) are being synthesized and characterized. Infrared (IR) smoke compositions effective in high IR region (8-14 /*) have been developed. However, the thrust areas in all the wings of HEM are development of more powerful, less sensitive, and environmentally friendly compositions and fully automatic, remote control processing technologies.

Novel cathode for field-emission applications

A CsI salt-based cathode which is capable of producing a modest perveance, 10 s of A/cm2 electron beam for several microseconds pulse lengths, and has little susceptibility to diode closure has been experimentally characterized. This explosive field-emission CsI-coated carbon fiber cathode has operated in modest 10−5 Torr vacuums at voltages up to 160 kV, and can easily be configured to provide space-charge-limited solid or annular electron beams in arbitrarily large diameter configurations. The CsI cathode has demonstrated negligible closure for 2 μs pulses, and has operated for 200 shots with no degradation in cathode performance. Data on the operating performance of this salt cathode, including effective gap time history and streak photographs demonstrating uniformity of the current density, are presented. A comparison of CsI cathode performance with a velvet explosive field emitting cathode used in electron-beam production is also presented.

Coercive field of explosion sintered Sm2Fe17Nx magnets

The coercive field of explosion sintered Sm2Fe17Nx magnet is analysed on the basis of micromagnetism. Results indicate that the coercivity of explosion sintered Sm2Fe17Nx magnet is controlled by the nucleation field near the grain surface. The large demagnetization factor originates from the strong strayfields of nonmagnetic poles, magnetically soft phases, and the very sharp corners of Sm2Fe17Nx grains.

Effect of an ensemble of explosions on the Galactic dynamo. I - General formulation

view Abstract Citations (43) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effect of an Ensemble of Explosions on the Galactic Dynamo. I. General Formulation Ferriere, Katia Abstract A general analytical expression for the electromotive force produced by a field of explosions in the Galactic disk is derived. Spherically symmetric and axisymmetric explosions successively are considered. The velocity and magnetic fields associated with one explosion and average their cross-product over a spatial distribution of explosions are first calculated. Simple expressions for the magnetic flux escape velocity and for the so-called alpha parameter are then provided. Publication: The Astrophysical Journal Pub Date: April 1992 DOI: 10.1086/171204 Bibcode: 1992ApJ...389..286F Keywords: Dynamo Theory; Electromotive Forces; Interstellar Magnetic Fields; Interstellar Matter; Magnetohydrodynamics; Milky Way Galaxy; Coriolis Effect; Galactic Rotation; Galactic Structure; Lines Of Force; Supernovae; Synchrotron Radiation; Astrophysics; ISM: MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS: MHD full text sources ADS |

Operating regime of an explosive magnetic field compression generator with a capacitive load with a consideration of magnetic flux losses

Operating regime of an explosive magnetic field compression generator with a capacitive load with a consideration of magnetic flux losses