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The magnetised spherical Couette (MSC) problem, a three dimensional magnetohydrodynamic paradigmatic model in geo- and astrophysics, is considered to investigate bifurcations to high-dimensional invariant tori and chaotic flows in large scale dissipative dynamical systems with symmetry. The main goal of the present study is to elucidate the origin of chaotic transients and intermittent behaviour from two different sequences of Hopf bifurcations involving invariant tori with four fundamental frequencies, which may be resonant. Numerical evidence of the existence of a crisis event destroying chaotic attractors and giving rise to the chaotic transients is provided. It is also shown that unstable invariant tori take part in the time evolution of these chaotic transients. For one sequence of bifurcations, the study demonstrates that chaotic transients display on-off intermittent behaviour. A possible explanatory mechanism is discussed., 19 pages and 18 figures

The transition between the stable base state of the magnetized spherical Couette (MSC) flow and the return flow instability is experimentally investigated. The experiments are conducted using an MSC setup consisting of insulating spheres with the ratio of the inner to the outer radii r i/r o = 0.5, Reynolds number Re = 1000 and Hartmann number Ha ∈ [25, 29]. The transition is characterized by changes in the power spectra of the azimuthal modes in the flow as Ha is dynamically changed. The transition occurs in the interval Ha ∈ [26.5, 27.5]. The evolution of the power spectra of the azimuthal modes exhibits hysteretic effect depending on whether Ha is increased or decreased within the experimental interval. The power spectra in the azimuthal modes m ∈ {3, 4} increases and remains dominant as Ha is increased, while the power spectra in m ∈ {2, 4} are dominant while the flow is time dependent due to return flow instability as Ha is decreased.

The properties of rotating turbulence driven by precession are studied using direct numerical simulations and analysis of the underlying dynamical processes in Fourier space. The study is carried out in the local rotating coordinate frame, where precession gives rise to a background shear flow, which becomes linearly unstable and breaks down into turbulence. We observe that this precession-driven turbulence is in general characterized by coexisting two dimensional (2D) columnar vortices and three dimensional (3D) inertial waves, whose relative energies depend on the precession parameter $Po$. The vortices resemble the typical condensates of geostrophic turbulence, are aligned along the rotation axis (with zero wavenumber in this direction, $k_z=0$) and are fed by the 3D waves through nonlinear transfer of energy, while the waves (with $k_z\neq0$) in turn are directly fed by the precessional instability of the background flow. The vortices themselves undergo inverse cascade of energy and exhibit anisotropy in Fourier space. For small $Po0.1$ turbulence is quasi-steady with only mild fluctuations, the coexisting columnar vortices and waves in this state give rise to a split (simultaneous inverse and forward) cascade. Increasing the precession magnitude causes a reinforcement of waves relative to vortices with the energy spectrum approaching Kolmogorov scaling and, therefore, the precession mechanism counteracts the effects of the rotation., 20 pages, 16 figures, 1 table, submitted to Physics of Fluids

We report Alfvén-wave experiments with liquid rubidium at the Dresden High Magnetic Field Laboratory (HLD). Reaching up to 63 T, the pulsed magnetic field exceeds the critical value of 54 T at which the Alfvén speed coincides with the sound speed. At this threshold we observe a period doubling of an applied 8 kHz CW excitation, which is consistent with the theoretical expectation of a parametric resonance between magnetosonic waves and Alfvén waves. Similar mode conversions are discussed as a possible mechanism for heating the solar corona.

A nonequilibrium atmospheric streamer discharge was investigated as a means to seed a large-gap arc breakdown. The dynamics of the streamer were analyzed with high speed imaging, photodiode light intensity, and current–voltage measurements. The temporal evolution of the discharge included a localized surface corona and a positive surface streamer. With the addition of an impurity gas (methane), the ionization was suppressed, which inhibited surface streamer propagation. The electron temperature was determined from time and spatially averaged spectra, coupled with a collisional–radiative model. The electron temperature in argon was measured at 1.25 eV for an electron density range of 1019 – 1020 m –3. Partial local thermodynamic equilibrium calculations showed that the Ar II 4p states followed a Boltzmann distribution with an excitation temperature of 0.7 eV. The gas temperature was estimated at 815 K from a black-body distribution. The velocity of the surface streamer in argon was estimated at ~100 km/s with a diameter of $\sim 500~\mu \text{m}$ .

Background. Bamboos are typical plants in tropical forests acting as resource for different animals and influencing forest structure and dynamics, but our knowledge about their influence on ecosystem functioning remains in its infancy. Nitrogen (N) is an important macronutrient that exerts control over productivity of ecosystems. Biological Nitrogen Fixation (BNF) is one of the main N input to terrestrial systems, more specifically by free-living BNF in tropical forests. In these forests, the dominant presence of bamboo and the occurrence of free-living N-fixers in its leaf surfaces appear to play a relevant role in N cycling. Here, we explore the importance of a native bamboo to N cycle in a Neotropical forest to provide insights on bamboo’s role on ecosystem functioning. Methods. The study was conducted in a pristine montane Atlantic Forest, Brazil. We select 100 sample units (100 m² each) and counted all bamboo clumps and live culms, estimated the leaf area, the litter production and its N content. We estimated the potential N input mediated by bamboo based on available data of free-living BNF rates for this species and then we contextualized it with information about N cycling components in the study area. Results. We counted 4000 live culms ha-1 and estimated 4.3 x 104 m² ha-1 of bamboo leaf area for free-living N-fixers colonization. Considering the free-living BNF rates quantified for the same study area, the native bamboo may contribute up to 61.6 kg N ha-1 y-1 which represents a reduction in the N deficit of at least 25% in the Neotropical forest area we studied. Through its litter, the bamboo adds 9 kg N ha-1 y-1 to the system (15% of the total fixed on its leaf surface). Discussion. The bamboo’s contribution for N input we found may be explained by free-living community composition on its leaf as previously indicated by other studies, as well as by the bamboo overabundance, and thus habitat availability for colonization. Such N input represents a reduction in N deficit in the study area where the rates of symbiotic BNF (0.2 kg N ha-1y-1) are lower than values in Amazon Forest (3 kg N ha-1y-1). This native bamboo may supply N to the system probably through the decomposition of its leaves which represents a return of 15% of the total N fixed. Although it is still necessary to evaluate the characteristics of bamboo that will further elucidate its role in the functioning of the forests, our findings suggest that it closely regulates N inputs and may better explain high diversity and carbon stocks of the area.

Background Nitrogen (N) is an important macronutrient that controls the productivity of ecosystems and biological nitrogen fixation (BNF) is a major source of N in terrestrial systems, particularly tropical forests. Bamboo dominates theses forests, but our knowledge regarding the role of bamboo in ecosystem functioning remains in its infancy. We investigated the importance of a native bamboo species to the N cycle of a Neotropical forest. Methods We selected 100 sample units (100 m2 each) in a pristine montane Atlantic Forest, in Brazil. We counted all the clumps and live culms of Merostachys neesii bamboo and calculated the specific and total leaf area, as well as litter production and respective N content. Potential N input was estimated based on available data on BNF rates for the same bamboo species, whose N input was then contextualized using information on N cycling components in the study area. Results With 4,000 live culms ha−1, the native bamboo may contribute up to 11.7 kg N ha−1 during summer (January to March) and 19.6 kg N ha−1 in winter (July to September). When extrapolated for annual values, M. neesii could contribute more than 60 kg N ha−1y−1. Discussion The bamboo species’ contribution to N input may be due to its abundance (habitat availability for microbial colonization) and the composition of the free-living N fixer community on its leaves (demonstrated in previous studies). Although some N is lost during decomposition, this input could mitigate the N deficit in the Atlantic Forest studied by at least 27%. Our findings suggest that M. neesii closely regulates N input and may better explain the high diversity and carbon stocks in the area. This is the first time that a study has investigated BNF using free-living N fixers on the phyllosphere of bamboo.

This paper proposes that in some instances, the magnetic saw effect is purely a melt phenomena and describes a series of experiments where the magnetic saw effect was studied by varying the sub-millimeter radius of the enhancement point on a set of highly-controlled samples. The magnetic saw effect is an instability that can occur when high currents are required to make sharp bends, thereby greatly enhancing the magnetic field and current densities. These sharp bends generally occur at joints, cuts, or other flaws and can cause failure of high-current conductors. The magnetic saw effect has also been proposed as a possible metal-cutting mechanism for manufacturing purposes. Due to the very small length scales of the features associated with magnetic sawing, it is difficult to make samples that are controlled enough to get reproducible experimental results for systematic studies. In these experiments, printed circuit board technology was used to accurately make samples with very fine enhancement features. The conditions for the magnetic saw effect were modeled using an electromagnetic code to establish the important parameters that predict the onset of damage.

A survey of members of the International Society of Burn Injuries (ISBI) and the American Burn Association (ABA) indicated that although there was difference in burn resuscitation protocols, they all fulfilled their functions. This study presents the findings of the same survey replicated in Africa, the only continent not included in the original survey. One hundred and eight responses were received. The mean annual number of admissions per unit was ninety-eight. Fluid resuscitation was usually initiated with total body surface area burns of either more than ten or more than fifteen percent. Twenty-six respondents made use of enteral resuscitation. The preferred resuscitation formula was the Parkland formula, and Ringer's Lactate was the favoured intravenous fluid. Despite satisfaction with the formula, many respondents believed that patients received volumes that differed from that predicted. Urine output was the principle guide to adequate resuscitation, with only twenty-one using the evolving clinical picture and thirty using invasive monitoring methods. Only fifty-one respondents replied to the question relating to the method of adjusting resuscitation. While colloids are not available in many parts of the African continent on account of cost, one might infer than African burn surgeons make better use of enteral resuscitation.

A gas-fed capillary plasma source has been developed to study plasma-surface interactions under pulsed high pressure arc conditions, without the use of an exploding fuse wire or ablative liner. A nonintrusive preionization source has been developed to break down relatively large interelectrode gaps at low charge voltages of 2-6 kV. The preionization source comprises a nonequilibrium surface streamer discharge that forms a conducting channel through which the main thermal arc discharge is initiated. The arc electron temperature and number density are estimated to be T e ~ 1-2 eV and n e ~ 10 23 m -3 . Silicon and sapphire samples were exposed to the arc plasma and revealed deposition of electrode and wall materials. Substitution of Elkonite 50W3 for brass electrodes reduced plasma contamination to acceptable levels. The plasma-material interactions were examined and quantified using scanning electron microscopy and energy dispersive X-ray spectroscopy.

This paper describes a novel technique for making fine cuts in metal plates and foils. The process is based on a variant of the magnetic saw effect, a magnetic instability associated with high pulsed magnetic fields. When not controlled, the magnetic saw effect can lead to failure in high-field pulsed conductors. By limiting energy and pulse duration, the authors have produced controlled cuts with kerfs as fine as 0.01 mm. These promising results motivated a systematic study of the parameters that govern the quality and reproducibility of magnetic saw cuts. This paper provides an overview of the technique, of some key experiments, and of the practical issues associated with using pulsed magnetic fields for manufacturing.

This paper describes a novel method for determining the specific action to melt the metals, and reports the values for action to melt that measure for several elements and three alloys: aluminum 2024, aluminum 6061, and C27400 brass. We electrically heat small diameter wires (127 $\mu{\rm m}$ ) to the point of vaporization using a slow regime exploding wire experiment. Using high-resolution voltage and current data, we compute the derivative of electrical resistivity with respect to specific electrical action. Features in the plot of this derivative clearly show the onset of melting for many of the materials we tested. We compare our results for copper, silver, aluminum, molybdenum, and titanium to those published by Tucker and Toth in the 1970s. Our data agree with their published values for silver and molybdenum, but not with those for copper, aluminum, and titanium. This paper presents our results and discusses possible reasons for the discrepancies between some of our measurements and those of Tucker and Toth.

This paper describes temperature measurements made on the high-energy medium-caliber launcher at the Institute for Advanced Technology. Simulations performed in Maxwell 3-D and E-Physics showed that Joule heating from current diffusing into the rails accounts for most of the temperature rise in the conductors. Temporal skin effects increase thermal dissipation significantly over what would be expected by the ohmic losses under fully diffused conditions. Based on this analysis, Joule heating is the overwhelmingly dominant source of heating in low-speed tests. As the velocity of the armature increases, Joule heating remains the dominant source of heat; however, additional mechanisms-which may include frictional heating, arcing energy, aluminum deposition, and temperature-dependent properties-are required to more satisfactorily explain the temperature profile obtained.

The Institute for Advanced Technology has been conducting experiments on plasma-armature railguns for the past several years. To control the damage mechanisms associated with this class of railgun, an electrothermal (ET) launcher preinjects the projectile into the plasma railgun bore milliseconds before the arc is energized. During the initial testing of the ET launcher, it was found that injection velocities were significantly lower than expected for a given input energy. Subsequent experiments using photonic Doppler velocimetry to resolve velocity as a function of time confirmed this finding. Using an ablative capillary discharge code written by Powell and Zielinski in 1991, the velocity discrepancy is analyzed, and the properties of the propellant gas-such as ionization fractions, the mass of the propellant gas, and the magnitude and decay of the pressure pulse seen by the projectile-are determined.

This paper describes how small permanent magnets embedded in a launch package can be used to locate armature position during launch with greater accuracy than can be achieved by detecting the dipole field of the armature itself. Using this technique, we determined armature position in a medium-caliber launcher to within a millimeter of the value measured with photonic Doppler velocimetry, a precise optical technique. This paper describes the technique and some experimental results.

This paper provides a brief overview of railgun armature development undertaken at the Institute for Advanced Technology (IAT) and elsewhere over the last decade. The fundamental physics issues that govern the armature requirements are described. These include the operating requirements, minimum parasitic mass, material action limits, contact interface pressures, electromagnetic skin effects and current nonuniformities, magnetic sawing, launch package interactions, material surface treatments, melt lubrication, gouging, and transition to arcing contact. Different bore geometries-square, rectangular, round, augmented, -turn-are also described and require matching armature and launch package designs. Novel designs are discussed, including forward tabs, magnetic obturators, splined armatures, fiber contacts, pseudoliquid armatures, plasma, and hybrid armatures.

The Institute for Advanced Technology (IAT) at The University of Texas at Austin has been conducting research aimed at achieving muzzle velocities in excess of 7 km/s using a plasma-driven electromagnetic launcher. Plasma-driven railguns are susceptible to bore effects that limit muzzle speeds to about 6 km/s. The velocity ceiling is believed to be the result of the viscous drag of material ablated from the bore of the launcher. Ablation of bore materials, especially the insulators, is caused by the intense heat radiated by plasma armatures, especially at low speeds. The experiments reported in this paper are focused on overcoming the velocity ceiling of 6 km/s. A proof-of-principle experiment has been designed to launch 5-10 g polycarbonate projectiles to muzzle velocities in excess of 7 km/s with modest acceleration loads of 500 kG. Our approach to controlling bore ablation involves the following elements: (1) using magnetic augmentation to reduce power dissipation in the plasma; (2) using high-purity alumina insulators to raise the ablation resistance of the bore; (3) using pre-acceleration to prevent ablation of the bore materials at low velocity; and (4) using a synchronously driven distributed power supply to electrically isolate stages. This paper describes the consequences of excessive bore ablation, the rationale for the IAT experiment, and the results obtained during testing in 2009.

A host–parasite relationship was observed, for the first time, between a piscicolid leech and a species of amphibious goby (Scartelaos tenuis) from an intertidal mud flat in southern Iran. Morphological and molecular investigations assign the leech to Zeylanicobdella arugamensis. Of the 3 endemic and sympatric mudskipper species living in the Persian Gulf (S. tenuis, Boleophthalmus dussumieri, and Periophthalmus waltoni), leeches were only found on S. tenuis (prevalence and mean intensity = 71.4% and 2.3 ± 2.5, respectively), which is also the most-aquatic mudskipper species. Scartelaos tenuis is not the largest species, but more leeches (≥4 leeches/host) were found on larger specimens (>12 cm standard length [SL]). Nonetheless, in aquaria, leeches also attached on P. waltoni. This suggests either an ecological partitioning of host–parasite complexes, determined by host habitat selection, or leech limited-resistance to air exposure, or both.

This paper deals with a theoretical study of the hot-spot onset (HSO) in cellular bipolar power transistors. This well-known phenomenon consists of a current crowding within few cells occurring for high power conditions, which significantly decreases the forward safe operating area (FSOA) of the device. The study was performed on a virtual sample by means of a fast, fully analytical electrothermal simulator operating in the steady state regime and under the condition of imposed input base current. The purpose was to study the dependence of the phenomenon on several thermal and geometrical factors and to test suitable counter measures able to impinge this phenomenon at higher biases or to completely eliminate it. The power threshold of HSO and its localization within the silicon die were observed as a function of the electrical bias conditions as for instance the collector voltage, the equivalent thermal resistance of the assembling structure underlying the silicon die, the value of the ballasting resistances purposely added in the emitter metal interconnections, and the thickness of the copper heat spreader placed on the die top just to the aim of making more uniform the temperature of the silicon surface.

The Remote Didactic Laboratory Laboratorio Didattico Remoto - LA.DI.RE. ldquoG. Savastanordquo is the e-learning measurement laboratory supported by the Italian Ministry of Education and University. It involves about 20 Italian universities and provides students of electric and electronic measurement courses with access to remote measurement laboratories delivering different didactic activities related to measurement experiments. In order to demonstrate the versatility for didactic use, the overview of some experiments is given. The didactic experiments summarized in this paper concern measurement characterization of instruments and communication systems, measurement devices for remote laboratories, basic electrical measurements, magnetic measurements, electromagnetic-interference measurements, and signal processing for measurement applications.

This paper describes work investigating a rail damage mechanism observed in solid-armature railguns at the Institute for Advanced Technology, The University of Texas at Austin. The damage occurs in the starting section of rails and is not associated with high-speed phenomena such as hypervelocity gouging or transition to arcing contact. The damage, which we call grooving, is localized to the region of the rails adjacent to the insulators. In this paper, we describe grooving damage observed in multiple tests using copper rails. In one series, in which we tested up to 20 shots on one pair of copper rails, we obtained grooves on the order of a millimeter deep and several millimeters wide. We present evidence that grooving is caused by liquid erosion and is not a result of plasma heating or mechanical deformation

The Institute for Advanced Technology has recently been awarded a grant by the U.S. Air Force Office of Scientific Research to undertake research in the area of hypervelocity, plasma-driven electromagnetic launch. This paper describes our technical approach to achieving 7 km/s at accelerations that are lower than typical for plasma launchers. Our approach to managing plasma restrike and secondary arc formation is to reduce the emission of ablated materials into the gun bore through the use of augmentation and ceramic sidewalls. In addition, preinjection to 1 km/s is used to provide an initial velocity for the launch package while introducing minimal dense cold gas, since such gas can also contribute to restrike. This paper describes the hardware that is being put in place for this experiment

This paper summarizes experiments to investigate the possibility of delaying the onset of transition by using armatures with high melting temperatures. The rationale for the experiments was that armature materials with high melting points might outperform aluminum by reducing melting and erosion at the rail-armature interface. The tests used a novel technique that consists of placing thin claddings on the rail-contacting surfaces of the armatures. Two low-speed tests with copper armature claddings proved that claddings affixed to the trailing arms can survive a railgun launch. These tests were followed by four tests at about 2 km/s that used titanium armature claddings. These tests showed that titanium-clad armatures can perform well; however, this performance cannot be maintained beyond the first shot because the titanium armature claddings cause significant damage to the rails

A non-equilibrium atmospheric streamer discharge has been investigated as a means to seed a large gap arc breakdown. The dynamics of a surface streamer discharge has been analyzed with high speed imaging, voltage and current characterization as well as light intensity measurements. The temporal evolution of the discharge has been examined and a mechanism for the formation and propagation of the discharge presented. Thermodynamic properties of the atmospheric streamer discharge were determined from a time and spatially averaged spectra and a collisional-radiative model. The electron temperature and density were measured to be 1.25 eV and 10 m respectively. The velocity of the surface streamer was estimated to be ~110 km/s with a diameter of ~400 μm.

The Sinewave Histogram Test (SHT) is a widely used standard technique to estimate the threshold voltages and the Integral NonLinearity (INL) pattern of Analog-to-Digital Converters (ADCs). The main drawback of the SHT is that, for a given target accuracy, the number of test samples as well as the testing time tend to increase exponentially as the resolution of the converter grows. The strategy described in this paper still relies on the SHT, but it enables a major reduction in testing time without affecting estimator accuracy provided that the INL pattern exhibits prevailingly a low code frequency content. This result can be achieved simply by suitably filtering the INL pattern obtained using a standard SHT over a reduced amount of test samples. After being justified theoretically, the proposed solution is validated by means of both simulations and experimental evidences.

In this paper, the performance of a code-division multiple-access (CDMA) system using digital frequency conversion is analyzed and modeled. The joint effect of channel noise, sampling jitter, and analog-digital converter (ADC) errors is discussed and modeled assuming uniform converters. Variable resolution memoryless and /spl Sigma//spl Delta/ ADCs are considered, and the sensitivities of the resulting systems to channel noise, sampling jitter, and overloading errors are compared.

The majority of published work on the transition to arcing contact in solid-armature railguns has focused on modeling the current melt wave, a localized front of molten material that erodes the perimeter of the armature. The most frequently cited models are based on two-dimensional (2-D) analytical formulations in which the dominant source of current concentration is due to the velocity skin effect (VSE). This paper compares the predictions of three such models with a 2-D numerical simulation that includes the effects of VSE, melting, and loss of molten material.

The muzzle voltage in a solid-armature railgun is an important diagnostic because it can provide information about the state of the rail/armature interface. Large changes in amplitude in the muzzle voltage usually indicate a transition from nonarcing to arcing contact. Medium and small amplitude changes are also observed in the muzzle voltage. Although the meaning of these is less understood, it has been suggested that these too might contain information about the condition of the rail/armature interface. This paper reports on an experiment in which features of the muzzle voltage trace were correlated to physical features of the launcher that most likely perturbed the inductance gradient of the gun. Although we could not conclusively determine which specific physical features make the majority of the contributions, our results suggest that physical features of the railgun do account for aspects of the secondary structure in our muzzle voltage traces.

This report describes a melt-lubrication model of the liquid film interface in solid-armature railguns that includes the effects of turbulence. The liquid film is modeled as a high-speed Couette flow with viscous heating. The model focuses strictly on mechanical wear and does not include magnetohydrodynamic body forces or Joule heating. The effects of turbulence are incorporated by using Prandtl's mixing-length theory and semiempirical methods. Turbulent viscosity is added to the laminar viscosity for the solution of the momentum equations. Thermal resistance is accounted for at the thermal contact between the melt film and rail. Expressions are obtained for the quantities of film thickness and melt wear rate. Results from the model are compared with experimental data that measured high-speed mechanical wear of 7075 aluminum sliding against ETP copper for face pressures ranging from 45 to 150 MPa in railguns. Although the model reproduces general trends in the data, it predicts melt speeds that are significantly greater than measured; in this regard, a previously published laminar model provides a better fit. However, given the shortcomings of both models, it is quite possible that a purely thermal hydraulic formulation is inadequate to the task of modeling high-speed, high-pressure mechanical wear, and that viscoplastic processes in the slider should be considered next.

The separation of air components, oxygen and nitrogen, to obtain oxygen-enriched mixtures was studied by molecular modeling, utilising the Grand Canonical Monte Carlo method. Since a good gas-chromatographic resolution between oxygen and nitrogen was obtained on acid mordenite, with oxygen eluted first, the differential adsorption of air components was studied on this porous material in selected ranges of pressures (120–200 kPa) and temperatures (300–383 K). The results, which show that an oxygen enrichment up to 80% can be obtained, cannot be explained only by the small molecular volume difference, but quadrupolar interactions must also be considered. These strong interactions between molecules and within the host change considerably the amount of sorbed molecules, which depends on the mixture pressure and composition and on the acidity of the sorbent as well.

This paper presents two numerical approaches to modeling melt-wave erosion in conductors. The first uses a one-dimensional finite-difference formulation and provides a starting point for understanding the effects of applied field and material properties on erosion speed. The second approach involves manipulating material properties in the three-dimensional (3-D) finite-element analysis (FEA) code EMAP3D. Because EMAP3D cannot eject material from the computation in the same manner as the finite-difference code, a "pseudoerosion" algorithm was devised to render cells above the melting temperature nonconducting. A comparison of the two approaches shows results that are in good agreement for depth of magnetic diffusion, current density, and temperature rise, as well as for depth and speed of erosion. These results are encouraging because they suggest that EMAP3D could be used to model melt-wave erosion in realistic 3-D armatures.

This paper describes experiments to measure melt-wave erosion (MWE) damage in railgun armatures. C-armatures were launched on the Medium Caliber Launcher at the Institute for Advanced Technology (IAT), Leander, TX, and recovered by "soft catching" in a tube filled with cotton rags. Recovered armatures were mechanically profiled to measure erosion depth as a function of position on the contacting surfaces. Although recovered armatures provide only a final snapshot of the erosion process, one can show how erosion proceeds by recovering armatures at different stages of the launch. This type of study requires the use of a crowbar switch at the breech of the railgun to rapidly shunt current out of the armature, thereby interrupting the erosion process at the desired point in the launch. We describe the experimental setup and summarize the results of 11 experiments. The experiments investigated the effect of linear current density (ranging from 35 to 45 kA/mm) and rail material (copper and aluminum) on MWE damage. Lastly, we present some thoughts on contact transition mechanisms that, in some cases, prevented the collection of MWE data.

An analytic model of the current melt wave in a one-dimensional stationary conductor has been developed to gain insight into the complex problem of melt-wave erosion contact wear in railgun armatures. Unlike two-dimensional models with motion, in which the dominant driving mechanism for the erosion front is current concentration from the velocity skin effect, in one dimension, the driving mechanism is a concentration of current caused by the electrodynamics at the melt-wave erosion boundary. Specifically, as molten material is ejected, current is driven into the remaining solid material at the interface, resulting in a local concentration of current and joule heating at the interface. We derive an expression for the velocity of the melt-wave front in one dimension, and assess the importance of this effect by comparing the erosion speed we obtain with erosion speeds predicted by previous models. The comparison suggests that the electrodynamics of the moving melt-wave boundary has an insignificant effect on melt-wave erosion in solid armature railguns, and as such can be justifiably neglected.

Over the last two decades, several mechanisms of armature transition have been postulated. Each mechanism has been subjected to experimental and analytic examination with the objective of "explaining" transition. However, no single mechanism has emerged as the consensus cause of transition. In this paper, we take the position that all the mechanisms that have been identified, and perhaps others which have not yet been, contribute to and may be mutually influenced by those processes leading to the onset of transition. Our inability to correlate transition with a single mechanism, or model, results from the multiplicity of mechanisms at work. We examine all the mechanisms identified to date, suggest several additional ones, and explore a more complex transition world in which each transition event is potentially caused by a different mechanism or combination of mechanisms. With this picture, control of transition requires that we control all of the mechanisms which can result in transition, not just one.

A reconfigurable audio amplifier is proposed. It can be configured to operate as a standard dual-bridge, a high-efficiency dual-bridge, or a high-power mono-bridge amplifier. Moreover, all these configurations can be set in low-noise mode. Built-in diagnostics with an I/sup 2/C-bus interface completes the features. The device, realized in a 0.6 /spl mu/m technology, delivers to 1 /spl Omega/ up to 150 W in high-power mode, saves 50% of the dissipated power in the high-efficiency configuration, and has a 16 /spl mu/V RMS in-band output noise in the low-noise configuration.

Summary Two new single-nucleotide polymorphisms are described within the human interferon gamma (IFN-γ) promoter in a Sudanese population. One is a G to T transition at position −183 from the transcription start. The other is a A to G transition at position −155. Allelic frequency analysis indicated frequencies of 0.927 (G) and 0.073 (T) at position −183 and 0.977 (A) and 0.023 (G) at position −155. These two polymorphisms have not been detected in the Centre d’Etude du Polymorphisme Humain (CEPH) reference population. The polymorphism −183(G T) may alter the AP-1 binding domain and the regulation of transcription. The polymorphism −155(A G) is located close to the nuclear factor-activated T-cell site (NFAT site) (−168 TAAAGGAAA-160) and may affect the stability of this region.

Molecular modelling, utilising the Grand Canonical Monte Carlo method, was employed to study the interactions between nitrous oxide (N2O) and nitric oxide (NO) with a solid matrix, namely mordenite, before and after charge equilibration. The results indicate that the charge equilibration leads to an adsorption which is independent of the pressure. Comparison with gas-chromatographic experiments indicate that charge equilibration is not allowed in acid mordenite. Therefore, in the case of polar molecules, their adsorption depends on the charge in the zeolite micropores.