Current Instability Research Articles

Acoustic diagnostics of electrical origin fault modes with readily available consumer‐grade sensors

Acoustic diagnostics, traditionally associated with mechanical fault modes, can potentially solve a wider range of monitoring applications. Typically, fault modes are induced purposefully by the researcher through physical component damage whilst the system is shutdown. This study presents low-cost real-time fault diagnostics of a previously unreported acute electrical origin fault that manifests sporadically during system operation with no triggering intervention. A suitability study into acoustic measurements from readily available consumer-grade sensors for low-cost real-time diagnostics of audible faults, and a brief overview of the theory and configuration of the wavelet packet transform (including optimal wavelet selection methods) and empirical mode decomposition processing algorithms is also included. The example electrical origin fault studied here is an unpredictable current instability arising with the pulse-width modulation controller of a BrushLess DC motor. Experimental trials positively detect 99.9% of the 1160 resultant high-bandwidth torque transients using acoustic measurements from a USB microphone and a smartphone. While the use of acoustic techniques for detecting emerging electrical origin faults remains largely unexplored, the techniques demonstrated here can be readily adopted for the prevention of catastrophic failure of drive and power electronic components.

Eddies in the Tropical Atlantic Ocean and Their Seasonal Variability

AbstractWe study mesoscale eddy characteristics in the tropical Atlantic Ocean, using 23 years of daily altimetry sea level anomalies. Eddies are mainly generated in the eastern boundary upwelling systems and in the Brazil Current region. Their westward propagation speed reaches 20 cm/s in equatorial areas, decreasing with latitudes. They present typical amplitudes of 1‐5 cm. The largest and most energetic eddies are observed in the equatorial region, especially in the North Brazil Current (NBC) retroflection. The seasonal cycle of the eddy characteristics shows higher amplitudes along the NBC retroflection and the western part of the North Equatorial Countercurrent. A new criterion, based on altimetry data, determines the probability that barotropic instability of mean surface currents is responsible for eddy generation. We find a strong likelihood that the latter plays a key role along the North Equatorial Countercurrent, whereas other mechanisms must be invoked for the NBC region.

Effect of Oxygen Content on Current Stress-Induced Instability in Bottom-Gate Amorphous InGaZnO Thin-Film Transistors.

The effect of oxygen content on current-stress-induced instability was investigated in bottom-gate amorphous InGaZnO (a-IGZO) thin-film transistors. The observed positive threshold voltage shift (ΔVT) was dominated by electron trapping in the gate insulator (GI), whereas it was compensated by donor creation in a-IGZO active regions when both current flows and a high lateral electric field were present. Stress-induced ΔVT increased with increasing oxygen content irrespective of the type of stress because oxygen content influenced GI quality, i.e., higher density of GI electron traps, as well as typical direct current (DC) performance like threshold voltage, mobility, and subthreshold swing. It was also found that self-heating became another important mechanism, especially when the vertical electric field and channel current were the same, independent of the oxygen content. The increased ΔVT with oxygen content under positive gate bias stress, positive gate and drain bias stress, and target current stress was consistently explained by considering a combination of the density of GI electron traps, electric field relaxation, and self-heating-assisted electron trapping.

Physics of relativistic collisionless shocks. II. Dynamics of the background plasma.

In this second paper of a series, we discuss the dynamics of a plasma entering the precursor of an unmagnetized, relativistic collisionless pair shock. We discuss how this background plasma is decelerated and heated through its interaction with a microturbulence that results from the growth of a current filamentation instability in the shock precursor. We make use, in particular, of the reference frame R_{w} in which the turbulence is mostly magnetic. This frame moves at relativistic velocities towards the shock front at rest, decelerating gradually from the far to the near precursor. In a first part, we construct a fluid model to derive the deceleration law of the background plasma expected from the scattering of suprathermal particles off the microturbulence. This law leads to the relationship γ_{p}∼ξ_{b}^{-1/2} between the background plasma Lorentz factor γ_{p} and the normalized pressure of the beam ξ_{b}; it is found to match nicely the spatial profiles observed in large-scale 2D3V particle-in-cell simulations. In a second part, we model the dynamics of the background plasma at the kinetic level, incorporating the inertial effects associated with the deceleration of R_{w} into a Vlasov-Fokker-Planck equation for pitch-angle diffusion. We show how the effective gravity in R_{w} drives the background plasma particles through friction on the microturbulence, leading to efficient plasma heating. Finally, we compare a Monte Carlo simulation of our model with dedicated PIC simulations and conclude that it can satisfactorily reproduce both the heating and the deceleration of the background plasma in the shock precursor, thereby providing a successful one-dimensional description of the shock transition at the microscopic level.

Influence of the Gate/Drain Voltage Configuration on the Current Stress Instability in Amorphous Indium-Zinc-Oxide Thin-Film Transistors With Self-Aligned Top-Gate Structure

The influence of ${V}_{\text {GS}}/{V}_{\text {DS}}$ condition on the current stress (CS) instability in amorphous InZnO thin-film transistors (TFTs) with the self-aligned top-gate structure is comprehensively analyzed and quantitatively validated by consolidating: 1) the ${I}$ – ${V}$ and ${C}$ – ${V}$ characteristics; 2) the extraction of density of states; 3) the decomposition of threshold voltage shift ( $\Delta {V} _{T}$ ); and 4) the computer-aided design simulation. It has been found that in a high ${V}_{\text {GS}}$ and low ${V}_{\text {DS}}$ CS condition, electron trapping into the gate insulator globally occurs. However, these effects are combined with a local electron trapping into gate insulator in the source region and the generated peroxide defects in the drain region under a high ${V}_{\text {DS}}$ and low ${V}_{\text {GS}}$ CS condition. The peroxide formation that is followed by the donor generation is clearly distinguished by the activation energy of 0.49 eV from the oxygen vacancies ionization which has been widely modeled for explaining the donor creation in amorphous oxide semiconductor TFTs.

Ditadura, transição e democracia na Constituição de 1988

This essay discusses the process of transition from dictatorship to a new regime and how social and political disputes materialized in the persistence of authoritarian elements in the new Constitution. It is tried to discuss different aspects that show the limits of the political regime constituted after the end of the dictatorship and how they are related the current institutional instability experienced in Brazil. Recebido: 17/04/2019Aceito: 06/06/2019

Physics of relativistic collisionless shocks: The scattering-center frame.

In this first paper of a series dedicated to the microphysics of unmagnetized, relativistic collisionless pair shocks, we discuss the physics of the Weibel-type transverse current filamentation instability that develops in the shock precursor, through the interaction of an ultrarelativistic suprathermal particle beam with the background plasma. We introduce in particular the notion of the "Weibel frame," or scattering center frame, in which the microturbulence is of mostly magnetic nature. We calculate the properties of this frame, using first a kinetic formulation of the linear phase of the instability, relying on Maxwell-Jüttner distribution functions, then using a quasistatic model of the nonlinear stage of the instability. Both methods show that (i) the Weibel frame moves at subrelativistic velocities relative to the background plasma, therefore at relativistic velocities relative to the shock front; (ii) the velocity of the Weibel frame relative to the background plasma scales with ξ_{b}, i.e., the pressure of the suprathermal particle beam in units of the momentum flux density incoming into the shock; and (iii) the Weibel frame moves slightly less fast than the background plasma relative to the shock front. Our theoretical results are found to be in satisfactory agreement with the measurements carried out in dedicated large-scale 2D3V particle-in-cell simulations.

Physics of Weibel-Mediated Relativistic Collisionless Shocks.

We develop a comprehensive theoretical model of relativistic collisionless pair shocks mediated by the current filamentation instability. We notably characterize the noninertial frame in which this instability is of a mostly magnetic nature, and describe at a microscopic level the deceleration and heating of the incoming background plasma through its collisionless interaction with the electromagnetic turbulence. Our model compares well to large-scale 2D3V particle-in-cell simulations, and provides an important touchstone for the phenomenology of such plasma systems.

Experimental study on the excitation of thermoacoustic instability of hydrogen-methane/air premixed flames under atmospheric and elevated pressure conditions

The current work examines the excitation of thermoacoustic instability of lean premixed hydrogen-methane/air low swirl flames under both atmospheric and elevated pressure conditions (up to 0.3 MPa). Under a given pressure condition, The tests were conducted at different bulk velocities (U), hydrogen proportions (ηH), and equivalence ratios (Ф). Results show that thermoacoustic instability can be excited by increasing one of these variables while keeping others the same. It was found that pressure elevation has a minor effect on the oscillation frequency. Moreover, it was demonstrated that the current instability is induced by large coherent structures. The effect of pressure elevation on the excitation of thermoacoustic instability is found to be Φ dependent. As indicators of the flame response to impinging vortices, the curvature and local flame surface area features were calculated with images captured with the planar laser-induced fluorescence of the OH radical (OH-PLIF) method. Results demonstrate a great similarity between the flame front evolution and the instability trend, implying that the effect of the chamber pressure on the instability trend can be indicated by the change in the flame front curvature and local flame surface area.

Research of static and dynamic characteristics of a voltage converter with soft switching running on arc load

Purpose. The purpose of the work is to ensure the improvement of the regulation quality of pulsed energy converters with simultaneous exclusion of undesirable dynamic modes from its dynamics. Construction of a simulation model of a bridge converter with phase control and soft switching operating on an arc load, studying static and dynamic characteristics, analyzing static and dynamic modes of a closed current stabilization system, studying the nonlinear dynamic properties of the considered converter with PWM, comparative analysis of the frequency characteristics of a real closed pulse system with the characteristics of its linear model.Methodology. Fundamental principles of feedback theory, frequency analysis of electrical circuit stability and control, mathematical modeling and spectral analysis of processes in nonlinear discrete systems, digital signal processing and experimental determination of characteristics and parameters of automatic control systems. The use of engineering techniques and universal computer programs based on new solutions of the matrix equations of circuits, which allow to perform time-consuming calculations of frequency characteristics that take into account the nonlinear nature of processes in closed modern high-power pulse systems at a qualitatively different level.Findings. The proposed current control system based on a converter with soft switching transistors has sufficient reliability and service life, allows to obtain high efficiency as well as quality and accuracy indicators in conditions of uncertainty of object parameters and disturbances. A technique has been developed for designing the Bode-optimal frequency characteristics of the loop gain of the considered converter with PWM, in the possibility of controlling the static and choosing the dynamic instability of the output current, in ensuring stable operation and eliminating the self-oscillating mode of the stabilized converter operating on an arbitrary complex load.Originality. The theory of frequency control has been further developed by extending it to a new class of objects — power sources for electrical technologies with improved quality and accuracy indicators, which makes it possible to raise welding technologies and related technologies to a higher level, to solve many problems, including the problem of improving the final product quality.Practical value. The analysis of static and dynamic characteristics considered in the article and the use of new methods for calculating and measuring the equivalent frequency characteristics of converting devices with PWM should be considered as one of the stages of creating engineering methods for synthesizing power supply regulators, considering the latter as essentially nonlinear systems and taking into account the possibility of undesirable dynamic modes . The closed-loop method allows to determine the frequency characteristics of the loop gain function of a PWM converter that adequately reflects the stability conditions and possible lasing modes.

Current instability phenomena in a tunnel diode and electron self-organization processes

Current instability phenomena in a tunnel diode and electron self-organization processes

Temperature dependence improvement of polycrystalline-silicon tunnel field-effect thin-film transistor

Trap-assisted tunneling (TAT) mechanism dominates the subthreshold leakage current and ambipolar transport behavior of tunnel field-effect transistor (TFET) with polycrystalline-silicon (poly-Si) channel. The unwanted ambipolar transport behavior of poly-Si TFET can be suppressed by the employment of gate-to-drain underlap structure to increase the tunneling distance near the drain side when the TFET is operated at the negative gate voltage. The gate-to-drain underlap length of TFET would also exhibit series resistance effect and strong temperature effect on the on-state current (ION). The strong temperature dependent series resistance effect and bandgap narrowing effect cause the poly-Si TFET to exhibit temperature instability of the on-state current. In order to improve the temperature instability of the on-state current, the ammonia plasma treatment has been performed to passivate the defects and reduce the trap state density of poly-Si film. The trap state density reduction of poly-Si can improve the field-effect mobility of transport carrier and reduce the series resistance effect, resulting in the reduction of the temperature dependence of ION. Consequently, the improved temperature dependent ION of poly-Si TFET can be obtained, and it is useful for the development of poly-Si thin-film transistor and its applications in the high resolution display industry and three-dimensional integrated-circuit.

Structural and electronic optimization of ring-graphene cathodes and their field emission properties

Cold field emission sources are capable of providing highly coherent and bright electron beams. However, they suffer from some well-known practical difficulties that have prevented their widespread use: unmanageably stringent ultrahigh vacuum requirements, relatively large current instabilities, and rapid emission decays in periods as short as 1–2 h, requiring regular flashing (Joule heating). This paper presents the microfabrication and a successful emission test of a micron-sized concentric ring-graphene cathode cold field emitter, without any observable wall collapse. The cathode is designed to have a nanometer size ring-edge, while its radius can measure several microns or larger, providing stable electron emission under high vacuum conditions. The turn-on electric field of a ring-cathode source can be dramatically lowered by introducing a thin layer of nickel nanoparticles by an in-situ focused ion beam process. The dependence of field enhancement on the anode-cathode distance and the ring-cathode radius has been systematically studied.

Fluid description of collisional current filamentation instability of a weakly ionized plasma in the presence of magnetic field

In this paper, the collisional filamentation instability of an electron beam-weakly magnetized and ionized plasma has been investigated in the presence of background plasma, using the fluid description. By describing the equilibrium configuration in the presence of binary collision terms between charged and neutral particles and using the local approximation method, the dispersion relation (DR) of instable mode (filamentation instability) has been obtained and the effect of collision and magnetic field driven-destabilization and current-driven stabilization on the growth rate of instability has been studied. The results show the cut-off wave number and the magnetic threshold for the filamentation instability in the collisional magnetized plasma in which the instability will disappear for the larger wave number and larger magnetic fields. Studies show that the value of cut-off wave number and magnetic threshold are raised by increasing the electron beam current density.

Molecular phylogeny of the large South American genus Eriosyce (Notocacteae, Cactaceae): Generic delimitation and proposed changes in infrageneric and species ranks

Eriosyce is one of most species‐rich genera within Notocacteae (Cactaceae) harboring a variety of stem and flower morphologies, and fruits with basal abscission. The lack of a well‐sampled molecular phylogeny contributes to the current taxonomic instability of the genus, where its circumscription and infrageneric classification has been questioned. Specimens of Eriosyce (63 taxa) plus 19 outgroups were analyzed through sequencing three plastid noncoding introns (rpl32‐trnL, trnL‐trnF, trnH‐psbA), one plastid gene (ycf1), and one nuclear gene (PHYC). Individual markers and concatenated matrices were analyzed using maximum likelihood and Bayesian approaches. Phylogenetic analyses strongly support the monophyly of Eriosyce s.l. Furthermore, seven clades within Eriosyce s.l. were defined based on supported branches, although one of them was weakly supported. Our results suggest that some past taxonomic proposals have low phylogenetic support and should no longer be used, e.g., based on their scattered positions in the phylogenetic reconstruction, several infraspecific taxa appear unrelated to the typical form of the species in which they had been placed. We present a phylogeny‐informed infrageneric classification of the genus Eriosyce, and new combinations are proposed to update the nomenclature of species and sections.

Enhanced Device Stability of Ionic Gating Molybdenum Disulfide Transistors

The application of ion gels as gate dielectrics having an excellent mechanical flexibility and high capacitance to molybdenum disulfide (MoS2) devices has been extensively studied; however, some issues remain unaddressed with regard to device stability such as gate leakage current, hysteresis, and bias stress instability. This study suggests a fabrication process for the ionic gating of the MoS2 device to enhance the device stability by laminating the ion gel film onto the MoS2 transistor using a cut‐and‐stick method and adding a passivation layer to remove unintentional parasitic capacitances generated by the capacitive coupling effect. The ionic gating MoS2 transistor fabricated via this process operates at a low voltage (<1 V) and exhibits superior electrical characteristics such as low gate leakage currents (≈10−11 A), high on–off ratio (>106), and low hysteresis (<0.05 V). To further investigate the device stability, bias stress instability of the ionic gating MoS2 transistor is examined. The charge‐trapping mechanism is the main cause of threshold voltage shifts under gate bias stress.

Citizenship education for Lesotho’s context: Towards a stable democracy

This article represents a contextual analysis that seeks a lasting solution to Lesotho’s current democratic instability, which has typified its political landscape since independence. This article analyses the literature to argue that the optimised political and institutional reforms that have recently raised people’s hopes are not likely to bring about a lasting solution of democratic stability. They are likely to suffer the same fate as those that came before, which only managed to provide a temporary solution. This article contends that the sustainability of democracy in Lesotho can ideally be achieved through the cultivation of the ‘spirit of democracy’, which can be nurtured through citizenship education. The article concludes by exploring possibilities for the introduction of citizenship education in Lesotho.

Política Nacional de Saúde Bucal Brasileira (Brasil Sorridente): Um resgate da história, aprendizados e futuro*

Antecedentes: no Sistema Único de Saúde do Brasil (SUS) a atenção básica é ordenadora do cuidado em saúde e representa o eixo estruturante do sistema. Em 2004, quando o Brasil Sorridente (BS) foi criado, o governo federal assumiu um importante papel de indução do crescimento da oferta de serviços de saúde bucal nos municípios e estados, criando linhas de financiamento específicos para criação de novas equipes de saúde bucal, para construção e implantação de centros de atenção secundária (Centros de Especialidades Odontológicas – CEO) e terciária dentre outras ações, que fizeram do BS uma rede de atenção à saúde e uma das maiores políticas públicas de saúde bucal do mundo. Objetivo: este artigo tem como objetivo compartilhar parte dessa experiência, a fim de promover reflexão em torno da inserção da saúde bucal nos sistemas de saúde ao redor do mundo, com especial interesse nos países da América Latina. Métodos: este é um estudo de caso do Brasil e as informações apresentadas nesse estudo foram coletas por meio de relatórios governamentais, base de dados secundários, artigos publicados e informantes chave. Resultados e conclusão: a experiência do Brasil vem sendo motivadora e mostrou que, além de necessário é possível inserir a saúde bucal nos sistemas de saúde universais. Porém, após 14 anos e com a instabilidade política atual do país, o futuro do Brasil Sorridente dependerá do compromisso dos formuladores de políticas, a continuidade do engajamento dos diferentes atores envolvidos, objetivos claros e estratégias transparentes e fundamentadas em evidências científicas para alcançá-los.

Bubble cascade in Guinness beer is caused by gravity current instability

The downward movement of the bubble-texture in a glass of Guinness beer is a fascinating fluid flow driven by the buoyant force of a large number of small-diameter bubbles. This texture motion is a frequently observed phenomenon on pub tables. The physical mechanism of the texture-formation has been discussed previously, but inconsistencies exist between these studies. We performed experiments on the bubble distribution in Guinness poured in an inclined container, and observed how the texture forms. We also report the texture-formation in controllable experiments using particle suspensions with precisely specified diameters and volume-concentrations. Our specific measurement methods based on laser-induced-fluorescence provide details of the spatio-temporal profile of the liquid phase velocity. The hydrodynamic condition for the texture-formation is analogous to the critical point of the roll-wave instability in a fluid film, which can be commonly observed in water films sliding downhill on a rainy day. Here, we identify the critical condition for the texture-formation and conclude that the roll-wave instability of the gravity current is responsible for the texture-formation in a glass of Guinness beer.

‘Our wee country’: national identity, golf and ‘Ireland’

The careers of professional golfers offer a noteworthy opportunity to examine the ways in which national identity is imagined, constructed and experienced. This paper examines the nexus of national identity and golf in ‘Ireland’, and more particularly, Rory McIlroy’s public statements on the identity politics surrounding his country of choice for the 2016 Olympic Games. In so doing, it reveals aspects of McIlroy’s personal and sporting habitus, the value he associated with being ‘Irish’, his legitimization of Northern Ireland (NI), and the possible effect of his decision on others’ orientation towards his sporting identity on the island of Ireland. Given the current political instability in NI, it remains to be seen whether McIlroy’s talent as a golfer and his transnational social status will translate into him being a powerful cultural intermediary for sport in/and NI.