Local Breakdown Research Articles

Local destabilization of the ideal or infinite-n ballooning mode in the H-mode pedestal region with high triangularity

Unlike the usual belief that the access to the 2nd stability regime of the H-mode pedestal is facilitated by strong plasma shaping, high triangularity is shown to induce a local breakdown of the access by destabilizing the ideal or infinite-n ballooning mode in the outer-well region, where n is the toroidal mode number. This destabilization in high triangularity regime is conjectured mainly due to the substantial increase of the magnetic shear, which can then make the stabilizing magnetic well effect sub-dominant. A large similarity observed in the mode excitation condition and location suggests that these locally-destabilized modes may be a possible source of the broadband fluctuations which are believed to trigger the wide pedestal quiescent-H (QH) mode in low toroidal rotation limit.

Experimental study on erosion-corrosion of carbon steel in flowing NaCl solution of different pH

The pure corrosion, pure erosion and erosion-corrosion behaviours of X65 carbon steel in corrosive fluid and slurry of different pH were studied at low flow velocity. Results show that the corrosion pattern changed from general corrosion to pitting damage along with the occurrence of erosion in pH = 3 solution and it is postulated that this feature might be associated with changes in the cathodic reaction. The sand impacts would cause the transient of main cathodic reaction from hydrogen evolution to oxygen reduction. Density and connected small pits generated in pH = 7 solution containing sand particles. While, the increase of the solution pH from 7 to 11 resulted in more scattered and serious pitting damage, which is caused by more focused initial anodes. The occurrence of pitting damage has complicated the analysis of the electrochemical results. The sand impacts in pH = 13 solution could cause the local breakdown of passive film, leading to a significant decrease of pitting resistance.

Tributary World-Ecologies, Part II

This essay—the second in a two part series—reconceptualizes the High Medieval Mediterranean World as a tributary world-ecology. Area Studies view the High Medieval Mediterranean as a culturally fragmented world, while the Commercialization Theorists only unite these fragments externally through trade relations. In contrast, Marxist Theorists almost exclusively focus on Medieval Europe through production relations. I argue that the High Medieval Mediterranean can be theorized as a tributary world-ecology. I advance two interrelated arguments. First, I underline the socio-ecological similarities and differences between the North Sea (addressed in Part I) and the Mediterranean Worlds. The Mediterranean world-ecology was premised upon the breakdown of world-imperial redistribution mechanisms and localization of peasant exploitation. This was exemplified by the development of iqta’, pronoia and similar land-tenure regimes across the Mediterranean World. The localization peasant exploitation, however, did not result in autarchy, but rather in the formation of a world-market. In fact, novel agrarian relations, coupled with the climatological upturn and technological innovations, led to the growth of surpluses in the hands of the aristocracies. This in turn stimulated artisanal production and revival of trade. Consequently, the Mediterranean World, like the North Sea World, witnessed further geographical integration and economic growth. Second, I emphasize the similarities and differences between the crises of the North Sea and the Mediterranean Worlds. Their socio-ecological relations reached their limits when the climate began to cool and destabilize, and organizational innovations could no longer produce sufficient surpluses. Consequently, both world-ecologies collapsed, finding its clearest expression in the Black Death. In turn, the Mediterranean, just like the end of the Roman period, disintegrated. The Western Mediterranean and the North Seas were integrated on the basis of capitalist productive and commercial networks, resulting in the birth of European capitalist world-ecology. In contrast, the Eastern Mediterranean would be reintegrated on the basis of the tributary networks of the Ottoman World-Empire.

Greatly enhanced discharged energy density and efficiency of BiFeO3-Based ceramics by regulating insulation performance

BiFeO3 (BFO) with excellent intrinsic polarization (Ps > 100 μC/cm2) receives a great deal of attention for advanced lead-free ferroelectric materials. However, the low resistance and high leakage current density in its naturally seriously hinder the improvement of energy storage performances. Herein, we demonstrate that superb comprehensive performances of large discharged energy density (Wrec ∼ 6 J/cm3) along with superior discharged efficiency (η ∼ 90%), impressive frequency stability (1–500 Hz), fatigue resistance testing (1-105 cycles) and ultrafast discharged time (t0.9–0.08 μs) could be attained in 0.3BiFeO3-0.7(0.88BaTiO3-0.12Bi(Li1/3Hf2/3)O3 [BFO-(BT-BLH)] relaxor ferroelectric ceramics. Remarkably improved breakdown strength (Eb) of BFO-based solid solutions could be primarily contributed to the substitution of BT-BLH, which provides refined grain sizes, as well as increased bandgap, resistivity, and activation energy. The phase-field simulations further indicate that the refined grain sizes are beneficial to improving local electric field distribution and breakdown path, resulting in enhanced Eb. The uncovered BFO-based ceramics design method provides a platform for high-performance capacitors development for extreme conditions.

Imaging the Breakdown of Ohmic Transport in Graphene.

Ohm's law describes the proportionality of the current density and electric field. In solid-state conductors, Ohm's law emerges due to electron scattering processes that relax the electrical current. Here, we use nitrogen-vacancy center magnetometry to directly image the local breakdown of Ohm's law in a narrow constriction fabricated in a high mobility graphene monolayer. Ohmic flow is visible at room temperature as current concentration on the constriction edges, with flow profiles entirely determined by sample geometry. However, as the temperature is lowered below 200K, the current concentrates near the constriction center. The change in the flow pattern is consistent with a crossover from diffusive to viscous electron transport dominated by electron-electron scattering processes that do not relax current.

Three-dimensional fine structure of fibroblasts and other mesodermally derived tissues in the dermis of adults of the Bahamas lancelet (Chordata, Cephalohordata), as seen by serial block-face scanning electron microscopy.

Tissues of adult cephalochordates include sparsely distributed fibroblasts. Previous work on these cells has left unsettled such questions as their developmental origin, range of functions, and even their overall shape. Here, we describe fibroblasts of a cephalochordate, the Bahamas lancelet, Asymmetron lucayanum, by serial block-face scanning electron microscopy to demonstrate their three-dimensional (3D) distribution and fine structure in a 0.56-mm length of the tail. The technique reveals in detail their position, abundance, and morphology. In the region studied, we found only 20 fibroblasts, well separated from one another. Each was strikingly stellate with long cytoplasmic processes rather similar to those of a vertebrate telocyte, a possibly fortuitous resemblance that is considered in the discussion section. In the cephalochordate dermis, the fibroblasts were never linked with one another, although they occasionally formed close associations of unknown significance with other cell types. The fibroblasts, in spite of their name, showed no signs of directly synthesizing fibrillar collagen. Instead, they appeared to be involved in the production of nonfibrous components of the extracellular matrix-both by the release of coarsely granular dense material and by secretion of more finely granular material by the local breakdown of their cytoplasmic processes. For context, the 3D structures of two other mesoderm-derived tissues (the midline mesoderm and the posteriormost somite) are also described for the region studied.

Stochastic Local Breakdown of Oxide Film on Ni from Identical-Location Imaging: One Single Site at a Time.

The electrochemical breakdown of a metal oxide film can directly affect the performance of functional electrochemical devices. However, revealing the structural insight into the breakdown sites is challenging because of heterogeneity: different breakdown sites are spatially distributed over the surface. Herein, we combine scanning electrochemical cell microscopy with identical-location microscopies to reveal the heterogeneity in the breakdown of NiO film on Ni in a site-by-site manner. Local critical breakdown potential varies by ∼500 mV, corresponding to an excess energy of 0.02-0.12 J/m2. Correlative composition imaging using time-of-flight secondary ion mass spectrometry shows Ni crystal grains with thinner NiO films are more resistant to breakdown. This high resistance is explained using classical nucleation theory, where the electrical energy is affected by the film thickness through the local interfacial capacitance. The correlative imaging approach overcomes the issue of heterogeneity, providing conclusive insight into the stability of the electrochemical interfaces.

Strain rate dependence of the effect of intergranular carbides on the resistance to stress corrosion cracking initiation of alloy 690 in simulated PWR primary water

The stress corrosion cracking (SCC) initiation of alloy 690 in simulated PWR primary water was evaluated in constant extension rate tensile tests with different strain rates. The SCC initiation was inhibited by intergranular carbide precipitation. However, the carbide effect weakened with increasing strain rate. The SCC mitigation effect of carbides mainly arises from the carbides’ ability to impede the transfer of dislocations across grain boundaries and delay the local break-down of surface oxide films. However, dislocations accumulated near the grain boundary can promote intergranular oxidation which induces cracking after the oxide film fails.

Dissipation-range fluid turbulence and thermal noise.

We revisit the issue of whether thermal fluctuations are relevant for incompressible fluid turbulence and estimate the scale at which they become important. As anticipated by Betchov in a prescient series of works more than six decades ago, this scale is about equal to the Kolmogorov length, even though that is several orders of magnitude above the mean free path. This result implies that the deterministic version of the incompressible Navier-Stokes equationis inadequate to describe the dissipation range of turbulence in molecular fluids. Within this range, the fluctuating hydrodynamics equationof Landau and Lifschitz is more appropriate. In particular, our analysis implies that both the exponentially decaying energy spectrum and the far-dissipation-range intermittency predicted by Kraichnan for deterministic Navier-Stokes will be generally replaced by Gaussian thermal equipartition at scales just below the Kolmogorov length. Stochastic shell model simulations at high Reynolds numbers verify our theoretical predictions and reveal furthermore that inertial-range intermittency can propagate deep into the dissipation range, leading to large fluctuations in the equipartition length scale. We explain the failure of previous scaling arguments for the validity of deterministic Navier-Stokes equationsat any Reynolds number and we provide a mathematical interpretation and physical justification of the fluctuating Navier-Stokes equation as an "effective field theory" valid below some high-wave-number cutoff Λ, rather than as a continuum stochastic partial differential equation. At Reynolds number around a million, comparable to that in Earth's atmospheric boundary layer, the strongest turbulent excitations observed in our simulation penetrate down to a length scale of about eight microns, still two orders of magnitude greater than the mean free path of air. However, for longer observation times or for higher Reynolds numbers, more extreme turbulent events could lead to a local breakdown of fluctuating hydrodynamics.

Corrosion of metallic bipolar plates accelerated by operating conditions in a simulated PEM fuel cell cathode environment

Metallic bipolar plates (BPs) are extensively applied in fuel cell stacks to achieve an extremely high power density, and are thus suitable for vehicle applications. Corrosion of metallic BPs and contamination on membrane electrode assemblies have become key issues. Automotive operating conditions significantly accelerate BP corrosion and surface destruction. Accelerated stress tests, simulating dynamic load and startup-shutdown, are conducted to reveal the operation-induced corrosion mechanisms of typical SS316L BPs. The dynamic potential in the normal range (0.6–0.95 V) accelerates the local breakdown and damage of the passivation layer, leaving pits rich in Cr-species. The abundant Cr-species in the pits prevents further dissolution of Fe-speices and relieves local corrosion. The startup-shutdown condition extends the cathode potential to the trans-passivation and secondary passivation regions. Startup-shutdown drives BP into complex evolution stages of corrosion and passivation. Frequent alternation of potentials between passivation and trans-passivation regions accelerates alternant dissolution of outer Fe-species and Cr-species, thus causing global destruction represented by flocculent microstructure and interlinked pits. The ultrahigh cathode potential in startup-shutdown may induce an obvious surface film via strong secondary passivation. This paper can further guide the test protocols to evaluate the durability of metallic BP and condition optimization to avoid extreme corrosion damage.

Electromagnetic Fields Due to an Electron Avalanche

In electric power and electronic systems, localized breakdown of air in the form of corona, partial discharge (PD), and electrostatic discharge (ESD) occurs frequently. These processes are known to be sources of electromagnetic fields. Corona on overhead line conductors produces electromagnetic noise over a wide frequency range leading to electromagnetic interference. The magnetic field due to PD can be employed to detect and possibly locate PD inside the apparatus. The fields due to ESD assume importance in designing suitable protection systems. Therefore, there is a need to relate the electromagnetic field produced by the discharge phenomenon to the physical ionization process, which constitutes them. The basic mechanism behind naturally initiated electrical discharges is the electron avalanche and an expression for its total electric field has been derived and validated in the literature. The next step in this direction is to obtain the magnetic field produced by it. An expression for the complete magnetic field, i.e., both near- and far-field components, is derived starting from Lienard–Wiechert potentials. The derived expression is validated with the numerical solution of Maxwell's equation. A simplified procedure to extend avalanche results for practical discharges is demonstrated. Close agreement is obtained between the estimated results and measured results reported in the literature.

Hamiltonian Truncation with larger dimensions

Hamiltonian Truncation (HT) is a numerical approach for calculating observables in a Quantum Field Theory non-perturbatively. This approach can be applied to theories constructed by deforming a conformal field theory with a relevant operator of scaling dimension ∆. UV divergences arise when ∆ is larger than half of the spacetime dimension d. These divergences can be regulated by HT or by using a more conventional local regulator. In this work we show that extra UV divergences appear when using HT rather than a local regulator for ∆ ≥ d/2 + 1/4, revealing a striking breakdown of locality. Our claim is based on the analysis of conformal perturbation theory up to fourth order. As an example we compute the Casimir energy of d = 2 Minimal Models perturbed by operators whose dimensions take values on either side of the threshold d/2 + 1/4.

Loss of Tcell tolerance in the skin following immunopathology is linked to failed restoration of the dermal niche by recruited macrophages.

T cell pathology in the skin leads to monocyte influx, but we have little understanding of the fate of recruited cells within the diseased niche, or the long-term impact on cutaneous immune homeostasis. By combining a murine model of acute graft-versus-host disease (aGVHD) with analysis of patient samples, we demonstrate that pathology initiates dermis-specific macrophage differentiation and show that aGVHD-primed macrophages continue to dominate the dermal compartment at the relative expense of quiescent MHCIIint cells. Exposure of the altered dermal niche to topical haptens after disease resolution results in hyper-activation of regulatory Tcells (Treg), but local breakdown in tolerance. Disease-imprinted macrophages express increased IL-1β and are predicted to elicit altered TNF superfamily interactions with cutaneous Treg, and we demonstrate the direct loss of Tcell regulation within the resolved skin. Thus, Tcell pathology leaves an immunological scar in the skin marked by failure to re-set immune homeostasis.

Measurement of Cyclic Variation of the Air-to-Fuel Ratio of Exhaust Gas in an SI Engine by Laser-Induced Breakdown Spectroscopy

Temporally and spatially resolved laser-induced breakdown spectroscopy (LIBS) was applied to a four-stroke, single-cylinder test engine’s cyclic exhaust gas to demonstrate engine performance. The LIBS technique provided quantitative air-to-fuel ratio (A/F) measurements by generating localized breakdown plasma during the compression and exhaust strokes. The results showed that the timing and duration settings of the emission energy ionization and molecular spectra affect the intensity peaks. Optimum measurements performed between 200 ns and 10 ms after breakdown resulted in observed atomic spectra of CI (248 nm), Hβ (485 nm), Hα (656 nm), NI (745, 824 nm), and OI (777, 844 nm). The intensities of CI (248 nm) and Hα (656 nm) decreased with increasing A/F, whereas the intensity ratios of NI and OI remained constant. A decrease in the intensity ratio of C/O and Hα/O was observed as the A/F increased. This study is a major step toward defining a means of using LIBS to control the A/F ratio in gasoline engines by focusing on the exhaust gas rather than the flame.

Control of the acoustic waves generated by intense laser filamentation in water

Experiments and simulations are performed to study filamentation and generation of acoustic waves in water by loosely focused multi-millijoules laser pulses. When the laser pulse duration is increased from femtosecond to nanosecond duration, a transition is observed from a filamentary propagation with extended and low energy density deposition to a localized breakdown, related to high energy density deposition. The transition suggests that Kerr self-focusing plays a major role in the beam propagation dynamics. As a result, the shape, the amplitude and the spectrum of the resulting pressure wave present a strong dependence on the laser pulse duration.

Design of a compact ultra-high frequency antenna for partial discharge detection in oil immersed power transformers

Oil-immersed Power transformers are considered to be one of the most essential parts of the electrical system and partial discharges (PD) are one of the dangerous problems that face transformer insulation. PD is localized dielectric breakdown of an insulation degradation or defect in an oil power transformer. Ultra-high frequency (UHF) detection technique can be utilized in PD detection inside oil-filled power transformers, by capturing the electromagnetic waves generated by PD using an antenna. This research presents a design for a compact double layer printed UHF antenna to detect and discriminate various types of PD activity in a power transformer. PD activity in the transformer has been proposed in this paper by using four different artificial insulation defects, metal protrusion defect in air medium, metal protrusion defect in an oil medium, surface discharge defect, and oil-paper void defect. The presented design guarantees a small-size wideband antenna for detection. The designed antenna has small/simple size and ultra-wide frequency passband from 1.4 to 1.85 GHz, 2.2 to 2.65 GHz, and 3.2 to 3.4 GHz. The normalized power frequency spectra of the captured UHF-PD signals of the four defective models showed that every PD type has its signature, and this could help in the discrimination among different types on site.

Degradation mechanisms in carbon fiber–epoxy laminates subjected to constant low-density direct current

This paper reports on an experimental investigation on the mechanical degradation of carbon fiber reinforced composites (CFRP) subjected to long-term exposure of low-density electric field by passing direct current (DC) through cross-ply laminates. In this study, an in-house fabricated experimental setup is used to study the effects of long-duration electrical conduction through cross-ply CFRP laminates. The experiments are conducted under constant current conditions. The change in laminate resistance is measured by monitoring the voltage across the specimen. Meanwhile, the effects of Joule heating are measured by monitoring the surface temperature across the length of the composite sample. Both voltage and temperature measurements are made continuously through the duration of the experiment. Experiments are conducted for continuous three (3) and twenty-four (24) hour duration and for cyclic three-hour repeated duration. Combined loading-compression (CLC) mechanical testing, ultrasonic pulse-echo test and dynamic mechanical analysis (DMA) is performed after electrical degradation to quantify mechanical and thermo-physical changes in the material. It is found that the electrical degradation leads to a 13%–15% reduction in compressive strength , 6%–7% reduction in Young’s modulus in thickness direction, 3%–4% decrease in glass transition temperature , and 41%–45% increase in tan δ . Post-degradation electrical resistivity change in through-thickness and in-plane directions is also correlated to damage. Comparison of change in resistivities of electrically degraded samples to thermally loaded samples indicates that the primary degradation is in the form of delamination caused by Joule heating and localized dielectric breakdown of epoxy near the carbon fiber–epoxy interface. Additionally, X-ray computed tomography (CT) images confirm that damage accumulated in electrically degraded samples is a combination of thermal damage due to Joule heating and dielectric degradation between plies due to passage of electric current.

Classification of Partial Discharges Originating From Multilevel PWM Using Machine Learning

A partial discharge (PD) is a small, localized breakdown which often appears in high-voltage insulation systems. PDs can be initiated within material defects such as voids or cracks when exposed to sufficiently high electric field levels. In this research work, we study and propose a machine learning (ML) approach to detect and classify PDs originating from multilevel pulsewidth modulation (PWM) waveforms as utilized in power electronic devices, such as inverters and variable frequency drives. Due to the increased use of power conversion units, new types of higher magnitude PDs have been observed to increase insulation degradation. By creating a cavity placed at different locations within the test object and exposing it to PWM voltage waveforms with two different rise times, a total of 345 660 PD events are recorded and organized into ten different classes. The maximum PD amplitude, duration, the time distance between consecutive PDs, and the area under the PD are used as features for PD classification. A unique way of concatenating a sequence of the extracted features to capture the temporal dependence of consecutive PDs is also presented. It is observed that when creating a sequence of information from consecutive observed PDs, a significant increase in classification accuracy can be obtained. Trained classifiers based on ensemble bagged decision trees (DTs) and long short-term memory (LSTM) architecture resulted in 95.3% and 98.5% average classification accuracy on test data respectively.

Armature Ejecta and Its Effects on Insulator Degradation in Solid-Armature Launchers

Armature ejecta, which erodes the insulator and reduces surface dielectric strength, has been considered as a threat to solid-armature launchers. Two series of experiments were carried out in a small caliber launcher (SCL) to investigate the sources of armature ejecta and degradation mechanisms of the insulator. In first series of experiment, the ejection of armature was imaged by a high-speed camera. The influence of linear current density on ejection was studied in second series of experiment. Mass difference of insulator samples was used to evaluate the output of armature ejecta. Insulation performance was judged from the surface flashover voltage. And the degradation mechanism resulted from armature ejecta was analyzed based on the process of surface flashover. Results show that the local breakdown at armature–rail interfaces is one of the sources of armature ejecta. The amount of ejecta on samples increased with linear current density. Average flashover voltages decreased when ejecta increased in Zone A (0–200 mm) and Zone B (200–600 mm). And there was little difference of average flashover voltages in Zone C (600–900 mm). The discharge channel of the post-shot sample was composed of arcs and ejecta. The distortion of the surface electric field and the shorten of creepage distance are degradation mechanisms of bore insulators in solid-armature launchers.

The Menstrual Endometrium: From Physiology to Future Treatments.

Abnormal uterine bleeding (AUB) is experienced by up to a third of women of reproductive age. It can cause anaemia and often results in decreased quality of life. A range of medical and surgical treatments are available but are associated with side effects and variable effectiveness. To improve the lives of those suffering from menstrual disorders, delineation of endometrial physiology is required. This allows an increased understanding of how this physiology may be disturbed, leading to uterine pathologies. In this way, more specific preventative and therapeutic strategies may be developed to personalise management of this common symptom. In this review, the impact of AUB globally is outlined, alongside the urgent clinical need for improved medical treatments. Current knowledge of endometrial physiology at menstruation is discussed, focusing on endocrine regulation of menstruation and local endometrial inflammation, tissue breakdown, hypoxia and endometrial repair. The contribution of the specialised endometrial vasculature and coagulation system during menstruation is highlighted. What is known regarding aberrations in endometrial physiology that result in AUB is discussed, with a focus on endometrial disorders (AUB-E) and adenomyosis (AUB-A). Gaps in existing knowledge and areas for future research are signposted throughout, with a focus on potential translational benefits for those experiencing abnormal uterine bleeding. Personalisation of treatment strategies for menstrual disorders is then examined, considering genetic, environmental and demographic characteristics of individuals to optimise their clinical management. Finally, an ideal model of future management of AUB is proposed. This would involve targeted diagnosis of specific endometrial aberrations in individuals, in the context of holistic medicine and with due consideration of personal circumstances and preferences.