Breakdown Region Research Articles

LOFAR Observations of Lightning Initial Breakdown Pulses

AbstractThis paper reports an observational study of lightning initial breakdown pulses (IBPs) using the low‐frequency array radio telescope and a broadband magnetic field sensor. The data show that the overall spatiotemporal evolution of the electrical breakdown causing an IBP is rather complex. During an IBP, spatially and temporally separated bursts of very high frequency (VHF) electromagnetic radiation occur in a volume on the order of 1003 m3, and they are coincident with brief magnetic field pulses, indicating that the location of the active breakdown region can change suddenly. Furthermore, recurrent breakdown activity is observed, especially at the location of the VHF burst. Interpreting each VHF burst as being generated by a corona burst, an IBP pulse appears to start off from an initial corona burst and subsequent corona bursts enhance it. We further suggest that the generation of IBPs likely involves multiple space stems/leaders and connections between them.

Quantification of Electric Fields in Fast Breakdown During Lightning Initiation From VHF‐UHF Power Spectra

AbstractThis work applies a new remote sensing approach to indirectly probe the electric fields in the discharge region of fast breakdown (FB) processes during lightning initiation. A VHF‐UHF broadband (1–750 MHz) measurement system is used to record the radiation spectra of FB. The average spectrum over 4 FB events exhibits a −10 dB spectral cutoff at 430 MHz, which yields an electrical field of 1.96–2.02 times the conventional breakdown field at 8–9 km altitudes. This in turn implies a localized field enhancement inside the region dominating the VHF‐UHF emissions of >7 times the expected pre‐discharge background field for lightning initiation. Such a high field has implications for the dynamics of individual streamers and their impact on the immediate environment, and also constrains the geometry of parts of the FB structure, suggesting that streamers initiate continuously in the enhanced field ahead of a narrow (aspect ratio >4) and conducting region.

A numerical tool to optimize voltage waveforms for plasma breakdown and early ramp-up in the presence of constraints

Plasma initiation is an important phase in a tokamak discharge. The main objective of magnetic control in this phase is to first obtain sufficiently high electric field and connection length (implying a low poloidal magnetic field) in the breakdown region, and then, once plasma is formed, to force the plasma current to increase, guaranteeing force balance equilibrium. The problem is dominated by the presence of significant eddy currents induced in the passive structures and by the many constraints due to the coil system and power supplies. The present paper shows how, with minor simplifying hypotheses, the magnetic control design problem can be converted into a Quadratic Programming problem with both linear and quadratic constraints that can be solved with an ad hoc iterative algorithm. On the other hand, the full design problem, including the choice of possible Switching Network resistors, becomes nonlinear and an evolutionary Genetic Algorithm solver is proposed. The algorithms are tested on DEMO breakdown and early ramp-up design which is a quite challenging problem because of the large distance between active coils and vacuum chamber, the presence of shielding passive structures with large time constants, and the active constraints on the CS/PF coil system.

On the Paradox of the Duality of Autoregressive and Moving Average Processes

A widely held view in time series analysis is the concept of duality that a finite order stationary autoregressive process of order p (AR(p)) is equivalent to an infinite order moving average (MA) process and a finite order invertible moving average of order q (MA(q)) is equivalent to an infinite order autoregressive (AR) process. The purpose of this paper is to demonstrate that the concept is not universally true. Thus, a finite order stationary autoregressive process of order p (AR(p)) can be written as an finite order moving average process and a finite order moving average process of order q (MA(q)) can be written as a finite order stationary autoregressive process. The regions of breakdown of concept of duality were determined for p = q = 1,2 using method of moments. The method involves equating non-zero autocovariances of the stationary AR(p) to the equivalent non-zero autocovariances of the invertible MA(p) to determine the region of non-duality. In such region of breakdown in duality, 1) both the Autocorrelation function and the Partial Autocorrelation function of the AR process and MA process cuts off after equal lags 2) a finite AR model can be adequately represented by a finite MA model of equal order and conversely with the same error variance and 3) negative values of the parameters of the AR process are equal in magnitude but opposite in direction to the parameters of the equivalent MA process and conversely. Empirical examples (simulation and real life examples) were used to illustrate these. Therefore, it has been recommended that caution should be exercised in using the concept of duality in time series analysis until future research proves otherwise.

Describing the Mechanism of Instability Suppression Using a Central Pilot Flame With Coupled Experiments and Simulations

Abstract Pilot flames are commonly used to extend combustor operability limits and suppress combustion oscillations in low-emissions gas turbines. Combustion oscillations, a coupling between heat release rate oscillations and combustor acoustics, can arise at the operability limits of low-emissions combustors where the flame is more susceptible to perturbations. While the use of pilot flames is common in land-based gas turbine combustors, the mechanism by which they suppress instability is still unclear. In this study, we consider the impact of a central jet pilot on the stability of a swirl-stabilized flame in a variable-length, single-nozzle combustor. Previously, the pilot flame was found to suppress the instability for a range of equivalence ratios and combustor lengths. We hypothesize that combustion oscillation suppression by the pilot occurs because the pilot provides hot gases to the vortex breakdown region of the flow that recirculate and improve the static, and hence dynamic, stability of the main flame. This hypothesis is based on a series of experimental results that show that pilot efficacy is a strong function of pilot equivalence ratio but not pilot flow rate, which would indicate that the temperature of the pilot products as well as the combustion intensity of the pilot flame play more of a role in oscillation stabilization than the length of the pilot flame relative to the main flame. Further, the pilot-flame efficacy increases with pilot-flame equivalence ratio until it matches the main-flame equivalence ratio; at pilot equivalence ratios greater than the main equivalence ratio, the pilot-flame efficacy does not change significantly with pilot equivalence ratio. To understand these results, we use large-eddy simulation (LES) to provide a detailed analysis of the flow in the region of the pilot flame and the transport of radical species in the region between the main flame and pilot flame. The simulation, using a flamelet/progress variable-based chemistry tabulation approach and standard eddy viscosity/diffusivity turbulence closure models, provides detailed information that is inaccessible through experimental measurements.

More realistic holographic model of color superconductivity with higher derivative corrections

In this paper, we have constructed a bottom-up holographic model for the color superconductivity (CSC) of the Yang-Mills theory with including the higher derivative corrections which allow us to study the CSC phase with the color number ${N}_{c}\ensuremath{\ge}2$. First, we consider the CSC phase transition in the context of Einstein-Gauss-Bonnet (EGB) gravity. We analyze the Cooper pair condensate in the deconfinement and confinement phases which are dual to the planar GB-RN-AdS black hole and GB-AdS soliton, respectively, where the backreaction of the matter part is taken into account. By examining the breakdown of the Breitenlohner-Freedman bound in the background of the planar GB-RN-AdS black hole, we find that the positive GB coupling parameter $\ensuremath{\alpha}>0$ leads to a lower upper bound of the color number in comparison to Einstein gravity where the CSC phase for ${N}_{c}\ensuremath{\ge}2$ is not realized. But, with the $\ensuremath{\alpha}<0$ case it is possible to observe the Cooper pair condensate for ${N}_{c}\ensuremath{\ge}2$ with the reasonable magnitude of $\ensuremath{\alpha}$. This is confirmed and the corresponding phase diagram is found by solving numerically the equations of motion for the gravitational system. In addition, we show that the CSC phase disappears in the confinement phase for the magnitude of $\ensuremath{\alpha}$ below a certain value which means that beyond that value it might lead to the breakdown region of the EGB gravity in investigating the CSC phase. However, the CSC phase transition occurring with ${N}_{c}\ensuremath{\ge}2$ requires the magnitude of the GB coupling parameter to be rather large. As a result, the GB term would no longer be considered as the correction and it also violates the boundary causality bound. We resolve this problem by including additionally the higher derivative correction for the Maxwell electrodynamics and the nonminimal coupled Maxwell field.

Supersonic Underexpanded Jet Features Extracted from Modal Analyses of High-Speed Optical Diagnostics

An experimental study concerning an underexpanded, screeching, Mach 1.5 jet operating at a stagnation-to-ambient pressure ratio of 4.4 is presented. Experimental data were acquired from high-speed schlieren imaging (100,000 frames per second) as well as pulse-burst particle image velocimetry (PIV) at 50 kHz. Spectral analyses of these data are presented and compared, as are the results from modal analyses using both proper orthogonal decomposition (POD) and spectral POD (SPOD). Comparisons revealed that both schlieren images and pulse-burst PIV vector fields could be decomposed to capture the same oscillatory phenomenon in the jet (screech) at (). Both techniques captured strong periodic behavior in the shear layer where the influence of screech is observed to be significant and in the turbulent breakdown region of the jet where the shock cells are unsteady. Excellent qualitative and quantitative agreement was found between the spectral and modal analyses of these data, despite the fundamental differences between schlieren and PIV. Challenges associated with the pulse-burst PIV experiment are discussed, and detailed uncertainty analyses of these data are presented, showing excellent convergence of the data with low statistical uncertainties. The comparisons between these diagnostics help to demonstrate the capabilities of pulse-burst PIV, which is still a relatively new diagnostic technique for studying high-speed flows.

Space-Charge breakdown phenomenon and spatio-temporal ion concentration and fluid flow patterns in overlimiting current electrodialysis

Overlimiting current regimes are of practical interest for electrodialysis with ion-exchange membranes (IEMs) operating at current densities significantly higher than the limiting current density. Such high ion currents across the membranes allows reducing the investments into expensive IEMs and stacks. Electroconvection (EC) is the major contributor to the overlimiting current and concepts to utilize EC are highly desired. Most of the known theoretical works describing EC in electrodialysis channels are performed when only a single cation exchange membrane is considered. The presence of a neighboring anion-exchange membrane has not been considered so far.We report the results of direct numerical simulations of ion and water transport involving EC in an electrodialysis channel formed by a cation- and an anion-exchange membrane. A 2D “basic” model involving the Nernst-Planck-Poisson-Navier-Stokes equations is introduced. Details of spatio-temporal changes in the distribution of concentrations and space charge patterns as well as the interactions of EC vortices are analyzed.The presence of the anion-exchange membrane in the transition to a close-to-chaotic regime at high potential difference is examined. For the first time, we discover the phenomenon of space-charge breakdown where the space charges of opposite sign formed at the two membranes are short-circuited throughout the desalination channel. The details of the mechanism of this phenomenon are analyzed using the 2D model and a simplified 1D model. Simulations show that the space-charge breakdown leads to a decrease in the size and number of EC vortices in the region of breakdown, which results in a reduction of the local current density. This phenomenon, apparently, determines the upper limit of the possible increase in the mass transfer rate by electroconvection.

Vortex Breakdown Control by the Plasma Swirl Injector

Vortex breakdown, observed in swirling flows, is an interesting physical phenomenon relevant to a wide range of engineering applications, including aerodynamics and combustion. The concept of using a plasma swirler to control vortex breakdown was proposed and tested in this study. The effect of plasma actuation on controlling the onset and development of the vortex breakdown was captured by particle image velocimetry. Flowfield measurement results suggested that, by varying the strength of the plasma actuation, the location and size of the vortex breakdown region was controlled effectively. The plasma swirl injector offers a method for optimal control and efficient utilization of vortex breakdown. The method being proposed here may represent an attractive way of controlling vortex breakdown using a small amount of energy input, without a moving or intrusive part.

Dielectric breakdown sizes of conducting plates

Abstract In this work, we propose mathematical formulations that detail the effect of the dielectric strength of dielectric material on the spatial distribution of electric field in an infinite space with a conducting plate. Using the dielectric strength of air as the maximum limit for the magnitude of electric field intensity and the equivalence of stored charge between two different zones, we determine the size of the dielectric breakdown region (the extended region with ionized material) for the conducting strip and the conducting disk charged to an electric voltage. The size of dielectric breakdown is proportional to the square of the applied voltage, and decreases with the increase of the width/radius of the conducting strip/disk.

Terahertz wave avalanche breakdown transistor for high-performance switching

There is still a lack of high-performance terahertz (THz) modulators with wide operation bandwidth and large modulation depth due to the underlying physics limitation behind existing approaches. Meanwhile, for many applications, simple compact THz modulators working straightforward in the transmission mode are also highly desired. Here, we demonstrate a THz modulator with a maximal transmission-amplitude modulation depth of 99.9% (switching ratio of 1000) based on a commonly used silica-on-silicon structure. Different from those reported graphene or metamaterials enhanced proposals, the device we proposed works within a reversible avalanche breakdown region of silicon that has not been studied yet and has the potential to modulate/switch THz waves efficiently. Further, we proved that the modulation depth exceeds 97% in the frequency range from 0.2 to 1 THz in the experiment. The simplicity and generality of this new type of near-perfect THz modulator will undoubtedly attract lots of attention of researchers in the near future due to its potential to be engineered into integrated devices.

Nonlinear optimal attitude control of spacecraft using novel state-dependent coefficient parameterizations

This paper addresses the attitude stabilization problem with the optimization objective regarding system performance and energy consumption for rigid spacecraft. Novel state-dependent coefficient (SDC) parameterizations are developed to reduce the optimization problem to solving the state-dependent Riccati equation (SDRE). To be explicit, a proper SDC parameterization is selected for the stabilization mission among infinite feasible SDC representations by utilizing an improved guideline. Subsequently, a novel analytical SDC reconstruction is evoked alternatively at the mission breakdown region in which the derived SDC parameterization fails to operate. Behind the scenes, the anti-unwinding behavior is supported by the explicit sampling SDC forms derived for the scalar part of the unit quaternion. The novelty of two developed SDC parameterizations lies in that they not only render the maximal pointwise controllable space to facilitate the control mission, but also cover the whole maneuvering domain without any breakdowns. Moreover, the computational burdens for online SDRE solvability checking and numerical SDC reconstruction are alleviated significantly. Finally, comparative numerical simulations are implemented to highlight the superior performance of the presented scheme.

Opacity from Loops in AdS

We investigate how quantum dynamics affects the propagation of a scalar field in Lorentzian AdS. We work in momentum space, in which the propagator admits two spectral representations (denoted “conformal” and “momentum”) in addition to a closed-form one, and all have a simple split structure. Focusing on scalar bubbles, we compute the imaginary part of the self-energy ImΠ in the three representations, which involves the evaluation of seemingly very different objects. We explicitly prove their equivalence in any dimension, and derive some elementary and asymptotic properties of ImΠ.Using a WKB-like approach in the timelike region, we evaluate the propagator dressed with the imaginary part of the self-energy. We find that the dressing from loops exponentially dampens the propagator when one of the endpoints is in the IR region, rendering this region opaque to propagation. This suppression may have implications for field-theoretical model-building in AdS. We argue that in the effective theory (EFT) paradigm, opacity of the IR region induced by higher dimensional operators censors the region of EFT breakdown. This confirms earlier expectations from the literature. Specializing to AdS5, we determine a universal contribution to opacity from gravity.

Analysis of characteristics and mechanism of flow unsteadiness in a transonic compressor

This paper reveals the flow mechanism of unsteady flow oscillations at small mass flow rate conditions in a transonic compressor rotor. A series of unsteady RANS simulations were performed to achieve time-accurate and time-averaged flow fields. The predicted results were validated by experimental data. The present analysis indicates that the tip leakage vortex (TLV) breakdown occurs with the operating condition approaching the stability limit due to an interaction between the TLV and the shock wave. A bubble-type vortex breakdown is first observed. However, the TLV breakdown does not directly cause flow oscillations. Only if the breakdown region develops beyond a threshold volume value (9.15e−9 m3 in current case), it will influence the static pressure distribution at the pressure side of blade tip region. After that the flux of tip leakage flow, the strength of TLV, the shock/TLV interaction are varied, which in return changed the size of the breakdown region in the adjacent passage. In such a way, a self-sustained system which causes periodical flow oscillations is established. When the compressor is further throttled, the bubble- and spiral-type TLV breakdown occur alternatively, which induces a change in the frequency of the periodical flow oscillation. Although the characteristic frequency of flow oscillation changes with the transition of operating conditions, the trigger for flow unsteadiness is the same as that in the bubble-type breakdown case, namely the changes of static pressure distribution at the pressure side of blade tip. Moreover, during the transition from bubble- to spiral-type breakdown, the breakdown region significantly increases, resulting in a more severe pressure fluctuation in the flow field.

Mechanistic insights into 2,4-D photoelectrocatalytic removal from water with TiO2 nanotubes under dark and solar light irradiation

Removal of recalcitrant pollutants from water is a major challenge, to which the photoelectrocatalytic processes may be a solution. Applied potential plays a key role in the photocatalytic activity of the semiconductor. This paper investigated the effect of applied potential on the photoelectrocatalytic oxidation of 2,4-Dichlorophenoxyacetic acid (2,4-D) with TiO2 nanotubular anodes under solar light irradiation. The process was investigated at constant potentials in different regions of the polarization curve: the ohmic region, the saturation region and in the region of the Schottky barrier breakdown. PEC tests were performed in aqueous solutions of 2,4-D, and in the presence of methanol or formic acid, as scavengers of OH• radicals and holes. Results showed the main mechanism is oxidation by OH• radicals from water oxidation, while runs with hole scavenger revealed a second mechanism of direct oxidation by holes photogenerated at the electrode surface, with high removal rates due to current doubling effect.

On the radiative heat loss and axis-switching phenomena of a decaying laser spark

A three-dimensional numerical simulation of a decaying laser spark in gaseous nitrogen has been carried out in Cartesian co-ordinate. The simulation starts with the introduction of a laser pulse in the domain. Thermal equilibrium has been assumed throughout the numerical simulation involving the species N2, N, N+, e−. The boundary conditions are treated following characteristic property of the waves to ensure smooth transmission of the blast wave out of the computational domain. The previously unexplained axis-switching phenomenon of the breakdown region has been explained, and a qualitative comparison of the same with experimental schlieren images has been made. The total radiation loss, including the loss during the breakdown, has been calculated. The variation of inverse Bremsstrahlung radiation, black body radiation, and spectral radiation of atomic nitrogen with time have been shown separately. Additionally, the temporal variation of radiation intensity of atomic nitrogen over a wavelength range has been plotted. The total radiation loss has been found to be small compared to the deposited laser energy.

Study of breakdown and plasma formation in the KTM tokamak with the massive conductive vacuum chamber

The article presents and discusses works on achieving ohmic breakdown at the KTM tokamak. The vacuum chamber of the KTM tokamak has a number of design features that significantly distinguish it from other installations - massive low-resistance vacuum chamber with unevenly distributed elements. The first experiments showed that the presence of massive asymmetrically distributed conductive elements of the KTM vacuum chamber leads to the complexity and inaccuracy of modeling the induced eddy currents and, accordingly, to the inaccuracy of calculating the field null configuration. Due to the limited value of the toroidal electric field of about 1.6 V / m and the toroidal magnetic field of 1 T, there are high requirements for the value of the stray poloidal magnetic field in the breakdown region for its implementation. So, it was required to carry out comprehensive work to achieve a breakdown at the KTM tokamak.

THz gas discharge in nitrogen as a source of ultraviolet radiation

This work presents the results of investigation of THz gas discharge in inhomogeneous flow of nitrogen. As a source of sub-millimetre radiation we used 40 kW@670 GHz gyrotron created in IAP RAS. It is shown that, despite the tendency of the discharge in electromagnetic wave beams to propagate towards the heating radiation from the breakdown region, in the case of a strongly inhomogeneous gas flow (nitrogen in our case), conditions can be created under which the discharge practically does not propagate and becomes point-like. The features of the glow of a THz discharge in a nitrogen flow in a wide range of background gas pressures are studied. The presence of a powerful afterglow in some bands of the second positive system of nitrogen at high pressures was demonstrated. The prospects of using such a discharge as a source of UV radiation are discussed.

The effect of turbulence on transitional flow in the FDA's benchmark nozzle model using large-eddy simulation.

The Food and Drug Administration's (FDA) benchmark nozzle model has been studied extensively both experimentally and computationally. Although considerable efforts have been made on validations of a variety of numerical models against available experimental data, the transitional flow cases are still not fully resolved, especially with regards to detailed comparison of predicted turbulence quantities with experimental measurements. This study aims to fill this gap by conducting large-eddy simulations (LES) of flow through the FDA's benchmark model, at a transitional Reynolds number of 2000. Numerical results are compared to previous interlaboratory experimental results, with an emphasis on turbulence characteristics. Our results show that the LES methodology can accurately capture laminar quantities throughout the model. In the pre-jet breakdown region, predicted turbulence quantities are generally larger than high resolution experimental data acquired with laser Doppler velocimetry. In the jet breakdown regions, where maximum Reynolds stresses occur, Reynolds shear stresses show excellent agreement. Differences of up to 4% and 20% are observed near the jet core in the axial and radial normal Reynolds stresses, respectively. Comparisons between viscous and Reynolds shear stresses show that peak viscous shear stresses occur in the nozzle throat reaching a value of 18 Pa in the boundary layer, whilst peak Reynolds shear stresses occur in the jet breakdown region reaching a maximum value of 87 Pa. Our results highlight the importance in considering both laminar and turbulent contributions towards shear stresses and that neglecting the turbulence effect can significantly underestimate the total shear force exerted on the fluid.

A Piezo-Avalanche Accelerometer

We are reporting on the first use of the piezo-avalanche effect in a micromachined sensor for the measurement of inertial forces. The device integrates a pn junction at the base of a beam that is used as the spring for the microstructure. Movements of a proof-mass at the opposite end of the beam produce stress at the location of the junction. To measure this stress, the junction is forced into its reverse breakdown region while monitoring its current-voltage relationship. The mechanical stress modifies the bandgap of the material, eventually leading to a change in its breakdown voltage. It is shown that this simple structure provides a high sensitivity of ~3 nA/g when the device is biased with a fixed voltage in its breakdown region. The device operation is modeled analytically, and results are found to be in good agreement with the experiments. The piezo-avalanche effect scales favorably and can be utilized for stress measurements at arbitrary locations on a structure as well as at nanoscales. [2019-0247]