Breakdown Conditions Research Articles

Breakdown path and condition of air-insulated rod–plane gap with polymeric barrier inserted under alternating voltages

In order to improve the insulating performance of air-insulated high voltage electrical apparatuses, solid barriers are always introduced in the interelectrode gaps. The breakdown paths and conditions of air-insulated rod-plane gaps in the range of 2–5 cm under alternating voltages were investigated for three different sizes of polyvinyl chloride barriers inserted at different gap positions. The breakdown processes were observed using two cameras placed in orthogonal directions, and two typical breakdown paths were captured. The statistical results showed that the occurrence probabilities of the two typical breakdown paths were related to the barrier position, but that the breakdown voltages were very similar in all breakdown events. Further, formulae to predict the breakdown voltage were deduced using a linear regression method. The results provide a reference for improving the insulation performance of gas–solid hybrid insulation systems based on the barrier effect as well as for modelling gas discharges in hybrid gaps.

Design of protected circuit models for nonlinear dielectric conductivity measurement system under high-energy impulse breakdown conditions

During high voltage tests for nonlinear dielectric samples, accidental breakdown may occur due to the influence of some objective factors (such as material defectiveness, ambient temperature, air humidity, etc.). Under the high voltage conditions, a high energy impulse causes breakdown discharge resulting in test equipment failure. In this article, several unique types of LC and RC filtering circuit models are proposed to protect the nonlinear dielectric conductivity measurement system. Simulation of the proposed design models are carried out using “MATLAB Simulink”. The simulation results revealed that all LC and RC filtering circuit models have a protective function for the test equipment, and can buffer the voltage-raising amplitude. By comparison, it is observed that the LC filtering circuit model has a better buffer-function than RC filtering circuit model. The LC filtering circuit can make the voltage-output curve more lenitive, and significantly increase the safety of measurement system. In addition, the comparison of single LC/RC and π LC/RC filtering circuit models show that the π LC/RC filtering circuit model has a better buffer function. Furthermore, in the high electric-field breakdown the π LC/RC model can make a less steeper voltage-output curve. The filtering principles of these kinds of filtering circuit models were discussed, and the correctness of simulation results were validated by experiments.

Split TiO2 nanotubes − Evidence of oxygen evolution during Ti anodization

The fabrication and growth mechanism of TiO2 nanotubes have attracted widespread interest for decades. Conventional Ti anodization generally produces only non-split TiO2 nanotubes. In this work, completely split TiO2 nanotubes, whose morphology is similar to that of porous anodic alumina, were produced under breakdown conditions. The appearance of so many split TiO2 nanotubes in such a short period of time (900 s) cannot be explained by the classical field-assisted dissolution theory, viscous flow model or dissolution equilibrium theory. Here, we use the oxygen bubble mould theory, separating the electronic current and ionic current from the total current. The results show that both ionic and electronic currents reach a steady value during conventional anodization, while the electronic current keeps increasing during anodization under breakdown conditions. We suggest that the split nanotubes are formed by intense oxygen evolution due to the increasing electronic current – the nanotube walls are cleaved by the pressure of oxygen bubbles. We believe that the split nanotube array provides solid evidence of oxygen evolution due to the electronic current.

Kinetic Control of [AuCl4]- Photochemical Reduction and Gold Nanoparticle Size with Hydroxyl Radical Scavengers.

Laser-induced photochemical reduction of aqueous [AuCl4]- is a green synthesis approach requiring no chemical reducing agents or stabilizers; but size control over the resulting gold nanoparticles remains a challenge. Under optical breakdown conditions producing hydrated electrons (eaq-) and hydroxyl radicals (OH•) through decomposition of water, [AuCl4]- reduction kinetics follow an autocatalytic rate law, which is governed by rate constants: nucleation rate k1, dependent on eaq-; and growth rate k2, dependent on the OH• recombination product, H2O2. In this work, we add the hydroxyl radical scavengers isopropyl alcohol and sodium acetate to limit H2O2 formation. Higher scavenger concentrations both lowered k2 values and produced smaller gold nanoparticles with Gaussian size distributions and remarkably narrow mass-weighted size distributions. With sufficiently high scavenger concentrations, the mean nanoparticle size could be tuned from 3.8 to 6.1 nm with polydispersity indices below 0.08. Both the higher surface area-normalized catalytic activity of the gold nanoparticles synthesized in the presence of scavengers, and FTIR measurements, indicate no capping ligands on the nanoparticle surfaces. These results demonstrate that the size distributions of "naked" gold nanoparticles produced by photochemical [AuCl4]- reduction can be effectively tuned by controlling the reaction kinetics.

Joint Ground and Aerial Package Delivery Services: A Stochastic Optimization Approach

Unmanned aerial vehicles (UAVs), also known as drones, have emerged as a promising mode of fast, energy-efficient, and cost-effective package delivery. A considerable number of works have studied different aspects of drone package delivery service by a supplier, one of which is delivery planning. However, existing works addressing the planning issues consider a simple case of perfect delivery without service interruption, e.g., due to accident which is common and realistic. Therefore, this paper introduces the joint ground and aerial delivery service optimization and planning (GADOP) framework. The framework explicitly incorporates uncertainty of drone package delivery, i.e., takeoff and breakdown conditions. The GADOP framework aims to minimize the total delivery cost given practical constraints, e.g., traveling distance limit. Specifically, we formulate the GADOP framework as a three-stage stochastic integer programming model. To deal with the high complexity issue of the problem, a decomposition method is adopted. Then, the performance of the GADOP framework is evaluated by using two data sets including Solomon benchmark suite and the real data from one of the Singapore logistics companies. The performance evaluation clearly shows that the GADOP framework can achieve significantly lower total payment than that of the baseline methods which do not take uncertainty into account.

Plasma and flow induced by single- and dual-pulse laser-induced breakdown in stationary air

The plasma and flow induced by laser-induced breakdown (LIB) were experimentally investigated in a stationary atmospheric air using deposited energy measurement, unfiltered plasma imaging, and high-speed schlieren imaging. Single- and dual-pulse LIB were studied to explore the effects of a laser pulsing strategy on the induced plasma and flow and their potential practical applications. For dual-pulsed LIB, various time intervals (50 ns, 150 ns, 40 μs, and 100 μs) and energy combinations (10–10, 20–20, 30–30, 30–10, and 10–30 mJ) were used to change the initial conditions for the second breakdown. Laser energy deposition, plasma generation, and hot plume generation were compared between SPLIB and DPLIB. For SPLIB, the plasma and flow were self-similar while the size of the plasma and flow increased with increasing laser energy. For DPLIB, the characteristics of the plasma and the flow (hot plume) and the laser energy deposition were strongly dictated by the time interval and energy combination. For a time interval of 150 ns, strong pulse-to-pulse coupling induced a second toroidal flow structure that suppressed third lobe generation. Depending on the time interval, the interactions between the hot plumes and shockwaves generated much finer flow structures compared to SPLIB. The uneven energy combinations (30–10 and 10–30 mJ) created coarser and finer flow structures, respectively, for relatively long time intervals (40 and 100 μs) compared to the other energy combinations.

The impacts of blanket on CFETR ohmic breakdown

China Fusion Engineering Test Reactor (CFETR) design is under way, it is necessary to develop an optimized scenario for plasma breakdown and current ramp-up, and to evaluate the consequent design constraints. A good initial magnetization (IM) and volt-seconds optimization are carried out for the ohmic breakdown based on the 2D TokSys model, giving the maximum volt-seconds that the magnetic coils can provide. Take IM as the initial condition, the ohmic breakdowns on the conditions that the blanket has different equivalent resistivities and positions are optimized. Comparisons of the optimized values and the target values are made. The ohmic breakdown without blanket is calculated as well, as a reference, the minimum equivalent resistivity of blanket required to meet the breakdown condition is obtained. Both initial magnetization and ohmic breakdown optimization could serve as preliminary references for the CFETR coils and blanket designs.

Effect of polarity on beam and plasma target formation in a dense plasma focus

Dense plasma focus (DPF) devices are conventionally operated with a polarity such that the inner electrode (IE) is the anode. It has been found that interchanging the polarity of the electrodes (i.e., IE as the cathode) can cause an order of magnitude decrease in the neutron yield. This polarity riddle has previously been studied empirically through several experiments and is yet not well understood. We have performed kinetic simulations using the particle-in-cell modeling to investigate the problem. This is the first time that both polarities have been studied with simulations in great detail. In our simulations, we have modeled the entire beam and plasma target formation processes, but we did not consider differences in break-down conditions caused by the two polarities. We have found that when using reverse polarity ions are still accelerated and, in fact, attain similar energy spectra as in the standard polarity case. The difference is that the fields are flipped and thus ions are accelerated in the opposite direction. So, in the reverse polarity case, the majority of the “plasma target” (formed by the imploding plasma) is in the opposite direction of the beam, and thus, the beam hits the IE and produces few neutrons. With a better inner electrode configuration, reverse polarity is able to create a high-quality ion beam as well as a high-density target. Both can be comparable to that generated by standard polarity. Furthermore, we will show that it is easier to add an additional solid catcher target to a DPF device with reverse polarity, potentially enabling it to generate more neutrons than standard polarity.

Numerical and Experimental Study of an Optimized p-SOS Diode

We report on the results of experimental investigation and numerical simulation of switching of SOS diode with p+P0n+ structure and with reduced thickness of P0-base. The proposed 1D diffusion-drift model of electron–hole plasma dynamics is found to be in good agreement with the experiment. The reduction of the P0-base thickness has allowed us to double the output pulse voltage with the same switching current density. This has been reached by a considerable reduction of switching losses as well as due to the formation of the domain of a strong quasi-rectangular electric field at the P0n+ junction during the current interruption. As a result, output pulse amplitude considerably exceeds the static breakdown voltage of P0n+ junction. This effect has been observed for the first time for high-voltage semiconductor opening switches.

Temperature State of a Hollow Cylinder Made of a Polymer Dielectric with Temperature-Dependent Characteristics

A mathematical model is constructed to describe a steady one-dimensional temperature distribution in a hollow circular cylinder made of a polymer dielectric with a constant difference in the electric field potentials on the cylinder surfaces. Based on systematized data on the dependence of the thermal conductivity and electrical conductivity of polymer materials used as dielectrics in engineering on temperature, a qualitative analysis of the model is performed for a prescribed density of the heat flux supplied to the inner surface of the cylinder and intensity of convective heat transfer on the outer surface The results obtained in the study allow one to determine the area of applicability of polymer dielectrics used in various electrical engineering applications, including electrical insulation of DC high-voltage cables, and to formulate the conditions for the thermal breakdown of the cylindrical layer of the dielectric.

Роль предпробойных токов в механизме статического пробоя двухсекционного тиратрона с холодным катодом

Роль предпробойных токов в механизме статического пробоя двухсекционного тиратрона с холодным катодом

Численное и экспериментальное исследования оптимизированного p-SOS-диода

AbstractWe report on the results of experimental investigation and numerical simulation of switching of SOS diode with p ^+ P _0 n ^+ structure and with reduced thickness of P _0-base. The proposed 1D diffusion-drift model of electron–hole plasma dynamics is found to be in good agreement with the experiment. The reduction of the P _0-base thickness has allowed us to double the output pulse voltage with the same switching current density. This has been reached by a considerable reduction of switching losses as well as due to the formation of the domain of a strong quasi-rectangular electric field at the P _0 n ^+ junction during the current interruption. As a result, output pulse amplitude considerably exceeds the static breakdown voltage of P _0 n ^+ junction. This effect has been observed for the first time for high-voltage semiconductor opening switches.

Switching Behavior and Forward Bias Degradation of 700V, 0.2A, β-Ga2O3 Vertical Geometry Rectifiers

We report the switching recovery characteristics of large area (contact dimension 0.04 × 0.04 cm2) vertical geometry β-Ga2O3 Schottky rectifiers, consisting of Si-doped epitaxial layers on conducting bulk substrates. Devices that were switched from forward current of 0.225 A to reverse off-state voltage of −700 V in an inductive load test circuit showed a recovery time (trr) of 82 ns, with a reverse recovery current (Irr) of 38 mA and dI/dt of −2.28 A. μsec−1. This shows the potential of Ga2O3 rectifiers for power switching applications, provided effective thermal management schemes can be implemented. Devices deliberately tested to failure under forward bias conditions exhibit delamination and cracking of the Ni/Au contact and underlying epitaxial Ga2O3 due to the low thermal conductivity of the Ga2O3. This failure mode is different to that under high reverse breakdown conditions, where pits formed by material failure under the high field generated at the edge of the rectifying contact occurs.

Bismuth Oxychloride Nanoplatelets by Breakdown Anodization.

Herein, the synthesis of BiOCl nanoplatelets of various dimensions is demonstrated. These materials were prepared by anodic oxidation of Bi ingots in diluted HCl under dielectric breakdown conditions, triggered by a sufficiently high anodic field. Additionally, it is shown that the use of several other common diluted acids (HNO3, H2SO4, lactic acid) resulted in the formation of various different nanostructures. The addition of NH4F to the acidic electrolytes accelerated the growth rate resulting in bismuth‐based nanostructures with comparably smaller dimensions and an enormous volume expansion observed during the growth. On the other hand, the addition of lactic acid to the acidic electrolytes decelerated the oxide growth rate. The resulting nanostructures were characterized using SEM, XRD and TEM. BiOCl nanoplatelets received by anodization in 1 M HCl were successfully employed for the photocatalytic decomposition of methylene blue dye and showed a superior performance compared to commercially available BiOCl powder with a similar crystalline structure, confirming its potential as a visible light photocatalyst.

The Mechanism of Superfast Switching of Avalanche <inline-formula> <tex-math notation="LaTeX">${S}$ </tex-math> </inline-formula>-Diodes Based on GaAs Doped With Cr and Fe

The results of theoretical and experimental investigation of charge carrier transport in avalanche S-diodes based on $\pi $ - $\nu $ -n and $\pi $ -n structures are presented. High-ohmic layers of the diodes were made by diffusion of deep chromium and iron acceptors into n-GaAs. It is shown that recharge of the deep acceptors in the avalanche regime should lead to expansion of the space charge region into the $\pi $ -layer and formation of step-type current–voltage characteristics rather than the S-type. It has been found experimentally that the switching of the S-diode is superfast (the time of switching is less than the transient time of the carriers through the active region). The obtained results are in contradiction with the earlier proposed mechanism of deep-level recharging. Thus, this mechanism has been revised. The comparison of the obtained results with the literature data allows one to find the only mechanism of superfast switching, which is associated with generation of collapsing field domains due to the Gunn effect under the avalanche breakdown condition. According to the experiment, the switching time of S-diodes depends on the applied voltage and the type of the deep-level impurity. The S-diodes can operate in relaxation oscillator and sharper circuits. The use of the S-diodes in a sharper circuit with a moderate voltage rate of 1011 V/s allows generating the voltage pulses with amplitude of 700 V and a rising edge of 250 ps at a load of $50~\Omega $ .

Monte Carlo modeling of radio-frequency breakdown in argon

This paper contains results of the detailed simulation study of the breakdown in low-pressure radio-frequency (RF) argon discharges. Calculations were performed by using a Monte Carlo code including electrons only, with the assumption that the influence of heavy particles was negligible. The obtained results are in a good qualitative agreement with the available experimental data and clearly show the multivalued nature of the left-hand branches of the breakdown voltage curves. The physical processes defining the breakdown conditions were analyzed based on the spatial profiles of electron density, local mean energy and the number of elastic and ionization collisions. Under the conditions where two breakdown values existed one could identify two regimes and two different balances between the electron losses and production. Using the dependence of the breakdown voltage on the product of the pressure and the interelectrode distance, and the product of the frequency and the interelectrode distance, similarity laws for RF breakdown have been reexamined.

Experimental Study on Swirling Jets

Experimental results on swirling flows are presented, contributing to the investigation and understanding of vortical flows close to vortex breakdown conditions. A swirling jet, issuing from a straight tube in a coaxial tube of larger diameter, is studied. Swirl is introduced by a rotating vane, located close to the jet outlet. Initial conditions, flow rates and swirl strengths can be parametrically controlled. In the present experiments two inner tube flow rates are combined with two inner jet swirls. The mean and turbulent flow fields are monitored on the axial central plane, based on measurements of all three velocity components, with stereoscopic 3D-PIV. Refractive index matching is utilized to eliminate optical distortions. A recirculation bubble stabilized downstream the exit of the swirl nozzle is the predominant coherent structure, formed due to the expansion of the swirling jet. The Reynolds number along with the swirl number are the nondimensional parameters used to interpret the measurements and to discuss the attributes of the flow field. The recirculation bubble topology is closely related to the rotation of the swirling jet, whereas the effect of the flowrate ιs of minor importance.

Avalanche breakdown evolution under hot-carrier stress: a new microscopic simulation approach applied to a vertical power MOSFET

During avalanche breakdown, hot carriers are generated due to impact ionization. In a MOSFET, some of these hot carriers impinge on the field oxide and can be captured in the oxide. Over time, this leads to an accumulation of fixed charges at the Si/SiO $$_2$$ interface and consequently to a degradation of device characteristics. Among those characteristics, the drift of the avalanche breakdown voltage is one of the most important aspects of device reliability. In this study, we present the first microscopic simulation of the oxide charge generation due to hot carriers and their field-driven recombination under avalanche breakdown conditions in a vertical power MOSFET. The distribution functions of electrons and holes in the device are calculated by solving their coupled Boltzmann equations. Based on the hot-carrier distribution functions, the injection rates of hot electrons and holes into the field oxide are evaluated. The resulting drift of the avalanche breakdown voltage in this simulation is in good agreement with experimental results. The underlying device internal processes are discussed, and the gained insight might help to improve the reliability of power transistors.

Flow structures in a swirl flow - vortex breakdown condition

This paper presents an experimental and numerical analysis of swirl flow in a circular tube. Swirl flow is important for technical and natural processes, where high heat transfer and good fluid mixing are needed. Flow and vortex structures respecting redistribution of momentum and possible occurrence of vortex breakdown are taken into account. The swirl flow is analysed experimentally by the measurement of the velocity field using Particle Image Velocimetry (PIV) and numerically via the commercial CFD code ANSYS CFX. Three cases are investigated: laminar flow regime (Re = 1,000), intermediate flow regime (Re = 2,000), and turbulent flow regime (Re = 5,000). The redistribution of the velocity field and the decrease of the swirl strength towards the outlet are shown. This redistribution affects the Reynolds number. Concerning the Rossby number, the occurrence of vortex breakdown in the swirl flow is determined. It is shown that the vortex breakdown takes place in the flow with higher Reynolds numbers, where an axial backflow may occur. Changes of the Reynolds numbers Reϕ and Rez along the tube length also confirm this statement. Furthermore, a thermodynamic perspective of vortex breakdown phenomena is presented.

Plasma Magnetic Control in Tokamak Devices

In tokamak experimental reactors, the magnetic control system is one of the main plasma control systems that is required, together with the density control, since the very beginning, even before first operations. Indeed, the magnetic control drives the current in the external poloidal circuits in order to first achieve the breakdown conditions and, after plasma formation, to track the desired plasma current, shape and position. Furthermore, when the plasma poloidal cross-section is vertically elongated, the magnetic control takes also care of the vertical stabilization of the plasma column, and therefore it is an essential system for operation. This chapter introduces a reference architecture for plasma magnetic control in tokamaks. Given the proposed architecture, the techniques to design all the required control algorithms is also presented. Experimental results obtained on the JET and EAST tokamaks and simulations for machines currently under construction are shown to prove the effectiveness of the proposed architecture and control algorithms.