Current Rise Time Research Articles

Mykonos: A pulsed power driver for science and innovation

Sandia National Laboratories has been operating the Mykonos linear transformer driver (LTD) in a five-cavity configuration since 2014. The machine operates at 1MA output current, 500kV output voltage, with a 10–90% current rise time of 85ns, which enables small scale physics and engineering pulsed power experiments. Mykonos provides hands-on pulsed power experimental training for students and staff alongside senior Sandia scientists in an environment that is more accessible than the Z Facility. Over the years, we have fielded and accumulated a wide variety of optical, x-ray and electrical diagnostics and we are preparing to open this facility to outside users. Here, we are presenting the pulsed power and diagnostic capability of Mykonos as well as some recent experiments that have been performed on the facility. The goal of this publication is to attract researchers across the pulsed power and high energy density (HED) community to collaborate with Sandia on exciting, innovative science and to train the next generation of researchers for the National Nuclear Security Agency (NNSA) and the nation. As such, we have established a Mykonos Academic Access Program (MAAP) as part of ZNetUS to enable academic utilization of the Mykonos Pulsed Power Facility.

Synergistic Enhancement of Electron Dynamics and Optical Properties in Zeolitic Imidazolate Framework-8-Derived Zinc Oxide via Surface Plasmon Resonance Effects of Silver Nanoparticles under UV Irradiation.

This study investigates the surface plasmon resonance (SPR)-induced UV photoresponse of zinc oxide (ZnO) derived from zeolitic imidazolate framework-8 (ZIF-8) to assess the influence of silver nanoparticles (Ag NPs) on the photoresponse behavior of metal-organic framework (MOF)-derived ZnO. The initial synthesis involved a thermal treatment in air to convert ZIF-8 into ZnO. We noted enhanced optical absorption both in the UV and visible spectra with the deposition of Ag NPs onto the ZIF-8-derived ZnO. Additionally, the presence of Ag NPs in the ZnO resulted in a substantial increase in current, even without any light exposure. This increase is attributed to the transfer of electrons from the Ag NPs to the ZnO. Photocurrent measurements under UV illumination revealed that the photocurrent with Ag NPs was significantly higher-by two orders of magnitude-compared with that without Ag NPs. This demonstrates that SPR-induced absorption markedly boosted the photocurrent, although the current rise and decay time constants remained comparable to those observed with ZnO alone. Although Ag NPs contribute electrons to ZnO, creating a "pre-doping" effect that heightens baseline conductivity (even in the absence of light), this does not necessarily alter the recombination dynamics of the photogenerated carriers, as indicated by the similar rise and decay time constants. The electron transfer from Ag to ZnO increases the density of charge carriers but does not significantly influence their recombination.

ATP-EMTP simulation of lightning protection system for multi-storey building

Multi-storey buildings with occupied highly technological instruments have more attraction possibilities of a lightning stroke than ordinary residential buildings. A protection system against lightning is necessary for these buildings and the level of the protection system performance is an issue to be examined. In this study, a protection system for a three-storey building is simulated in Alternative Transient Program-Electromagnetic Transients Program (ATP-EMTP). Distributed-parameter line models are used for the conductors of the protective system by modelling vertical conductors as nonuniform lines. The currents flowing in the critical conductors and the voltages at some critical points resulting by a lightning stroke are computed. The conventional lighting parameters required for structural protection such as lightning peak current, current rise time and time interval for the current flowing are determined. The effect of the radius of conductors in the protection system on the surge response is investigated.

Optimised projection welding of steel Bars

The article presents tests aimed at optimising the welding of bars, made of steel AISI1005 (ø 4.2 mm), in a cruciform configuration (i.e. using the cross-wire welding process). The authors identified research gaps and carried out a detailed numerical analysis of these cases. The research-related calculations involved the use of SORPAS software. The new areas of research included (i) bars with protective zinc coatings having various thicknesses of 0 µm, 5 µm, 10 µm and 30 µm, (ii) various welding current rise times (up-slope) and (iii) single and multi-pulse welding cycles. The tests were carried out using a DC 1 kHz inverter welding machine; the results concerning welding up-slope times were compared with those obtainable using a conventional AC 50 Hz welding machine. The results of the study, in the form of a welding technology with various values of up-slope time, have been implemented in industry. The inventive idea is protected by patent no. P.229445 at the Polish Patent Office.

Transient turn-on characteristics of Si RBDT and SiC MOSFET under nanosecond current pulse range

With the development of pulse power supplies, solid-state semiconductor switches are required to have fast switching speeds and low losses. Both Reverse blocking diode thyristor (RBDT) and silicon carbide metal oxide semiconductor field effect transistor (SiC MOSFET) have fast turn-on speed, making them suitable candidates for short pulse generator. In this paper, the transient turn-on characteristics of Si RBDT and SiC MOSFET are analyzed theoretically and validated experimentally. Si RBDT and commercial 1.2 kV/81A SiC MOSFET are comparatively investigated in switching current pulse with a rise time of several hundreds of nanoseconds. (1) The anode current of RBDT could rise exponentially because of the latch-up effect of Si RBDT, while the drain current of SiC MOSFET tends to be saturated. (2) Utilizing the current rise time (Trise) to represent the switching speed, SiC MOSFET can switch faster when the current is lower than 400A. With the increase of the current, Si RBDT presents faster turn-on speed and lower switching loss due to the regenerative action of the two coupled transistors. (3) The transient characteristics of Si RBDT include a voltage-controlled inductance, while SiC MOSFET contains a current-controlled inductance based on the current rise time, respectively. With the increasing of the main voltage, the equivalent inductance of Si RBDT decreases and tends to be saturated when the main voltage is higher than half of the breakdown voltage of Si RBDT. For SiC MOSFET, the equivalent inductance increases until the drain current remains stable for a given gate voltage. This paper confirms the advantages of Si RBDT and SiC MOSFET in different situations.

Lightning current Risetime influence on inductive coupling effect of secondary shield cable in substation

Secondary cables are susceptible to lightning surge currents when substation is struck by lightning. Little work has been made to the effects of surge currents in substation secondary cables, particularly the effects of surge current rise time. Six types of simulated lightning surge currents with different rise times, ranging from 5.7 μs to 25.4 μs, were injected into the substation grounding grid, and the inductive voltage from core wire to ground and the shield current in the closed loop of the buried secondary cable were measured simultaneously. The inductive coupling characteristics and the effect of rise time on shielding factor (SF) of buried secondary cables were investigated using time-frequency domain method. The results indicate that the surge current with shorter rise time will induce higher core-wire voltage and higher the shield current on the secondary cable. Double-end grounding of the shielding layer can significantly reduce the cable coupling response, especially reduce the high-frequency component of the inductive voltage at 10 to 100 kHz with steeper surge currents. The SF of secondary cable increases exponentially from 1.91 dB to 8.15 dB as the rise time of the lightning surge current decreases.

Short rise- and decay-time Z-pinch currents for soft x-ray laser excitation

The article addresses how to create inductance-free plasma and use it to excite soft x-ray lasers. The method employs a bifilar phenomenon in which one part of the pulsed current flows via the plasma column while the other part runs in the opposite direction via the closely placed external conductor. The electromagnetic fields formed by the plasma and return conductor are adjusted by lowering the distance between them to neutralize the magnetic field of the bifilar. Because the net transient magnetic field is drastically reduced, the plasma-conductor mutual inductance is near zero during current rise and decay. The inductance-free (L ∼ 2 nH) 35 cm-long Z pinches in a 3.1 mm-diameter argon-filled alumina capillary with current rise and decay times of 15 ns, dI/dt > 1012 A/s, and amplitudes up to 17 kA were predicted, realized, and verified to be suitable for pumping soft x-ray Ar+8 lasers. Without the bifilar phenomenon, the 35 cm Z pinches obey a 200 nH inductance that restricts the rise and decay durations of currents to 150 ns. The 35 cm Z pinches with 2 nH inductance generated 46.9 nm laser pulses with up to 4 μJ of energy and a beam divergence of 2 mrad at a low operating voltage of 35–45 kV, compared to 0.1–0.8 MV for similar lasers. The bifilar method could find applications in many research and technological fields, where the rise and decay times of discharge currents play a key role.

Improvements the direct torque control performance for an induction machine using fuzzy logic controller

This article examines a solution to the major problems of induction machine control in order to achieve superior dynamic performance. Conventional direct torque control and indirect control with flux orientation have some drawbacks, such as current harmonics, torque ripples, flux ripples, and rise time. In this article, we propose a comparative analysis between previous approaches and the one using fuzzy logic. Results from the simulation show that the direct torque control method using fuzzy logic is more effective in providing a precise and fast response without overshooting, and it eliminates torque and flux fluctuations at low switching frequencies. The demonstrated improvements in dynamic performance contribute to increased operational efficiency and reliability in industrial applications.

Understanding the bonding mechanism in short-time resistance projection welding: a comprehensive analysis

Capacitor discharge welding (CDW) is characterized by a pulsed electrical current profile. It is primarily utilized for resistance projection welding tasks, offering high power densities and short welding times. According to the latest findings, the welding process can be divided into different phases: contacting, activating, material connection, and holding pressure. During the activation phase, high-speed video-imaging reveals the generation of metal vapor which effectively eliminates impurities and oxide layers from the contact zone. The result of this is an activated surface. The purpose of this paper is to describe the physical effects of the bonding mechanism during short-time resistance welding. The Chair of Joining Technology and Assembly at the Technische Universität Dresden has a laboratory facility that can interrupt the welding current at any desired time during capacitor discharge welding. This allows different welding current profiles with always the same current rise time to be scientifically investigated. The experimental findings were supplemented with simulative analyses to clarify the bonding mechanism in resistance projection welding. Three different surface conditions are considered to generalize the findings on the bonding mechanism. Temperature and current density distributions were assessed to provide a physical description of the activation phase. The power density in the joining zone at different interruption times is determined, which gives an indication of activation by metal vaporization. The material connection is determined experimentally for the same interruption times. The numerical simulation model can be used to describe the bonding mechanism in short-time resistance welding. In resistance welding, the bond is formed due to the molten phase (solidification structure). In short-time resistance welding, the bond is formed due to surface activation by metal vaporization.

Vesicular neurotransmitters exocytosis monitored by amperometry: theoretical quantitative links between experimental current spikes shapes and intravesicular structures.

Single cell amperometry has proven to be a powerful and well-established method for characterizing single vesicular exocytotic events elicited at the level of excitable cells under various experimental conditions. Nevertheless, most of the reported characteristics are descriptive, being mostly concerned with the morphological characteristics of the recorded current spikes (maximum current intensities, released charge, rise and fall times, etc.) which are certainly important but do not provide sufficient kinetic information on exocytotic mechanisms due to lack of quantitative models. Here, continuing our previous efforts to provide rigorous models rationalizing the kinetic structures of frequently encountered spike types (spikes with unique exponential decay tails and kiss-and-run events), we describe a new theoretical approach enabling a quantitative kinetic modeling of all types of exocytotic events giving rise to current spikes exhibiting exponential decay tails. This model follows directly from the fact that the condensation of long intravesicular polyelectrolytic strands by high concentrations of monocationic neurotransmitter molecules leads to a matrix structure involving two compartments in constant kinetic exchanges during release. This kinetic model has been validated theoretically (direct and inverse problems) and its experimental interest established by the analysis of the amperometric spikes relative to chromaffin and PC12 cells previously published by some of us.

Development of technology for creating high-voltage p0–n0 junctions based on GaAs

An optimal solution has been found to the problem of obtaining p0-n0 junctions based on lightly doped GaAs layers with high values of electrical parameters and specified thicknesses of base layers to create ultra-fast high-voltage pulsed three-electrode switches with a photon injection mechanism of minority charge carriers. A technology has been developed for the formation of high-voltage, powerful subnano-second photonic injection switches based on gallium arsenide and its solid solutions. The dependence of the current rise time, switching voltage and switching stability relative to the control pulse of high-voltage photonic injection switches in a wide current and frequency mode of their operation, its sensitivity to various external influences, as well as dependence on the thickness of the p0-layer, on the transmission coefficient, on breakdown voltage Uprobe of the high-voltage p0-n0 junction. The carried out studies and the obtained results indicate the prospects of using the developed high-voltage pulsed semiconductor devices in picosecond optoelectronics for pumping high-power laser and LED structures.

Direct Observation of the Stretching of the Photoinduced Current Relaxation in α-Ga2O3 Schottky Diodes

Relaxation of photoinduced current in α-Ga2O3-based Schottky diodes was measured. Such Schottky diodes were fabricated using α-Ga2O3 films grown by Halide Vapor Phase Epitaxy (HVPE) on basal plane sapphire. Their photosensitivity in UV-C region was very high and several orders of magnitude higher than for visible light. The photoinduced current rise and decay times are found to be quite long, which is attributed to the hole capture by deep acceptors and to the effects of potential fluctuations. Due to these problems the photoinduced current relaxation process consists of several steps and takes more than hundreds of seconds. The characteristic relaxation times were calculated using the logarithmic differential analysis that demonstrated that the build-up and decay processes can be accurately described by stretched exponents with different broadening factors β. It was shown that additional illumination by long-wave light can help to observe the slowing of the relaxation speed. The temperature activation energies for photoinduced current decay and rise times are 0.37 ± 0.03 eV and 0.06 ± 0.005 eV respectively.

The effect of current rise time on the acceleration of thick flyers to hypervelocities using an electric gun

The electric gun is a projectile launcher which utilises both the rapid expansion of an ohmically heated exploding foil and strong electromagnetic forces to accelerate an insulating flyer up to 20 km/s. The gun’s acceleration mechanism is highly efficient at converting stored electrical energy in the pulsed-power device driving the load to kinetic energy in the flyer, with values of up to 25% reported (Osher et al., 1990). This high efficiency would allow capacitor banks to use less energy than conventional drives to generate higher pressure states in materials of interest to extreme state research. Despite its promising efficiency, the electric gun is rarely used, as the process of launching flyers above 0.5 mm thickness in this manner is highly variable, often resulting in uncontrolled launch characteristics and premature failure of the flyer. The gun is also difficult to optimise without use of a sophisticated multi-physics hydrocode, further limiting its take-up. This work presents experimental results from the successful launch of 24 × 24 mm flyers up to 2.0 mm thick to 10 km/s using an electric gun load on 1.2 MJ pulsed-power device: Machine 3. In combination with results from a 0D model, these findings are used to identify the mechanism for the destruction of thick flyers accelerated using electric guns, including strategies for mitigating their break-up. The results support the existing idea that flyer failure can be avoided by limiting the maximum pressure within the flyer. They also reveal a second mechanism by which the flyer integrity can be maintained when the current rise time is longer than the flight time; high pressures in thick flyers caused their density to remain high enough to prevent the driving foil plasma from breaking through, averting their disassembly. This second mechanism provides a route to accelerate thick electric gun flyers to higher velocities with efficiencies up to around 9% whilst lengthening the shock duration on impact, broadening the gun’s potential applications in extreme state research.

Impact-Ionization Switching of High-Voltage Thyristors Connected in Parallel

Thyristors triggered in the impact-ionization wave mode is a promising switching method in pulsed power technology. This triggering approach enhances the current capability of standard thyristors, allowing switching a current pulse with an amplitude of 200 kA and a current rise time rate-of-change (dI/dt) of 58 kA/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> s. Thyristor wafer area can be considered as one of the factors that are limiting the peak current amplitude and dI/dt. Switching thyristors in parallel is a possible solution to overcome these limitations. However, no evidence was found in the literature for successful parallel switching of standard thyristors in the impact-ionization wave mode. In this research, the possibility of parallel switching of two standard thyristors rated for 2.2 kV with a wafer diameter of 32 mm was investigated. A Marx generator equipped with a peaking module was used to charge the thyristor equivalent capacitance (1 nF) up to double its static breakdown voltage with a voltage rise time rate-of-change (dV/dt) of more than 1 kV/ns, as required for impact-ionization triggering. The diagnostic system includes two wideband voltage probes (VPs) with a subnanosecond time response, which allows simultaneous measurement of a voltage drop across each thyristor connected in parallel. Two stages of thyristor operation were investigated: 1) a triggering stage, without energy switching, when a dc bias voltage is applied; and 2) a current flow stage, with energy switching, when an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$RLC$</tex-math> </inline-formula> discharge circuit is connected. In these experiments, thyristors change from blocking to conducting state within 400 ps, switching a current pulse with an amplitude of 11 kA and a dI/dt of 6 kA/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> s (circuit limits). The current imbalance between the thyristors was less than 10%. To the best of the authors’ knowledge, it is the first time reported that high-voltage thyristors connected in parallel have been successfully triggered in the impact-ionization wave mode.

A Numerical Simulation Study on DC Positive Corona Discharge Characteristics at the Conductor’s Tip Defect

For investigating the relationship between the surface corona discharge of a DC wire and other influencing factors, a hybrid numerical model based on a fluid-chemical reaction was proposed to simulate the discharge process at the tip defect of the wire. Under different defect geometries and gas pressures achieved via simulation, the microscopic process of the reaction and movement of electrons and heavy particles during a positive corona discharge was studied, and characteristic parameters such as corona inception voltage and discharge current were analyzed. Furthermore, through the corona cage test, for a specific electrode configuration, corona inception voltages under different pressures were compared and verified, which showed that the model was reasonable. The results showed that the maximum electron density of the streamer head was about 1 × 1020 m−3, the rise time of the pulse current was about 10 ns, and the decay time was about 300–500 ns. The corona inception voltage decreased with an increase in the tip height and decreases in the tip curvature radius, conductor radius, and background air pressure; the amplitude of the pulse current increased with increases in the wire radius and curvature radius of the defect tip and decreases in tip height and background air pressure. The experimental results are consistent with the simulation results, which verifies the reasonability of the model.

Investigation on the scaling of magneto-Rayleigh–Taylor instability to the current rise time of Z-pinch plasmas

Understanding how the magneto-Rayleigh–Taylor instability (MRTI) scales to the current rise time is vital for Z-pinch dynamic hohlraum driven inertial confinement fusion. Wang et al discovered in prior theoretical work that the perturbation amplitude of MRTI before stagnation increases linearly with the current rise time when the implosion velocity of Z-pinch plasma is held constant. In the present work, three types of wire-array experiments with similar implosion dynamics and constant implosion velocity are performed on an 8 MA pulse power generator to investigate the scaling of MRTI to the rise time. It is successfully accomplished for the first time to obtain the similar wire-array Z-pinch implosions in which the current rise time is scaled up to three times on the generator by controlling the trigger time of its 24 modules. Both the experimental results, which include x-ray radiation pulses and x-ray images of imploding plasmas, and the related numerical analysis have shown that the MRTI before stagnation grows linearly with the rise time, as predicted by the theoretical model.

Natural sensitizers-mesoporous TiO2 hybrid nanomaterial for future optoelectronic applications

Optoelectronics deals with the design and development of electronic devices including photodetector (PD), solar cells and LEDs for light detection, generation and application for a variety of purposes. It includes X-rays, Gamma rays, Infrared, Ultraviolet and Visible light. In the current work, we developed a self-powered and efficient UV–Visible PD by sensitizing mesoporous TiO2 powder with a natural sensitizer Ficus Benghalensis (Banyan) and Rubia Cordifolia (Manjishtha). Prominent enhancement of visible light absorption was noted due to sensitizers as compared to pure TiO2 with the decrease in band gap from 3.13 eV to 3.01 eV. TiO2 photoanodes fabricated with and without dye loading were characterized using XRD, FESEM and UV–Visible and FTIR spectroscopy and used to fabricate a PD device with an active area of 0. 25 cm2. At zero bias, the Banyan-loaded TiO2 PD (B-TiO2) demonstrates enhanced photo response by nearly three times than Manjishtha-loaded PD (M-TiO2). At zero bias voltage, the PD (B-TiO2) displayed very high photosensitivity (8665), Dark current density (126 nA), Photocurrent density (158 µA), Photoresponsivity (1.88 mA/W), Rise time (0.31S) and Decay time (0.35S), respectively. Therefore, the use of novel dye for electricity generation in this study opens new routes to design future optoelectronics devices.

Nanocrystalline β-Ga2O3 thin film prepared by electron beam evaporation for enhanced photodetection

In this study, e-beam evaporation technique was used to fabricate thin-film (TF) of β-Ga2O3 on Si-substrate. The average crystallite size was determined to be roughly ∼39.8 nm. The deposition of β-Ga2O3 TF appears smooth and devoid of cracks or pits, with an average film thickness of ∼375 nm. From the ultraviolet (UV)–Vis absorption, the optical bandgap value for β-Ga2O3 TF was found to be about ∼4.84 eV. Our study found that the photodetector (PD) had excellent performance, characterized by an extremely low dark current of 4.02 × 10−8 A at −1 V bias, an I L/I D ratio over nine times, and the short rise and fall times of .27 s and .059 s. The simultaneous achievement of minimal dark current and quick rise and fall times is remarkable. The β-Ga2O3 TF PD’s remarkable consistency and reproducibility suggest promising practical applications for UV PDs.

On the Possibility of Achieving Thermonuclear Ignition During Magnetic Compression of High-Temperature Magnetized Plasma by the Current of a Disk Explosive Magnetic Generator

One of the directions for achieving thermonuclear ignition is compression of a heated, magnetized plasma by a liner. This concept was developed in the USA at the Z Machine (MagLIF project). To achieve ignition, it is necessary to create a current pulse with an amplitude of 60 MA or higher. The Z Machine produces currents with amplitudes up to 25 MA. The development of more powerful installations is a problem for the future. At the same time, today already, the explosive magnetic generators create the required currents with long current rise times. In this work, based on calculation results of the compression of a hot magnetized plasma, the possibilities of achieving ignition using modern disc explosive magnetic generators are discussed.

Impulse Characteristics of Ground Electrodes in Low Resistivity Soil

Soil properties have been known to be the factors that influence the ionization process in soil, which cause the impulse impedance, Zimpulse to be lower than that at steady state resistance, RDC. Many studies have established the relationship between the percentage reduction of Zimpulse from RDC in such that the percentage reduction is more pronounced in high resistivity soil, or in high RDC, than that at low resistivity soils. Nevertheless, Zimpulse is not the only parameter in representing the characteristics of soil under high impulse conditions, where from the voltage and current waveforms, the current rise and discharged times can also be established, where more accurate and extensive analysis of the electrical characteristics of ground electrodes subjected to high impulse currents can be achieved. This paper presents the experimental results of four ground electrodes, installed in agricultural land, with soil resistivity, of below 3 Ωm, subjected to high impulse currents, where the parameters of current rise times, Zimpulse and current discharged times were investigated. Two of these ground electrodes are having close RDC values, despite having different electrode's configurations, were also investigated. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.