Pulse Rise Time Research Articles

Gain-switching in CsPbBr3 microwire lasers

All-inorganic perovskite microwire lasers, which have intrinsic high material gain and short cavity, especially favor the generation of ultrashort optical pulses via gain switching for various potential applications. Particularly, the ultrashort gain-switched pulses may extend perovskite microwires to previously inaccessible areas, such as ultrafast switches, and chipscale microcombs pumping souces in photonic integrated circuits. Here, we show 13.6-ps ultrashort single-mode green pulses from the gain-switched CsPbBr3 microwire lasers under femtosecond optical pumping. The gain-switching dynamics is experimentally investigated by a streak camera system. The excitation fluence dependences of pulse width, delay time and rise time of the output pulses show good agreements with the rate equation simulations with taking gain nonlinearities and carrier recombination ABC model into account. Our results reveal that perovskite microwire lasers have potential for ultrashort pulse generation, while the low transient saturated gain, which may result from the high transient carrier temperature under femtosecond pumping is a significant limitation for further pulse shortening.

Thermal Field Effect in Resistive Random Access Memory With Sidewall Structures of Different Thermal Conductivity

The effect of different pulse rise time in resistive random access memory (RRAM) is presented in this study. The result shows that the length of pulse rise time will lead to variations in RRAM OFF-state resistance that occur due to which of two distinct effects governs the reaction, either the electric field or the thermal field. Different sidewall materials are adopted in RRAM devices to verify these phenomena, and a physical model supplemented with COMSOL simulation is proposed.

Particle in cell simulations of the pulsed plasma sheath: Dependence on pulse parameters

The spatial and temporal evolution of the pulsed plasma sheath is simulated using the particle-in-cell method. The effects of pulse parameters such as the rise time, duration, and maximum voltage on the sheath characteristics are discussed. In addition, it is considered how the pulse parameters affect ion current density, and ion kinetic energy at the cathode surface. It is found that even in the absence of collisions, the pulse rise time can broaden the energy distribution of ions at the cathode surface and the ion energy distribution is determined by the ratio of the pulse duration to the rise time.

Preliminary benchmarks and analysis of boundary conditions in a trenched microstructured silicon radiation detector

Microstructured neutron detectors have the benefit of enhanced neutron detection efficiency as compared to planar devices, achieved by etching 6LiF-filled trenches on the top surface of a silicon PIN diode. This sensor geometry results in a complex electric field distribution and depletion characteristics within the diode under reverse bias. For the first time on record, the effects of a fixed oxide charge on the microstructured device depletion characteristics and mobile carrier transport is investigated. Prototype detectors were fabricated with non-conformal surface doping. Capacitance voltage and current voltage measurements were performed for these prototypes and compared with COMSOL Multiphysics simulations. A spectral response from an 241Am alpha particle source was acquired and analyzed. It was found that monoenergetic alpha particles produce three prominent peaks in the pulse height spectrum output by the device. The peaks were confirmed by simulations to correlate with dead layers and incident trajectories into the microstructure. It was also found that significant differences in pulse rise time result, corresponding with events arriving in a low-field region in the fins and a high-field region in the bulk. Geant4 was utilized for radiation transport, interaction modeling, and benchmarking the spectral data. The results of this simulation work provide confidence in the ability to attain and benchmark electrical characteristics and spectral data for semiconductor radiation detectors employing complex microstructures.

An Ultra-Short Dense Paraxial Bunch of Sub-Relativistic Runaway Electrons

This letter reports new results on the formation of a paraxial flow of runaway electrons emitted in an elongated air gap near the tip of a conical steel cathode. In the axial magnetic field > 4 T, the peak current density of ≈ 0.6 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> has been achieved in the bunch core with a 0.7 mm diameter. Similarly to the vacuum case, dependence of the bunch parameters on the magnetic field was obtained and interpreted. Analysis of recorded current waveforms and oscilloscope transient response shows that the original current pulse rise time and width did not exceed 5 and 10 ps, respectively. Using the time-of-flight method, we have estimated the particle kinetic energy (≈200 keV) at the peak of the distribution function.

The Effects of Pulse Shape on the Selectivity and Production Rate in Non-oxidative Coupling of Methane by a Micro-DBD Reactor

The conversion of methane to ethylene has been investigated in a micro-DBD reactor with electrodes containing charge injector parts and excited with a negatively nano-second pulse voltage superimposed on a positive dc voltage. The effect of changing the characteristics of pulsed voltage such as pulse rise time (5–7 ns), total pulse width (12–14 ns), and pulse fall time (5–7 ns) on generation rate and products selectivity of the methane plasma has been studied. The kinetic model includes twenty species (electron, ions, radicals, and neutrals). The results showed that change in input pulse shape changes the generation rate and selectivity of neutral products. The rate of voltage change during pulse on-time significantly changed the instant C2H4 selectivity. With increasing the pulse rise and fall times the ethylene selectivity decreases, while the hydrogen selectivity increases. Results also showed that the electron reactions are dominant conversion channels during pulse on-time, while they had lower contributions in conversion progress during pulse off-time and the conversion process during this period is mainly governed by the radical reactions.

Influence of electron beam generation on the parameters and emission characteristics of a constricted arc discharge in a pulsed forevacuum plasma-cathode electron source

We have explored the influence of accelerating voltage, gas pressure, and accelerating gap length on the discharge parameters and emission characteristics of the plasma of a constricted arc discharge which forms the emission plasma in a forevacuum-pressure, plasma-cathode, broad-beam electron source. We find that the back-streaming ion flux from beam plasma, formed by electron beam in the accelerating gap and beam propagation region, leads to increase in the maximum current and pulse duration of the constricted arc. The rise-time of the emission current pulse (and electron beam current pulse) depends not only on the formation of the emission plasma but also on changes in arc current flow conditions in the constricting channel, which occur during the pulse duration due to the back-streaming ion flux. There is a significant influence of accelerating gap length on the emission characteristics. Varying the accelerating gap length provides control of the delay time and rise time of the emission current pulse (e-beam current pulse), as well as the electron emission efficiency.

Skyrmion velocities in FIB irradiated W/CoFeB/MgO thin films

In recent years magnetic skyrmions attracted great attention for the possibility to move them with low current density, their intrinsic stability and their robustness against defects and edge roughness compared to other magnetic textures. For applications, it is very important to be able to influence the behaviour of skyrmions locally. In this article, we present an evaluation on the effects of FIB Ga+ irradiation on skyrmion motion in W/CoFeB/MgO thin films. The influence of FIB irradiation is evaluated both, in terms of modification of the skyrmion Hall angle and the skyrmion velocity. An overview of the effects of the pulsing parameters on the skyrmion motion, shows low influence of the pulses rise-time and an external magnetic field. In addition the analysis after the irradiation shows that it influences notably the dynamics of skyrmions. In the irradiated zone the speed and angle of motion of these magnetic textures are strongly reduced.

Analysis of circuit current in electrochemical micromachining process under the application of different waveforms of pulsed voltage

Selection of optimal pulsed voltage parameters such as waveform, pulse rise and fall time, frequency, and duty ratio is very important to achieve the desired machining efficiency and accuracy in electrochemical machining (ECM). At present, this selection is done by conducting time-consuming and costly trial experiments. It is well understood that the volume and rate of material dissolution in ECM is dependent on the current flowing between the two electrodes. However, only a part of this current, called as Faradaic current, is responsible for the anodic dissolution and the rest of it, called as capacitive current, is consumed in charging the electrical double layer (EDL) capacitance which forms at the interface. Therefore, it is vital to understand the variation of Faradaic current in the system to arrive at optimal pulse parameters. An increase in the current flowing between the two electrodes does not ensures an increase in the Faradaic component of that current. To the best of authors' knowledge, there does not exists any method or device, to date, which can break the system current into its components and provide their variation explicitly. This research attempts to fill in this gap by provide a method of breaking the overall system current into its constituents by investigating the system under consideration using electrochemical impedance spectroscopy (EIS). The results of EIS are used to virtually construct an equivalent electrical circuit in MATLAB® Simscape and the current is simulated for different waveforms of pulsed voltage input. To access the correctness of the trends of Faradaic current obtained through simulations, an experimental study is conducted. Both simulation and experimental study is conducted for three waveforms of pulsed voltage namely: half wave rectified rectangular (RVW), sinusoidal (SVW), and triangular (TVW). For experimentation, these waveforms are obtained with the help of a rectification-cum-amplification circuit which is designed and developed in lab. Further optimization of triangular waveform is performed by varying its rise and fall time in five different cases. It is finally concluded that the trends of Faradaic current obtained from simulation study are in good agreement with the experimental results.

Определение потерь в стали бесконтактного двигателя постоянного тока с использованием линейной аппроксимации импульсов напряжения

Analysis of iron loss of a BLDC motor is of particular interest at the design stage or when researching methods to improve energy efficiency. Most of the existing methods for determining losses in steel are based on the analysis of losses for individual harmonic components of induction with their subsequent summation. However, the application of these methods for various forms of voltage obtained at the output of the converter is incorrect due to the non-linearity of magnetic circuits. This paper proposes a method for determining steel losses of BLDC motor using a linear approximation of voltage pulses, which makes it possible to take into account the influence of the voltage shape on the magnitude of magnetic induction directly, without expansion into a Fourier series. The dependences of steel losses on the switching frequency for various values of the duty cycle, and on the rise time of voltage pulses are obtained. Comparison of the simulation results of the 5 kW BLDC Motor with the experimental data of the idling mode given by the manufacturer proved the adequacy of the considered method.

Electronic mechanism of propagation of nanosecond breakdown channel in liquid organic dielectrics

The mechanism of anode-initiated breakdown in liquid organic dielectrics with long molecular chains is proposed on the basis of the experimental data on high velocities of the breakdown channel propagation in organosilicon and organofluorine liquids (~107 cm/s), which are comparable to that obtained earlier in crystals in the same conditions. The high velocities of the anode-initiated breakdown channel are satisfactory explained within the model of the cascade Auger transitions, developed for the crystalline materials. According to this model, velocity of the breakdown channel propagation is proportional to the electrical field strength. The time delay in breakdown channel formation relative to the voltage pulse rise time does not exceed ~5·10-10 s within the margin of error Keywords: electrical breakdown in liquid, nanosecond breakdown, breakdown mechanism.

Design and Experiment of a 4-MV Self-Triggered Switch

A 4-MV self-triggered switch based on ultraviolet (UV) illumination technology is designed and tested under approximate 800-ns rise time pulse voltage. The switch is composed of five sections in series, and each section comprises an UV-illuminating gap (UV gap) and a main gap. The voltage charged on the UV gap is obtained by a resistance–capacitance coupling circuit, which enhances synchronization with the main gap voltage. The gap distance of the 4-MV switch is determined according to the performance of the same type of switch under a shorter pulse (about 300 ns), while the characteristics of the gap under 800-ns pulse voltage is still unclear. The experimental results show that low jitter breakdown of the switch can be realized effectively when two UV gaps are functioning. The breakdown jitter percentage achieves less than 1.55% within the range of 1.0–3.2 MV. When the working scope is narrowed down to 1.4–2.1 MV, the breakdown jitter is less than 1%.

The Impact of Fast-Rise-Time Electromagnetic Field and Pressure on the Aggregation of Peroxidase upon Its Adsorption onto Mica

Our present study concerns the influence of the picosecond rise-time-pulsed electromagnetic field, and the impact of nanosecond pulsed pressure on the aggregation state of horseradish peroxidase (HRP) as a model enzyme. The influence of a 640 kV/m pulsed electromagnetic field with a pulse rise-time of ~200 ps on the activity and aggregation state of an enzyme is studied by the single-molecule atomic force microscopy (AFM) method. The influence of such a field is shown to lead to aggregation of the protein and to a decrease in its enzymatic activity. Moreover, the effect of a shock wave with a pressure front rise-time of 80 ns on the increase in the HRP aggregation is demonstrated. The results obtained herein can be of use in modeling the impact of electromagnetic and pressure pulses on enzymes and on whole living organisms. Our results are also important for taking into account the effect of pulsed fields on the body in the development of drugs, therapeutic procedures, and novel highly sensitive medical diagnosticums.

Influence of pulse rise time and duty cycle on distribution of transients along the wind turbine step-up transformer windings

A transformer is one of the most important assets in an electric power generation system. Wind turbine step-up transformers have recently seen premature failures due to high-frequency repetitive transients. The high-frequency transients under resonance can severely damage the insulation of the transformer. Thus the paper initially analyses the distribution of transformer frequency response to identify critical resonance frequencies for the two transfer function frequency responses; namely, impedance frequency response and voltage ratio response. Transformer frequency response is compared with transient response to understand the influence of repetitive transient parameters; namely applied voltage amplitude, rise time, and duty cycle, on transient distribution along the transformer winding. Based on three variable experiments, the range of rise time was considered as less than 300 ns and 1000 ns, applied voltage considered was 100 V and 1000 V, and duty cycle considered was 10% and 50%. For the current transformer model, both the experimental and PSCAD simulation results indicated that a duty cycle of 50% and rise time < 300 ns were critically damaging to transformer insulation. The research helps in designing transformer insulation stressed under different transient parameters.

High-voltage pulse rise time decreasing using transmission coaxial and spiral lines with ferrite

This article presents the results of a study of the formation of a short rise time of a powerful nanosecond 410 kV pulse. Ferrite filled coaxial transmission lines with standard inner conductor and construction in the form of a spiral are designed. The sharpening of the pulse occurs due to the appearance of an electromagnetic shock wave in the ferrite. The value of the rise time at the level of 0.1 - 0.9 has been decreased from 4.5 to 2.5 ns.

Multicavity linear transformer driver facility for Z -pinch and high-power microwave research

BLUE is a four-cavity linear transformer driver system in the Plasma, Pulsed Power, and Microwave Laboratory at the University of Michigan. Each BLUE cavity can store up to 2 kJ of electrical energy and provide an open-circuit voltage of 200 kV when using the maximum bipolar charge voltage of $\ifmmode\pm\else\textpm\fi{}100\text{ }\text{ }\mathrm{kV}$. The impedance of each cavity is approximately $0.5\text{ }\text{ }\mathrm{\ensuremath{\Omega}}$, and the rise time of the current pulse into a reasonably matched load is $\ensuremath{\sim}100\text{ }\text{ }\mathrm{ns}$. The system will be used to study high-energy-density $Z$-pinch plasmas and high-power radiation sources. The facility has been equipped to handle intense bursts of x rays, nuclear fusion neutrons, and high-power microwaves (HPM). This paper reports on the construction and performance of the first (prototype) BLUE cavity and its auxiliary systems, as well as preliminary experiments using the cavity to drive resistive loads and a magnetically insulated line oscillator (MILO) HPM device. In these initial single cavity, BLUE-driven MILO experiments, 1.19 GHz microwave oscillations were successfully generated.

Effective streamer discharge control by tailored nanosecond-pulsed high-voltage waveforms

In this paper we present our solid-state nanosecond pulse source (the solid-state impedance-matched Marx generator) which can generate arbitrary waveforms and which can be used for pulsed discharge generation. The purpose of the development of such a generator is twofold: by being able to change the waveform at will, we aim to control the discharge generated by such pulses very precisely which can be very useful for plasma applications, but also for more fundamental studies. In the presented study, we applied the arbitrary-waveform pulse source for streamer discharge generation in a cylinder-wire-like arrangement and used the arbitrary-waveform capability to change the rise time (in our experiments we used 6.8–26.2 ns) of unipolar positive pulses with 6-10 kV amplitude and 80 ns duration. Additionally, we introduced variations of a step in the rising edge of the waveform. We performed measurements both in air and nitrogen to electrically characterize the discharge while analyzing the streamer propagation in the plasma reactor with intensified charge-coupled device imaging and measured ozone generation (in air). The results show that we can indeed control the propagation of the streamer discharge with the stepped waveform, but that the rise-time variation has little effect on the streamer propagation in our system. However, the streamer velocity and structure differs significantly comparing discharges in nitrogen and air for the same applied voltage waveform. Additionally, for some of the stepped waveforms we found a slight increase of the ozone yield for air at low overall energy densities.

A calculation model for breakdown time delay and jitter of gas switches under hundred-nanosecond pulses and its application in a self-triggered pre-ionized switch

In this paper, a calculation model for the breakdown time delay and jitter of gas switches under hundred-nanosecond pulses is proposed and applied in a self-triggered pre-ionized switch. The effects of injection time of pre-ionization, pulse rise time, and the pre-ionization jitter are discussed and verified through experiments. It indicates that the pre-ionization should be injected when the electric field is high enough in the gap, injection after 80% peak-time can ensure its effectiveness. Then the statistical time delay jitter will be determined by the pre-ionization jitter, which is an intrinsic restriction of the self-triggered switch. However, when the changing rate of the pulsed electric field exceeds a certain value, the breakdown time delay jitter can be partly offset in the formative stage because the formative time delay has an exponential relationship with the electric field. Therefore, lower time jitter can be obtained under pulses with a shorter pulse rise time. In general, the results of the calculation model agree with the experimental results, and the experimental parameters which lead to a low jitter can also be used as a reference.

Compact generators of high-power fast-rising pulses with closing switches in the form of reversely switched dynistors.

This paper presents the results of a study of reversely switched dynistors (RSDs) with an operating voltage of 2.3 kV and a structure diameter of 12mm in an unconventional mode, when they commutate current pulses with very short durations (a few microseconds). In this mode, we studied the dependence of the switching energy loss on the amount of charge that passes through the RSDs during the flow of the control current, which is reversed with respect to the main current. It is shown that a sufficiently powerful control action will result in the RSDs under study having a very low energy loss and being able to effectively commutate microsecond current pulses with an amplitude of >10 kA. It is found that if a rise time of the commutated current pulses is less than 200ns, the efficiency of theRSDswitching significantly decreases. The results concerning the use of the RSDs under study in high-voltage generators are presented. The generators are able to commutate a pulsed power of tens of MW in less than 1 µs at a repetition rate of hundreds of Hz. Finally, this paper suggests ways to increase the power of generators.

Nonlinear transmission line implemented as a combined pulse forming line and high-power microwave source.

Nonlinear transmission lines (NLTLs) are typically driven by pulse forming lines (PFLs) or Marx generators to generate high repetition rate, high power microwaves (HPMs) with fewer auxiliary systems than conventional sources. This paper reports the development of an even more compact HPM system that utilizes a composite-based hybrid NLTL as the PFL and HPM generator in a single device. We designed the following three different combinations of nickel zinc ferrite (NZF) and barium strontium titanate (BST) inclusion volume loads in a polydimethylsiloxane host material to provide magnetic field dependent permeability and electric field dependent permittivity, respectively: 25% NZF, 10% BST/15% NZF, and 15% BST/10% NZF. By constructing the NLTL in a coaxial geometry, this device uses the capacitance and length of the NLTL to generate a fast rise-time high voltage pulse with microwave oscillations that occurred both during and after the pulse after exceeding a threshold charging voltage. The output frequency of the NLTLs ranged from 950MHz to 2.2GHz during the pulse for all volume loadings and was 1GHz after the pulse for the 10% BST/15% NZF and 15% BST/10% NZF volume loadings. The oscillations generated after the pulse were much higher in amplitude and achieved 160 kW at a 15 kV charging voltage for the 15% BST/10% NZF composite-based NLTL.