Pulse Rise Time Research Articles

Retarded field model for fast polarization rotations caused by lightning events

We study the fast state of polarization (SOP) rotation of lightwaves propagating in the fiber of optical ground wire under lightning-induced events because of the Faraday effect. Theoretical descriptions have been completed previously based on an instantaneously established uniform magnetic field time-profile. Here we describe more accurately the SOP rotation time-profile, using the retarded field model, taking into account the finiteness of the speed of light both in vacuum and in fiber. An expression for SOP rotation angle is found using the isotropic fiber approximation. Comparison of the retarded field model with the instantaneous field model is given for both sharp (pulse rise time ∼1 μs) and slow (∼10 μs) current pulses.

Free space Thomson scattering to study high energy density shocks

A free space collective Thomson scattering system has been developed to study pulsed power produced plasmas. While most Thomson scattering diagnostics on pulsed power machines use a bundle of fibers to couple scattered light from the plasma to the spectrometer, this system used free space coupling of the light, which enabled a spatially continuous image of the plasma. Initial experiments with this diagnostic were performed on an inverse wire array generated by a 200 kA, 1100ns rise time pulse power generator. The capabilities of this diagnostic were demonstrated by using the low frequency ion acoustic wave feature of the Thomson scattering spectra to measure the plasma flow velocity. The diagnostic was demonstrated to measure velocities between 20 and 40 km/s with an error of 4.7 km/s when fitting with a 600 μm spatial resolution or 8.9 km/s when fitting with a 150 μm spatial resolution. In some experiments, the diagnostic observed a bow shock in the plasma flow as the scattering intensity increased and flow velocity decreased.

Simulating Plasma Formation in Pores under Short Electric Pulses for Plasma Pulse Geo Drilling (PPGD)

Plasma Pulse Geo Drilling (PPGD) is a contact-less drilling technique, where an electric discharge across a rock sample causes the rock to fracture. Experimental results have shown PPGD drilling operations are successful if certain electrode spacings, pulse voltages, and pulse rise times are given. However, the underlying physics of the electric breakdown within the rock, which cause damage in the process, are still poorly understood. This study presents a novel methodology to numerically study plasma generation for electric pulses between 200 and 500 kV in rock pores with a width between 10 and 100 μm. We further investigate whether the pressure increase, induced by the plasma generation, is sufficient to cause rock fracturing, which is indicative of the onset of drilling success. We find that rock fracturing occurs in simulations with a 100 μm pore size and an imposed pulse voltage of approximately 400 kV. Furthermore, pulses with voltages lower than 400 kV induce damage near the electrodes, which expands from pulse to pulse, and eventually, rock fracturing occurs. Additionally, we find that the likelihood for fracturing increases with increasing pore voltage drop, which increases with pore size, electric pulse voltage, and rock effective relative permittivity while being inversely proportional to the rock porosity and pulse rise time.

Stochastic heating threshold of electrons in field-ionized plasma

In this paper, simulation study of electron stochastic heating arising from density fluctuation which in turn appears during laser interaction with Hydrogen atoms is considered by use of a massively parallel particle-in-cell (PIC) code. In order to examine the effect of ionization, different mechanisms of electron stochastic heating have been considered and analyzed. Our results reveal that since the ionization affects the pulse scattering process and the medium refractive index, the stochastic heating threshold of the electrons can be changed considerably by the ionization effects. Also, it is shown that when there is a long rise-time laser pulse (here, 80 fs) which induces a weak space charge field, the Raman backscattered radiations are seeded by a strong initial noise at the earlier times. As a result, the Mendonca condition for chaos is happened sooner which in turn causes the electron stochastic heating starts quickly. While, in a short rise-time laser pulse case (here, 40 fs) which produces a stronger electrostatic space charge field, the rapid nonlinear wave breaking because of changes in the density inhomogeneity, leads to early stochastic heating of electron. Moreover, according to our results it turns out that at the boundary between the vacuum and field-ionized plasma, the nonlinear wave breaking occurs sooner and onset of the electron stochastic heating becomes faster.

Response characteristics and optimization of electroporation: simulation based on finite element method

ABSTRACT Electroporation has been widely used in biology, medicine, and the food industry as a means to transport various molecules through the cell membrane. The phenomenon of electroporation is the result of cell membrane damage caused by the application of an electric field. In order to understand more precisely how cells function, we established a dielectric model of a spherical cell and analyzed its characteristics by the finite element method. The effects of altering different electrical parameters were determined. The results showed that the electric field strength was positively related to the transmembrane voltage (TMV) and pore density. There was a minimum electric field strength necessary to induce a critical TMV for the formation of pores. Pulse width also had to be long enough to charge the cell membrane, compared with the normal membrane charging time constant of about 1 μs. When the pulse width was shorter than the charging time constant, it was necessary to increase pulse frequency to create a high enough TMV. The rise-time of the electric pulse also affected electroporation: a fast rise-time pulse not only allowed penetration of the plasma membrane but also the organelle membrane. With slow rise-time pulse, the organelle was shielded from electroporation. This study defines the response characteristics of electrical parameters on the electric load cell and establishes the specificity of parameters for different purposes.

Evaluation of Feasibility of High-Power Long-Pulse Transformers Using Single-Layer and Pancake Winding Techniques

Solid-state klystron modulators are typically based on oil-immersed high-voltage (HV) pulse transformers because of their high performance, robustness, simplicity, and straightforward design. However, pulse transformer size is fundamentally linked to application pulse length, pulse power, and pulse rise time. For high-power applications transformer size quickly becomes very problematic when approaching pulse lengths on the order of one millisecond. This article presents a systematic study of the applicability of HV pulse transformers for such long-pulse high-power applications. Both the single-layer and pancake winding techniques are evaluated, keeping reduction of transformer volume as the main design objective. First, design models and efficient optimization procedures are developed. The proposed models are validated through circuit simulation, 3-D finite element analysis, and comparison with a commercial HV long-pulse transformer. Then, the developed optimization procedure is used in studying the evolution of pulse transformer size when pulse length is varied from 500 μs to 5 ms assuming peak pulse power requirements corresponding to that of the European Spallation Source (ESS) klystron modulators (115 kV, 100 A, 14 Hz). Finally, the developed trends are used to derive general analytical equations expressing maximum attainable pulse length as a function of application parameters and system constraints.

A Medium Power, Self-Sustaining, and Configurable RF Pulse Generation Circuit Using a Nonlinear Transmission Line and Power Amplifier in Open and Closed-Loop Configurations

Nonlinear transmission lines (NLTLs) are a class of pulse forming networks (PFNs) for modulation of pulsed power to generate radio frequency (RF) signals. Primarily, NLTL designs assume a two-port configuration, which requires ones-to-tens of nanosecond input pulse rise time for each modulated pulse out. In this article, a closed-loop or single-port pulser that yields continuous pulses based on prior pulse stimulation is presented. The pulser is comprised of an NLTL, a power amplifier (PA), and a solid-state HV pulse generator. The capability advantage of the closedloop system is its stability and center frequency tunability. Pragmatically, the solid-state-based design of this system enhances its probability of fielded use due to volume and mass savings compared to the prior art. The validity of the proposed design is empirically shown by a 16-section NLTL coupled with a solid-state laterally diffused metal-oxide-semiconductor (LDMOS)-based PA and a custom HV pulse generator. The closed-loop configuration constructed from these components demonstrated a peak power of 2.1 kW into 50 Ω at 90 MHz. This is the first report of a solid-state switch-based closed-loop NLTL configuration capable of producing a continuous pulse stream. Furthermore, we propose a dual-diode-based NLTL design, which can operate and output at higher center frequencies compared to its single-diode counterpart.

Influence of Rogowski coil structure for sub-ns current pulses.

Short current pulses are very diffuse and have also been used in many electronic devices for biological stress recently. In order to measure these current pulses, Rogowski coils are applied. In this work, we focus our efforts on the structure of the Rogowski box, which has a narrow slit, needed to correctly lead the current to be diagnosed. The attenuation coefficient depends mainly on the inductance values, the load resistance, and the virtual capacitance between coil and ground. Until now, the influence of the slit length and its width was never considered. We have studied, either theoretically or experimentally, the influence of the slit dimensions on calibration factor variations. The attenuation factor ranged from 11.3 to 16.3A/V for s ranging from 0.8 to 0.2mm, respectively. The device we realized is able to perform precise measurements of sub-nanosecond rise time pulses (∼100ps).

Differences Between Flashes With and Without Return Strokes in Rocket‐Triggered Lightning

AbstractBased on simultaneous observation of currents and electric field changes, the differences have been identified between flashes with and without return strokes in rocket‐triggered lightning. The triggered lightning events without return stroke have an obvious no‐current period up to some milliseconds before the current flow is reestablished, then a relatively large amplitude (1.2 kA or so) and fast risetime (0.7 μs or less) pulse similar to those of the return stroke current are reestablished between the upward positive leader and the ground. The triggered lightning events with return strokes have no no‐current period in the initial current variation, and the current pulse of wire destruction and plasma channel reestablishment process is similar to the M‐component pulse, featuring a small amplitude (0.17 kA or so) and slow risetime (57 μs or more). The stronger initial stage is usually followed by return strokes.

Main modes of runaway electron generation during a breakdown of high-pressure gases in an inhomogeneous electric field

Using our recently proposed method, we have identified three modes of runaway electron generation. This method is based on the recording of a dynamic displacement current using a current collector. It makes it possible to unambiguously interpret the dynamics of ionization processes occurring at the stage of formation of a high-voltage nanosecond discharge in a strongly inhomogeneous electric field and dense gaseous media. Simultaneous and separate recording by the current collector of a runaway electron beam current and the dynamic displacement current as well as the recording of discharge characteristics and high-speed framing of plasma glow at the breakdown stage made it possible to interpret the runaway electron generation modes in terms of the rate of ionization processes. It has been shown how the reduced electric field strength, the gas kind, the design and material of a high-voltage cathode with a small radius of curvature, the amplitude and rise time of a voltage pulse as well as the delay in the onset of the explosive emission process at the cathode affect the implementation of one or another generation mode.

Vacuum breakdown in microgaps between stainless-steel electrodes powered by direct-current and pulsed electric field

This paper reports on the variation of the physical properties of vacuum breakdown in microgaps from the static to the pulsed electric fields in a quasi-homogeneous electric field. The measurements were performed between the spherical cathode and planar anode, both made of stainless-steel separated from 1 μm up to 10 μm at the pressure of ∼1×10−4 Pa. Pulse rise time was varied gradually from fast 1011 V/s down to 3 V/s. Several stages during pre-breakdown have been distinguished dependent on the pulse rise time from the recorded current–voltage characteristics. A detailed analysis of each pre-breakdown stage was based on the field emission theory. From the Fowler–Nordheim plots, the enhancement factor, the radius of the emitting area, and the maximum current density were calculated and subsequently substituted to the General Thermal-Field emission equation to estimate the dominating mechanism of the electron emission.

Generation of a Pulsed Electron Beam by a Forevacuum Plasma-Cathode Source Based on a Constricted Arc Discharge

We describe the features of electron emission from the plasma formed by a constricted arc discharge, and the generation of a pulsed (100–150 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> ) low-energy (up to 10 keV) electron beam by a plasma-cathode electron source based on this type of arc in the forevacuum pressure range (5–20 Pa). The electron beam current pulse is delayed relative to the arc discharge current pulse. Limitation of the discharge current to the cylindrical part of the anode, increasing the gas pressure, and increasing the beam accelerating voltage all serve to reduce this delay time and provide a sharper rise time of the electron beam current pulse. The emission of electrons from the plasma formed by the constricted arc discharge is accompanied by a decrease in arc voltage and a corresponding increase in arc current. These characteristics are due to the specifics of operating the constricted arc discharge in the forevacuum pressure range and also the influence of backstreaming ions penetrating into the discharge system from the downstream electron beam region.

High voltage microsecond pulse generators with highly effective energy recuperation in cases of sharp changes in load impedance.

An experimental generator of microsecond pulses with an output current amplitude of ∼1 kA and an output voltage amplitude of ∼1 kV was developed. The generator is able to operate on a load whose electrical resistance changes significantly during operation. Under these conditions, the generator ensures high stability of output pulse energy and effectively uses energy that is reflected from the load. Using this generator, a high voltage generator with an output voltage pulse amplitude of ∼30 kV was developed. The high voltage generator is designed to be used in electric discharge technologies. The results of the experiments are given in which the high voltage generator produced discharges in water at a repetition rate of 500Hz. The amplitude of the discharge current pulse was ∼600A, and the pulse rise time was ∼150ns.

Dynamics of magnetic pulse compression circuit of metal vapor laser

Solid-state pulse power supply uses magnetic pulse compression circuit (MPC) to reduce the rise time of the voltage pulse applied across the Metal vapor Laser (MVL). In MPC, the nonlinear characteristics of ferromagnetic material is leveraged, to obtain large change in its inductance. These inductors get saturated during application of desired voltages and offer low impedance, which acts like a closed switch. It is desirable to fix the operating point in the B-H curve for smooth functioning of the switch, hence a current is applied to the magnetic switch to reposition the operating point on B-H curve from positive saturation flux to negative before appearance of next pulse. In MVL proper reset becomes extremely important because of small tolerance in the timing, for obtaining low jitter requirement.At low repetition rate, resetting of magnetic switch is easy as the oscillation in L-C circuit in MPC dies out before the next pulse. However, as the repetition rate increases there is very little time for reset dynamics to take place before the next pulse. In this paper we are presenting detailed dynamics of 9 kHz pulse repetition rate MPC for MVL. B-H curve have been derived using voltage, current waveform and Jiles Atherton equation in simulation model. We have shown how MPC oscillation affects the B-H curve of the magnetic switch. An experimentally validated simulation model is developed for detailed analysis of dynamics of MPC and to observe other implications like effect of reset on jitter in output of MPC.

Addressing the symmetry of a converging cylindrical shock wave in water close to implosion

We report experimental and numerical results relevant to the dynamics in the vicinity of the implosion axis of a strong shock wave generated by an underwater electrical explosion of a cylindrical array. Experiments were conducted using both sub-microsecond and microsecond timescale pulsed generators with stored energy up to ∼5 kJ, delivering ∼420/350 kA with a 320/1000 ns rise time pulses to the array, respectively. The backlighted images of the converging shock wave and the light emission around the cylindrical axis indicate strongly that the shock wave front keeps its azimuthal uniformity as far as r ≈30 μm. Also, images obtained almost simultaneously with the implosion suggest symmetric convergence &amp;lt;2 ns prior to the shockwave's arrival to the axis. In addition, the light emission obtained by a photo-multiplying tube suggests the existence of a ∼200 ns long, almost constant, strong afterglow immediately following the light emission peak related to the implosion.

Charge transfer in plasma assisted dry reforming of methane using a nanosecond pulsed packed-bed reactor discharge

This paper is aimed to investigate the effect of packing material on plasma characteristic from the viewpoint of charge transfer process. Both the charge accumulation and release processes in the dielectric barrier discharge reactor and packed-bed reactor were investigated by measuring voltage and current waveforms and taking ICCD images. The packing material was ZrO2 pellets and the reactors were driven by a parameterized nanosecond pulse source. The quantity of transferred charges in the dielectric barrier discharge reactor was enhanced when decreasing pulse rise time or increasing pulse width (within 150 ns), but reduced when the gas gap was packed with pellets. The quantity of accumulated charges in the primary discharge was larger than the quantity of released charges in the secondary discharges in the dielectric barrier discharge reactor, but they were almost equal in the packed-bed reactor. It indicates that the discharge behavior has been changed from the view of charge transfer process once the gas gap was packed with pellets, and the ICCD images confirmed it.

Electromagnetic transient modeling of form-wound stator coils with stress grading system under PWM excitation

The insulation system of a machine coil includes several layers made of materials with different characteristics. The effective insulation design of machine coils, especially in the machine end winding, depends upon an accurate model of the stress grading system. This paper proposes a modeling approach to predict the transient overvoltage, electric field, and heat generation in machine coils with a stress grading system, considering the variation of physical properties in the insulation layers. A non-uniform line model is used to divide the coil in different segments based on material properties and lengths: overhang, stress grading and slot. The cascaded connection of chain matrices is used to connect segments for the representation of the complete machine coil. The resulting model is able to simulate the transient overvoltages due to the application of fast rise time pulses such as those observed with pulse width modulation from adjustable speed drives, considering coils with different insulation topologies and under pulses with different rise times. The parameters in each coil region are calculated using the finite element method (FEM). Additionally, the resistive heat and electric field distribution in the machine coil are calculated for excitations with different rise times by means of 3-dimensional FEM simulations.

The effect of the laser pulse shape on the wakefield generation in field-ionized plasma

In this paper, the effect of the laser pulse shape on the generation and evolution of the wakefield during the interaction of the intense laser pulse with the gas have been studied utilizing the parallel relativistic PIC simulation code. In order to reach this aim, three pulses with length 300 fs and different rise-times 30, 45, and 60 are typically selected. Our results show that, the amplitude of the laser wakefield produced in the gas in comparison with the plasma strongly depends on the laser pulse shape. The simulation results indicate that for the high-slope laser pulse time (here 30 fs), ionization and thus density fluctuations have no significant effect on the wakefield generation because of rapid increase of the laser electric field. While by increasing the laser pulse rise-time to 45 fs, the rapid wave breaking due to the change in the medium refractive index during the gas ionization, prevents from the wakefield amplitude growth, so that the wakefield with larger amplitude is emerged in the plasma. For a slow-sloping pulse (here 60 fs), the ratio of the wakefield generation in the gas to the plasma is altered for the different gas densities and laser intensities. Moreover, it is represented that the longer the laser pulse rise-time, the sooner difference between the wakefield produced in the gas and plasma is observed. In fact, the larger the rise-time, the greater the density fluctuations and, consequently, the larger the initial noise is generated to seed the Raman instability.

Effects of pulse rise time and repetition frequency on nanosecond pulsed plasma ignition for combustion

Transient plasma ignition (TPI) employs highly non-equilibrium plasmas driven by nanosecond high-voltage pulses for combustion ignition. The effects of rise time and pulse repetition frequency (PRF) on combustion ignition are evaluated when transient plasmas, powered by 10-ns, kilovolt pulses, are employed to initiate combustion of lean, stoichiometric, or rich methane-dry air mixtures at atmospheric pressure. It was found that the plasmas driven by high voltage pulses with a faster rise time and PRF between 6–10 kHz enhanced combustion with higher peak pressure and/or shorter ignition delay. A mode transition in the plasma, corresponding to the combustion initiation, was observed based on voltage and current waveforms of four consecutive pulsed plasmas driven by a pulse burst at kilohertz PRFs. Although benefits in combustion ignition from a shorter rise time and optimal PRF were observed for all three equivalence mixture ratios, the highest peak pressure and shortest ignition delay were associated with the stoichiometric mixtures. In addition, the gas temperature of the transient plasma was measured by determining the rotational temperature of the 2nd positive system of nitrogen to be ∼1000 K for the first pulse and increased up to 2000 K after the application of the 4th pulse in a four-pulse burst TPI.

Simultaneous experimental evaluation of pulse shape and deadtime phenomenon of GM detector

Analysis of several pulse shape properties generated by a Geiger Mueller (GM) detector and its dependence on applied voltage was performed. The two-source method was utilized to measure deadtime while simultaneously capturing pulse shape parameters on an oscilloscope. A wide range of operating voltages (600–1200 V) beyond the recommended operating voltage of 900 V was investigated using three radioactive sources (204Tl, 137Cs, 22Na). This study investigates the relationship between operating voltage, pulse shape properties, and deadtime of the detector. Based on the data, it is found that deadtime decreases with increasing voltage from 600 to 650 V. At these low voltages (600–650 V), the collection time was long, allowing sufficient time for some recombination to take place. Increasing the voltage in this range decreased the collection time, and hence deadtime decreased. It is also observed that rise and fall time were at their highest at these applied voltages. Increasing the voltage further would result in gas multiplication, where deadtime and pulse width are observed to be increasing. After reaching the maximum point of deadtime (~ 250 µs at ~ 700 V), deadtime started to exponentially decrease until a plateau was reached. In this region, it is observed that detector deadtime and operating voltage show a strong correlation with positive pulse width, rise and fall time, cycle mean, and area. Therefore, this study confirms a correlation between detector deadtime, operating voltage, and pulse shape properties. The results will validate our hypothesis that deadtime phenomena at different operating voltages are phenomenologically different.