Unipolar Current Pulses Research Articles

Calibration of terahertz sampling detectors for intense unipolar picosecond current pulses in waveguides

Terahertz pulses are nowadays commonly used in many areas of condensed matter physics to access material properties and explore transport phenomena at fast timescales. However, little work has been devoted to the full characterization of such pulses when confined in waveguides. In this work, we fabricate terahertz photoconductive switch detectors, located at various points along a coplanar waveguide, and sample the electrical pulses that pass by the detectors. Two different but consistent methods were developed to calibrate the pulse amplitude. Notably, we synchronize a commercial pulse generator with the laser in order to sample nanosecond-wide pulses with the photoswitch detectors, effectively turning the setup into an on-chip high-frequency sampling oscilloscope. Both methods give identical results on pulse current, voltage, and field amplitudes and enable an absolute characterization of the electrical pulse propagation along the waveguide. These techniques constitute a reliable tool to explore (non-linear) phenomena such as current or field induced magnetization switching or phase transitions, which take place at high THz intensities.

The possibility of anesthesia stimulated by a train of current pulses

This study considers a simple theoretical model of blocking the passage of signals (action potentials) from sensory neurons, thereby affecting anesthesia without the use of anesthetics as a result of a sequence of unipolar current pulses generated by an external source. The proposed model allows the selection of parameters and the required frequency of the repetition of current pulses for the possible implementation of anesthesia, depending on the electrical characteristics of the skin and the conductivity of the saline solution in which the myelinated nerve fibers are located.

Characterization of the initial stage in upward lightning at the Gaisberg Tower: 2. Electric field signatures

• Near and far electric field signatures of IS current pulses were analyzed. • Electric field signatures of early-stage-IS bipolar current pulses were characterized. • Such bipolar pulses could be associated with upward leader inception. We examined the characteristics of electric field signatures occurring during the initial stage of 58 flashes measured simultaneously at near (170 m) and far (79 or 109 km) distances from the Gaisberg Tower located near Salzburg, Austria. Of the 340 field signatures measured at the near station, 68 (20%) were associated with current pulses occurring during the initiation and propagation phase (IPP) of the upward leader, and 272 (80%) were associated with pulses that occurred during the mature phase (MP) of the upward leader. Of the 68 field signatures of IPP pulses, 40 were associated with bipolar (IPP-B type) current pulses and 28 were associated with unipolar (IPP-U type) current pulses. Field signatures of IPP-B pulses were only detected at the near measurement station and appear to be associated with currents in relatively short (meter-scale) channel segments formed during the upward leader inception. At the far stations, field signatures of 84 IS pulses were recorded and analyzed. There was modest correlation between the background-to-peak current of IS pulses and near and far electric radiation field changes as well as between radiation field changes recorded at near and far distances.

Field-Free Deterministic Writing of Spin-Orbit Torque Magnetic Tunneling Junction by Unipolar Current

We propose a field-free magnetic switching method in a perpendicular spin-orbit torque magnetic tunneling junction on a 200 mm wafer. The field-free switching can be achieved by unipolar current pulses via engineering the interlayer exchange coupling in the thin ferromagnetic films. The proposed device can reach a high writing speed of up to 1 ns and work properly at temperature of 100°C. This novel unipolar switching scheme offers an alternative writing approach for spin-orbit torque magnetic random access memory.

Influence of Ag Electrodes Asymmetry Arrangement on Their Erosion Wear and Nanoparticle Synthesis in Spark Discharge

For nanoparticle synthesis in a spark discharge, the influence of the degree of electrode asymmetry in the rod-to-rod configuration, using the example of silver electrodes, on the energy efficiency and nanoparticle composition is studied. The asymmetry degree was determined by the angle between electrodes’ end faces. Two types of discharge current pulses were used: oscillation-damped and unipolar, in which electrodes changed their polarities and had a constant polarity during a single discharge, respectively. A significant influence of the asymmetry degree of the electrode arrangement on the synthesized nanoparticle size, agglomeration and concentration, and on the synthesis energy efficiency, has been established. An increase in the degree of the electrode asymmetry with the oscillation-damped discharge current pulse led to an increased mass production rate and energy efficiency of nanoparticle synthesis, a significant fraction of which had large dimensions of more than 40 nm. The effect of the transfer of synthesized nanoparticles to the opposite electrode at the unipolar discharge current pulse led to the appearance of electroerosive instability, manifested in the formation of a protrusion on the anode surface, around which spark discharges, leading to its further growth and electrode gap closure.

Electromechanical installation based on high power current pulse generator

Electromechanical installation containing electromagnetic s noid with a piston operating in reciprocating motion mode has been developed. Technologies with similar operating modes are widely used in various industries (mechanical engineering, metallurgy, mining, mechanics, robotics, as presses for forging and stamping, as well as jack-hammers in crushing devices for coal, ore and rock). An economically effective generator of high power unipolar current pulses (with unique systems that allow adjustment of the main parameters in a wide range and with high speed: the frequency of pulse reproduction, amplitude) was used as a power source for electromechanical installation. The principle of operation of the generator is based on periodic discharge of pre-charged capacitors to a low-resistance active-inductive load. Generator contains power unit, consisting of a capacitor discharge unit for load; a generator control system (GCS), consisting of a capacitor charging unit (reversible thyristor converter with counter-parallel connected thyristor bridges); a recharge unit and an automatic control system of the ACS. According to the known equations, parameters of mechanical and electrical parts of the electromechanical installation were calculated: initial coordinate of the piston; magnetomotive force arising from change in inductance L(x) ; spring force; the force of resistance to the piston, proportional to the speed of its movement; force action on the piston; amplitude, duration and frequency of reproduction of current pulses. A simulation model of the installation has been developed in the MATLAB-SIMULINK environment. The graphs of transient processes during operation of the installation at idle and under load were built. Analysis of the operating modes of the installation was carried out. Developed electromechanical installation for influencing load with the aim of its destruction or deformation with a system of automatic control of parameters makes it possible to regulate process parameters with high speed: force and distance traveled by the piston.

Electromechanical installation based on a powerful current pulse generator for materials treatment

The electromechanical device containing an electromagnetic solenoid with a piston operating in the reciprocating motion mode was developed. Technologies with similar operating modes are widely used in various industries. Economical generator of powerful unipolar current pulses with unique systems is used in the electromechanical device. It allows its main parameters to be regulated in a wide range and with high speed: pulse reproduction frequency and amplitude. The principle of generator operation is based on the periodic discharge of pre-charged capacitors to a low-resistance active-inductive load. The parameters of the mechanical and electrical parts of the electromechanical device are calculated: the initial coordinate of the piston, the magnetomotive force resulting from a change in the inductance L(x), the spring elasticity, the resistance force of the piston proportional to its speed, amplitude, duration and frequency of current pulses. A simulation model of the device in the environment of Matlab-Simulink was developed. Plots of transients during the device idling and under load were constructed. The analysis of the device operating modes was performed. The developed electromechanical installation allows the process parameters to be adjusted.

A compact and powerful EMAT design for contactless detection of inhomogeneities inside the liquid volume of metallic tanks

Abstract A simple and powerful design of an electromagnetic acoustic transducer (EMAT) without bulky permanent magnets is presented. The EMAT is operated in a pulse echo modality and generates longitudinal ultrasound at about 1 MHz. Unlike shear waves, these longitudinal ultrasound pulses can propagate in liquids. The generally addressed application scenario is the examination of a liquid volume inside a metallic container or tank, e. g., the detection of inhomogeneities within the liquid. The herein proposed EMAT operates for virtually all metallic containers, i. e., it succeeds for container walls made of aluminum or ferromagnetic steel, and even for non-ferromagnetic (stainless) steel. Moreover, unlike piezo transducers, EMAT techniques allow for a non-contacting ultrasound transduction: the air gap between the EMAT sensor coil and the tank’s metallic surface extends up to 2 mm. Even with this relatively large air gap, the biasing magnetic field approaches a flux density of 3.2 T at the surface, more than what is possible to achieve with the permanent magnets of conventional and bulkier EMATs. Strong fields improve the coupling efficiency of the principally low-efficiency EMAT mechanism, which is important for both ultrasound transmission and reception. For that superior field intensity, a unipolar current pulse of up to 3.6 kA is applied through the thin windings (0.5 mm) of the EMAT coil. This paper presents a novel solid-state EMAT circuitry for such strong currents and MHz pulsed voltages >1 kV. As a particularly delicate task, the powerful circuitry must also detect the rather weak echo signals in the μV range. A very short recovery time is required after such a strong emission burst. The discussed circuitry consists of three unipolar high-current modules, which can each be independently launched. This allows for received echo signals that can be timed independently, e. g., objects deep inside the liquid tank can be specifically addressed. In general, this work concentrates on the novel circuitry in parallel connection, the general pulse-echo functionality and the magnetic fields. A detailed analysis and shaping of the ultrasonic fields through different EMAT coil geometries would exceed the scope of this contribution and is to be reported separately.

Chiral Domain Wall Injector Driven by Spin-Orbit Torques.

Memory and logic devices that encode information in magnetic domains rely on the controlled injection of domain walls to reach their full potential. In this work, we exploit the chiral coupling, which is induced by the Dzyaloshinskii-Moriya interaction, between in-plane and out-of-plane magnetized regions of a Pt/Co/AlOx trilayer in combination with current-driven spin-orbit torques to control the injection of domain walls into magnetic conduits. We demonstrate that the current-induced domain nucleation is strongly inhibited for magnetic configurations stabilized by the chiral coupling and promoted for those that have the opposite chirality. These configurations allow for efficient domain wall injection using current densities of the order of 4 × 1011 A m-2, which are lower than those used in other injection schemes. Furthermore, by setting the orientation of the in-plane magnetization using an external field, we demonstrate the use of a chiral domain wall injector to create a controlled sequence of alternating domains in a racetrack structure driven by a steady stream of unipolar current pulses.

Investigation of Reverse Switch-On Dynistors Upgraded for Reducing the Energy Loss when Switching Reverse-Current Pulses

The results of investigations of new semiconductor devices, reverse switch-on dynistors dynistors–diodes (RSDDs), with an operating voltage of 2 kV and a 50-mm diameter of the structure are presented. Their comparison with conventional RSDs shows that the losses in RSDDs are approximately 2 times lower when high-power reverse-current pulses are switched, the losses are virtually the same during switching, and the losses are 15–20% higher when switching rapidly rising forward-current pulses. The limiting abilities of RSDDs are determined when switching unipolar current pulses with a duration of 300 μs: 30 and 100 kA in the reverse and forward directions, respectively. The possibility of reliable switching of alternating slowly damped current pulses with a duration of 40 μs and amplitudes of the first forward and reverse half-waves of 90 and 65 kA, respectively, with which conventional RSDs cannot operate, is shown.

Effect of applied voltage and inter-pulse delay in spark-assisted LIBS

We report the results obtained in an investigation on the effect of the time delay between the laser and electrical pulses in a spark-assisted laser-induced breakdown spectroscopy (LIBS) experiment. The electrical discharge is produced by the discharge of a charged coaxial cable. This arrangement produces a fast unipolar current pulse (500ns) that applies high power (~600kW) to the laser ablation plasma. The delay between the laser pulse and the electric pulse can be controlled at will in order to find the optimal time in terms of enhancement of the emitted lines. It was found that the application of the high voltage pulse enhances the ionic lines emitted by up to two orders of magnitude. An additional enhancement by a factor of 2–4 can be obtained delaying the application of the electric pulse by a time of 0.6–20μs. In the tests it was noticed that the ionic lines were found to be clearly responsive to increments in the applied electric energy while the neutral lines did so marginally. Our results show that the intensification of the lines is mainly due to reheating of the ablation plasma as the application of the electrical pulse increments the temperature of the ablation plasma by about 50%. It is demonstrated that the present technique is an efficient way of intensifying the lines emitted without incurring in additional damage to the sample.

Surface plasmons in superintense laser-solid interactions

We review studies of superintense laser interactions with solid targets where the generation of propagating surface plasmons (or surface waves) plays a key role. These studies include the onset of plasma instabilities at the irradiated surface, the enhancement of secondary emissions (protons, electrons, and photons as high harmonics in the XUV range) in femtosecond interactions with grating targets, and the generation of unipolar current pulses with picosecond duration. The experimental results give evidence of the existence of surface plasmons in the nonlinear regime of relativistic electron dynamics. These findings open up a route to the improvement of ultrashort laser-driven sources of energetic radiation and, more in general, to the extension of plasmonics in a high field regime.

Electrostimulated machining of metals

New mechanical equipment requires structural materials whose high performance cannot be ensured by traditional methods. A promising approach to the shaping of steel is the use of powerful unipolar current pulses with the following characteristics: amplitude 10–15 kA; pulse frequency up to 400 Hz; pulse length up to 100 μs. The widespread industrial use of this technique is hindered by the low efficiency of the corresponding pulse generators, which also draw considerable power from the ac grid and are not sufficiently controllable. In the present work, a generator of powerful unipolar current pulses that is free of those defects is described. It includes a charging system connected to power capacitors; and a thyristor switch that discharges the capacitors to a low-resistance load. To reduce the power drawn from the grid, the generator includes a recharging device based on a thyristor, which is connected to a reverse-parallel thyristor switch. To permit regulation of the pulse amplitude and increase its power, the uncontrollable dc source in the charging system is replaced by two irreversible thyristor converters in series. That permits control of the voltage at the power capacitors. To optimize capacitor charging, a two-loop subordinate control system regulates the parameters of the pulse generator: the external control loop governs the voltage charging the capacitors, while the internal control loop governs the charging current. MATLAB Simulink software is used to create a model of the proposed generator. The model corresponds to the actual pulse generator used at Siberian State Industrial University to investigate the electrostimulated plastic deformation of metals and alloys. The model permits improvement in the characteristics of the pulse generator and its operating conditions. A benefit of the proposed generator over its counterparts is that the power drawn from the grid is considerably reduced, while the voltage charging the capacitor may be regulated in the range up to 600 V, with pulse frequencies up to 400 Hz. The generator may be used industrially—in particular, in rolling mills when drawing steel wire that is hard to deform.

Flexible Supercapacitor Stackable with Solar Cells Based on PEDOT-Carbon Nanotube Nanocomposite Electrodes Using Ionic Liquid Gel Electrolytes for Solar Electricity Storage

Electrically conducting polymers (ECP) showing pseudo-capacitive property are actively researched as supercapacitor electrodes for energy storage. Nanocomposites with structured carbons contribute additional charge storage through electric double layer component enhancing the charge storage ability. The nanocomposites with CNTs hold additional promise to compensate for weak mechanical property of ECPs useful for flexible supercapacitor devices and enhance its electronic conductivity. Conventionally, the supercapacitor performance is evaluated using anion dopants in aqueous medium having voltage limitation and in organic electrolytes which are toxic. Ionic liquid gel electrolytes are promising but not widely studied for supercapacitor applications. Besides enabling stable high voltage operation, ionic liquid gel electrolyte makes supercapacitor assembly simple which can be configured as flat stackable layered device. These attributes are exploited in integration with the solar cells for the storage of solar electricity. We have investigated electrochemical and energy storage performance of poly(3,4-ethylenedioxy-thiophene) (PEDOT) and CNT nanocomposite electrodes based supercapacitors in the flexible solid-state format with ionic liquid gel electrolyte. In this work nanocomposite electrode was formed by co-deposition of PEDOT and CNT over the flexible graphite substrates using the galvanic pulsed electro-polymerization. The method involves ultrasonic dispersion of 1 wt% anionic CNT for 2 h in acetonitrile to which 0.1 M EDOT monomer and 0.2 M LiClO4 dopant was added. The PEDOT-CNT composite deposition is initiated as unipolar anodic current pulses of 4 mA.cm-2 amplitude with 10 ms ON time interspaced with 100 ms OFF time were sequentially applied for 20k cycles providing a unique way to control the electrode morphology. The symmetrical supercapacitor device was fabricated using ionic liquid 1-butyl-3-methylimidazolium tetrafluoro-borate ([BMIm]BF4) immobilized in poly(vinylidenefluoride -co-hexafluoropropylene (PVdF-HFP) forming a gel which was applied uniformly as ~50-70 μ layer sandwiched between the two PEDOT-CNT composite electrodes. With such stackable flat plate- supercapacitor device design, enhanced energy density is obtained and direct charge storage performance was realized when connected to solar cell module while mounted at the rear without affecting the form factor. The electrode morphology show randomly dispersed CNT of average diameter of 20-30 nm embedded within and encased by copolymerized PEDOT forming a 3-D network structure. The CNT is observed to enhance the PEDOT polymerization by facilitating charge transport at the substrate and also increase the electrode conductivity. Raman shifts in the 1425 cm−1 band assigned to symmetric Cα=Cβ (−O) stretching show in the formation of nanocomposite, the CNTs interact with the ethylenedioxy group of PEDOT resulting in high degree of conjugation and enhancrd charge transfer between CNTs and PEDOT molecular chains. Conjugated interaction is further confirmed by shift in disordered A1g mode of CNT. The cyclic voltammetry curves recorded over -1 to +1 V range showed fast and near vertical current switching at end potentials over wide 10-100 mV s-1 scan rates signifying highly capacitive property. Supercapacitor showed areal capacitance density of 357 mF.cm-2 yielding high single electrode specific capacitance of 297 F.g-1. The high areal specific capacitance in the PEDOT-CNT composite is due to dopant anion aggregation along the CNTs and enhanced conduction of dopant ions in ionic liquid through embedded CNT nanopores providing rapid pathways to delocalize the electrons from along the PEDOT molecular chain. Impedance data as Nyquist plots feature near vertical trend in the low frequency region and lower real impedance cutoff suggestive of highly capacitive property and low electrode-electrolyte resistance. Occurrence of high knee frequency of > 10 Hz suggests compared to PEDOT, the CNT nanocomposite has minimal diffusion limitation. Charge-discharge (CD) data at various currents cycled between 0.05-1.0V are triangular with low ESR of 20 Ohm.cm2 and Coulomb efficiency > 90%. Long-term CD cycling for 2000 cycles showed overall degradation of 21% with a nominal aggravation of merely 7% occurring in initial 400 cycles. These data show comparatively higher stability of CNT nanocomposite compared to PEDOT. The supercapacitor device functional properties were evaluated in terms of energy and power density parameters and presented as Ragone plots. Energy density of 18.3 Wh.kg-1 and corresponding power density of 5.1kW.kg-1 was recorded from CD data at 0.5 mA.cm-2 for 1V operation which is reasonably high for a gel electrolyte supercapacitor device. This paper would present detailed analysis of the pulse polymerization synthesis mechanism, microstructure and electrochemical properties of PEDOT-CNT nanocomposite. The supercapacitor device data on CV, CD, impedance and Ragone plots will be presented along with its efficacy in storing the solar electricity in combination with the solar cells.

ON MINIMAL INDUCTION OF THYRISTOR DRIVE SMOOTHING THROTTLE UNDER CONVERTER EDGE OPERATING CONDITIONS

The smoothing throttle operations in the DC-motor circuit of a thyristor drive are studied. The main mathematical and graphic dependences of the smoothing throttle Lс inductance modification on the control angle, and on the number of the thyristor converter (TC) phases are given. The situation of the technological machine (TM) executing mechanism (EM) low conveying speed and of the armature pulsating current zone (PCZ) reduction is considered. In this case, it is reasonable to determine the minimal induction value of the smoothing throttle (Lс) at the boundary control angle αboun equal to 50—60 electric degrees for the increase in the mechanical characteristic rigidity of the DC motor. Then, the throttle magnetic conductor condition is defined by the branches of the specific limit asymmetric hysteresis cycle characterizing the electromagnetic processes in the magnetic conductor under the influence of the unipolar current pulses. Thus, the efficiency of the throttle core usage is defined through the incremental induction, i.e. through the difference in the residual induction B(0) levels, and the magnetic conductor material saturation induction. The airgap injection (δ) stabilizes the core equivalent magnetic capacity which can be broken by heating the throttle magnetic conductor with the armature current, by the environmental temperature fluctuations, and by the operation conditions of the thyristor drive. At the minimum inductance Lc of a smoothing throttle (for a boundary angle αboun), effects of the stabilizing factors of the core equivalent magnetic capacity (with airgap δ) decrease, PCZ is excluded. This permits to reduce electric losses in the power contour of the TC — DC-motor system, and also to form the required rigidity of the DC-motor mechanical characteristics, and the TM EM movement steadiness in the anchor low rotation frequency zones.

Propagating features of upward positive leaders in the initial stage of rocket-triggered lightning

The propagation characteristics of upward positive leaders in the initial stage of rocket-triggered lightning flashes have been analyzed using high-speed video images, channel base currents, and electric field changes. The luminous intensity at the tip of leader steps was stronger than that in the channel behind it. The average 2-D speed of the leader was 1.0×105m/s with partial speeds ranging from 2.0×104m/s to 1.8×105m/s between 130m and 730m above ground. The leader speed showed an obvious irregularity and an accelerating tendency with height and time. Unipolar current pulses, with peak currents ranging from tens of amperes to about 150A, were observed at the ground during the initial stage of leader development. The electric field change showed stepped waveforms at this stage and it was evident that the leader involved a stepwise propagating mode. For 34 current pulses occurring in the initial stage of the leader development, the geometric mean values of the peak current, rise time, half peak width, duration, charge transfer and pulse interval were 45.0A, 0.49μs, 0.99μs, 3.2μs, 4.8μC and 19.9μs, respectively. After the leaders extended to several hundred meters above ground, fluctuations were found to be superimposed on the continuous current and the associated electric field.

Technological features of electrochemical polishing of gold alloys by pulse currents

The features of the electrochemical polishing of complex shaped jewelry made of different gold alloys by unipolar and bipolar current pulses have been considered. A comparison with the other polishing processes has been carried out.

Laser Radar Receiver Channel With Timing Detector Based on Front End Unipolar-to-Bipolar Pulse Shaping

An integrated receiver channel for a pulsed time-of-flight laser range finder is presented based on a timing discrimination principle in which the incoming unipolar detector current pulse is converted to a bipolar pulse at the front end of the receiver channel. Thus no optical or electrical gain control is needed within the dynamic range of the receiver, which according to measurements is 1:3000 with a timing walk error of plusmn 55 ps (plusmn 8 mm in distance). The minimum detectable input signal current is about 1.3 muA at an SNR of 10 with a bandwidth of 200 MHz. The circuit is realized in a 0.35 mum SiGe BiCMOS process and consumes 220 mW of power.

Properties and mechanism of solar absorber CdTe thin film synthesis by unipolar galvanic pulsed electrodeposition

Electrochemical deposition of CdTe semiconductor thin films over transparent conducting glass substrates by sequential unipolar current pulses is described. The magnitude of pulsed current and pulse periodicity affects the crystalline structure, morphology, optical absorbance and composition of CdTe films. CdTe films formed under high magnitude pulsed current density ~5–15 mA cm−2 are crystalline with dominant cubic structure having (111) plane oriented parallel to the substrate. Stoichiometric CdTe film growth occurs with current pulses of short 25–300 ms periodicity and 3–50 ms duration. A mechanism of the CdTe growth involving in situ cathodic tellurization process step involving H2Te formation and reaction with electrochemically deposited Cd monolayer is described. CdTe film growth in the pulsed electrodeposition occurs under mass transport conditions under strong influence of high magnitude pulsed current. This results in much higher growth rates ~5–8 μm h−1 for CdTe films which is attractive for CdTe solar cells in a production environment.

Microstructural investigations on CdTe thin films electrodeposited using high current pulses

CdTe thin films were electrochemically deposited over FTO-coated glass substrates by singular unipolar current pulses of high magnitude. A wide range of periodic current pulse parameters was investigated in CdTe thin film deposition and the growth properties of the deposited film were determined as a function of variables viz. applied current density, average current and ON/OFF pulse duration and the conditions were optimized for stoichiometric CdTe film growth. A detailed investigation has been carried out to understand the effect of pulse parameters on CdTe growth phases and film microstructure. As evidenced by the diffraction patterns of pulse electrodeposited CdTe films mean current density of 0.36 mA/cm 2 represents an upper limit to form crystalline—CdTe film without secondary phases. Increase in pulse current show inclusion of secondary growth phases. A highly oriented cubic phase was obtained at low current pulse, due to low nucleation rate and surface diffusion of adions on the substrate to growth sites. The crystalline structure and morphology of films were analyzed using powder X-ray diffraction and scanning electron microscopy suggest that OFF time contributes more on the grain size, whereas ON time affects the rate of deposition and grain morphology.