Negative Polarity Pulses Research Articles

Collision of Two Plasma Diffuse Jets with the Same and Different Front Polarity at an Air Pressure of 1 Torr

In air at a pressure of 1 Torr, the mode of collision of diffuse plasma jets (PDJs) consisting of ionization waves — red streamers have been studied. PDJ were formed in a quartz tube by a capacitive discharge from two identical generators with pulses of positive and negative polarity. It has been established that, with the same polarity of voltage pulses, counter-current PDJs suppress each other’s radiation. It is shown that for different polarity of voltage pulses, the intensity of the glow in the region where the PDJ meets increases significantly. Data are presented on the effect of delays between switching on generators of different polarity on the emission spectra of PDJ. It has been established that with an increase in air humidity, an atomic hydrogen line Ha appears in the emission spectrum, as well as OH and OH+ bands, the spectral energy density (W) of some of which is commensurate with the W bands of the second positive (2+) nitrogen system. The velocity of the PDJ front was measured when two generators of different polarity were turned on.

Studying of Filamentation Mechanism for Nanosecond Surface Dielectric Barrier Discharge. Part 1. Local Field Approximation

The goal of this work is to check numerically whether or not the previously proposed mechanism for surface barrier discharge filamentation in nitrogen in the case of positive polarity nanosecond voltage pulse is applicable for similar process in nitrogen and air in the case of negative voltage polarity pulse. The results have shown, that in this case some signs of successful filamentation modeling are present both in nitrogen and air, but the whole dynamics of discharge development is qualitatively different from that one observed in experiment. It is assumed, that the failure of simulation is due to the usage of local field approximation, which is too rough inside a region with steep electron density gradient relevant to filamentation zone.

Influence of the pulse polarity on micro-hollow cathode helium plasma jet

Helium plasma jets generated by micro-hollow cathode discharge (MHCD) with the square-wave power source of different polarities are investigated in this work. The effects of positive and negative polarity pulses on the MHCD and plasma jet were compared, and the time-resolved optical emission spectra of excited species ( and O) were studied. The results confirm that the electric field is a key factor for the propagation of the jet during the rising edge of the positive current pulse, while the gas expansion is mainly important for the jet propagation during the current stable phase. The time-resolved spectra show that the generation of specie O in the jet-driven by the electric field is more efficient.

Ion acoustic solitons collision in spin-polarized relativistic quantum plasma

The dynamics of electrostatic ion acoustic solitons collision in relativistic magnetized spin-polarized plasma is studied using a Magnetized Relativistic Quantum Hydrodynamics model. The plasmas components are relativistic degenerate electron with spin-up (n e↑) and spin-down (n e↓) concentrations, and relativistic inertial classical ions. In two-dimensional plasma geometry, a homogeneous external magnetic field is provided along the z-direction, i.e. . The extended Poincare Lighthill Kuo (PLK) approach is used to calculate the Kortewegde Vries equations for oppositely moving ion-acoustic solitons. Before and after collision, their trajectories and associated phase shifts are computed. Only negative polarity pulses of ion acoustic solitons are observed in our stated relativistic magnetized plasma. The spin density popularization ratio κ is shown to have a significant impact on the phase shifts of colliding solitons. It is found that when κ increases, one soliton's phase shift increases while the other soliton's phase shift decreases. The relativistic effects are also analyzed numerically and found that the phase shift of colliding ions acoustic solitons grows as the relativistic component rises from weakly to ultra-relativistic. It is observed that the phase shift of both solitons is to be dependent on their phase velocity with each other. Our research is noteworthy in that it focuses on, the astrophysical systems such as white dwarf, neutron star and pulsar.

Polarity dependence of CO2 conversion in nanosecond pulsed large gap dielectric barrier discharges

The splitting of carbon dioxide was investigated for a coaxial dielectric barrier discharge, which was operated with nanosecond high-voltage pulses of 500 ns and amplitudes up to 20 kV at ambient temperature and atmospheric pressure. A non-thermal plasma could be established across a gap distance of 4 mm and a length of 90 mm for gas flows of 30–210 sccm of pure CO2 and with admixtures of Argon. The application of high-voltage pulses of either positive or negative polarity resulted in distinct differences in effective conversion and CO production. The highest observed conversion of 6.6%, corresponding to a CO production of 7%, was achieved for positive high-voltage pulses of 20 kV that were applied with a repetition rate of 3 kHz for a ratio of CO2:Ar of 1:2 at a flow rate of 30 sccm. Conversely, an operation with negative high-voltage pulses, for otherwise the same operating parameters, resulted in an effective conversion of only 5.3% and CO production of 5.4%. The corresponding conversion rates for specific energy input (SEI), concerning different operating parameters, could be related to reaction enthalpies that were calculated from thermodynamic functions. The differences in polarity were associated with discharge characteristics, i.e., plasmas appeared more filamentary for positive high-voltage pulses. In this case, a visible plasma could also be established for much lower pulse amplitudes.Graphical

Electric charge build-up and dissipation at pulsed streamer corona by EFISH and probe measurements

This work examines the dynamics of volumetric electric charge deposited by a single pin electrode streamer corona in atmospheric air at ambient conditions. Recent studies show that, at pulse-periodic operation, electric charge remains in surrounding air after the first discharge. This residual charge significantly affects the electric field distribution and morphology of discharges at subsequent pulses. In this study, the residual electrical potential/field was measured during and after dissipation of a pulsed 80 kV streamer corona of alternating polarity. This was accomplished using electrostatic probes and the electric field induced second harmonic method (EFISH generation). The residual volumetric charge of a positive pulse polarity streamer corona occupies a zone of up to 80 mm from the electrode. The subsequent application of a negative polarity pulse leads to partial neutralization of the previous charge and the generation of a region with a high-amplitude electric field. The propagation of following positive polarity pulses is limited by the residual positive volumetric charge. The EFISH method reveals the magnitude of the electric field at discharge propagation and electrostatic probes inform the polarity. Intensified imaging acquisition was used to evaluate the spatial distribution and dynamics of corona discharge during the alternating polarity waveform.

Breakdown voltage in long tubes: the effect of surface charge

The study focuses on ignition processes in long discharge tubes (the length of which is large compared to the diameter) in rare gases Ne, Ar, and their mixture at a low pressure (∼1 Torr). Gas breakdown was caused by ramp voltage pulses of positive or negative polarity applied to the active electrode. The breakdown voltage was determined by the voltage drop at breakdown. The emission of the ionization wave (IW) preceding the breakdown was explored. The discharge tubes were exposed to two types of external influences. The first was illumination of the tube cathode with visible spectrum light, while the second was the constant or pulsed bias of the cathode potential by a value lower than that of maintaining discharge. In both cases the breakdown voltage increased up to doubling under some conditions. The observation of the IW revealed the presence of extra waves preceding the regular pre-breakdown IW. The extra wave velocity and emission intensity differed from those of the regular waves. Their main feature is that they do not overcome the entire inter-electrode gap, but weaken and disappear in between. It is assumed that the extra waves deposit the wall surface charge, which in turn affects the breakdown voltage. The increased breakdown voltage value remains for tens of minutes, which could indicate the surface charge lifetime of the same order. This was confirmed by direct wall-potential measurements using an electrostatic voltmeter.

On the slow ionization waves forming the breakdown in a long capillary tube with helium at low pressure

The results of studies of an electrical breakdown leading to the glow discharge ignition in a long capillary quartz tube are presented. Under such conditions, the breakdown completion is preceded by the development of direct, backward, and counter slow ionization waves (IWs) traveling in the tube. The initiation of the waves was created in helium at low pressure (P = 10 Torr) by the high-voltage pulses of positive and negative polarity with amplitude of several kilovolts. In the beginning, the regime without the breakdown completion in the tube was studied. In this regime, the propagation of only direct positive and direct negative IWs happens. The research on dynamics of the direct, backward, and counter positive and negative waves followed by a complete breakdown was done as well. The influence of the pre-existing plasma on the IWs propagation was also studied. The plasma was created in advance by low-current glow discharge being formed in the tube. The instant images of IWs were correlated with the electrical currents formed by the waves, that is, with the displacement current through the dielectric wall and the conductive current through the plasma column. In the experiments, the fine-sectioned electrode wrapped around the lateral tube surface was used. The usage of such electrode allowed one to study the dynamics of the surface charge deposition and deletion happening during the direct and backward wave propagation, respectively. Finally, a strong difference in the spatial structure and velocity of positive and negative direct waves traveling through non-ionized gas was revealed. Contrary, both the positive and negative backward waves traveling through the plasma formed by previous direct waves have the parameters close to each other.

Research on the characteristics of atmospheric air dielectric barrier discharge under different square wave pulse polarities

Energy efficiency limits the application of atmospheric pressure dielectric barrier discharge (DBD), such as air purification, water treatment and material surface modification. This article focuses on the electrical and optical effects of the DBD under three square wave pulses polarities-positive, negative and bipolar. The result shows that under the same voltage with the quartz glass medium, the discharge efficiency of bipolar polarity pulse is the highest due to the influence of deposited charge. With the increase of air gap distance from 0.5 to 1.5 mm, average power consumed by the discharge air gap and discharge efficiency decrease obviously under alumina, and increase, and then decrease under quartz glass and polymethyl methacrylate (PMMA). Through spectrum diagnosis, in the quartz glass medium, the vibration temperature is the highest under negative polarity pulse excitation. Under bipolar pulse, the vibration temperature does not change significantly with the change of air gap distance. For the three dielectric materials of quartz glass, alumina and PMMA, the molecular vibration temperature is the highest under the quartz glass medium with the same voltage. When the gap spacing, pulse polarity or dielectric material are changed, the rotational temperature does not change significantly.

Generation of runaway electrons in plasma after a breakdown of a gap with a sharply non-uniform electric field strength distribution

The paper is devoted to the study of the initiation and formation of a negative streamer in a sharply inhomogeneous electric field and the generation of runaway electrons (REs) in air and helium at atmospheric pressure and below, as well as in sulfur hexafluoride at low pressure. Nanosecond voltage pulses of negative polarity with an amplitude of 18 kV were applied across a point-to-plane gap 8.5 mm long. The studies were carried out using broadband measuring sensors and equipment with picosecond time resolution, as well as using a four-channel ICCD camera. Using a special method for measuring the dynamic displacement current caused by the redistribution of the electric field during streamer formation, the waveforms of voltage, discharge current, RE current, and dynamic displacement current were synchronized to each other, as well as to ICCD images. Data on the generation of REs with respect to the dynamics of streamer formation were obtained. It was found that REs are generated not only during the breakdown of the gap, but also after that. It has been found that the formation time of explosive emission centers affects the generation of REs after breakdown. Based on the measurement data of the voltage, discharge current, and dynamic displacement current, the electron concentration in the plasma channel after breakdown and the electric field strength near the surface of the grounded electrode were calculated.

Transient processes during an initial stage of breakdown in saline solution

This paper deals with the investigation of the discharge in the solution of 0.9% NaCl in water for the conditions, when the voltage at the gap is in a vicinity of the so-called critical voltage at which the discharge in a thin vapor layer at the surface of active electrode appears. In the experiments, the critical voltage is at a level of (180–200) V. Most of the data correspond to powering the discharge by the bipolar voltage pulses of 5 s duration. The main attention is centered on the nonsteady (transient) processes and on the forms of the discharge operation. For the voltage pulses of negative polarity, when the metal electrode is the cathode, the discharge is sustained in the glow regime with the occasional glow-to-spark transitions. At the positive polarity of the voltage, the surface of liquid inside the vapor layer plays a role of cathode in the gas-discharge gap. In these conditions, the process of the glow-to-spark transition is suppressed and the discharge is sustained in the glow regime.

Computational Study of Discharge Process in Plasma Actuator for Enhanced Electrohydrodynamic Force Generation toward Low-Voltage Operation

Numerical simulations of discharge process in two types of actuator modules were conducted to obtain better understanding of the process of electrohydrodynamic (EHD) force generation toward the low-voltage operation. The charge accumulated on the dielectric surface can be absorbed by the additional downstream-side exposed electrode, which is a component of the adjacent actuator module in a multi-module plasma actuator, when positively-DC-biased negative polarity pulses are applied. The elimination of the surface charge results in EHD force enhancement. The enhanced EHD force is also obtained when a negatively-DC-biased positive polarity pulses are applied. The applied voltage affects the surface charge profile and EHD force enhancement by the downstream-side exposed electrode. Under our simulation condition, the positively-DC-biased negative pulses obtain larger enhancement of EHD force compared to that obtained by the negatively-DC-biased positive pulses.

High-voltage 10 ns delayed paired or bipolar pulses for in vitro bioelectric experiments

Recent studies proved that classical bio-effects induced by nanosecond pulsed electric field (nsPEF) can be reduced by the delivery of a negative polarity pulse generated immediately after a positive polarity pulse. This phenomenon is known as “bipolar cancellation” and it was reported for a wide range of bipolar pulses with pulse duration from 2 ns to 900 ns. On the contrary, paired pulses, i.e., two identical pulses with the same polarity, increased traditional nsPEF outcomes. Herein, we propose a novel robust and flexible generator, based on the frozen-wave concept, able to produce a broad range of pulses with the duration of 10 ns and delay between 17 and 360 ns. Numerical simulations and experimental measurements were performed to fully characterize the proposed generator. YO-PROTM-1 uptake was investigated in the U87-MG human glioblastoma cell line as a marker of membrane permeabilization in response to 10 ns, 11.5MV/m nsPEF. Our results showed that bipolar cancellation occurred for delays of 0–30 ns and decreased as a function of the interphase interval. In addition, we observed that cellular response following the application of paired nsPEF was more than two-fold compared to the unipolar pulse response and was independent from the interphase interval.

Measurements of electric field in an atmospheric pressure helium plasma jet by the E-FISH method

Temporal and spatial distributions of the electric field in an atmospheric pressure, ns pulse, positive and negative polarity helium plasma jets are measured by ps electric field induced second harmonic generation. The measurements have been done in a quasi-two-dimensional plasma jet impinging on liquid water, using a laser sheet and a focused laser beam positioned at different heights above the water surface. Absolute calibration of the electric field is obtained by measuring a known Laplacian electric field distribution for the same geometry and at the same flow conditions. The vertical component of the electric field is determined by isolating the second harmonic signal with the vertical polarization. The measured electric field is averaged over the span of the plasma jet, in the direction of the laser sheet or the focused laser beam. The spatial resolution of the laser sheet measurements is approximately 15 μm across the sheet, with the temporal resolution of 10 ns. The spatial resolution of the focused laser beam measurements is approximately 180 μm across the beam, with the temporal resolution of 2.5 ns. The results show non-monotonous electric field distribution across the jet, with two maxima produced by the surface ionization waves propagating over water. Considerable electric field enhancement is detected near the surface. Residual charge accumulation on the water surface is detected only in the negative polarity pulse discharge. The results provide new insight into the charge species kinetics and transport in atmospheric pressure plasma jets, and produce data for detailed validation of high-fidelity kinetic models.

Characteristic Features of Electric Fields Radiated by Cloud Flashes in Himalayan Region

Electric fields radiated by cloud flashes that occurred over the rugged terrain of mountainous country Nepal were recorded, analysed, and compared with those from different geographical regions. The total duration of the flash varies from 80 to 469.5 ms. The majority of the cloud flashes were of two stages: the first stage of the majority of the flashes was found to consist of large microsecond scale bipolar pulses having negative initial polarity and the late stage consists of submicrosecond scale pulses having both positive and negative initial polarities. The average durations of the first and second stages are 11.23 and 66.79 ms, respectively, and the time gap between them is 53.57 ms. The cloud pulses led by the negative initial polarity pulses are more compactly distributed and are higher in mountainous countries as compared to those of flashes lead by positive initial polarity pulses, with the average values of interpulse interval being 211.42 and 309.79 μs, respectively.

Fast Ionization Waves in Extended Capillaries Initiated by High-Voltage Pulses With Varying Rise Rates: A Numerical Investigation

This article presents the results of numerical investigations of initial stages of a fast capillary discharge—initiation and propagation of a fast ionization wave and consequent formation of a conducting plasma channel. Simulations were performed for the case of a 10 cm-long narrow capillary filled with argon at 270 Pa. The wave was created by voltage pulses of negative polarity with an amplitude of 10 kV and rise times varying in the 5–40 ns range, which is comparable with switching times of conventional switching devices based on gas discharges. Obtained spatio-temporal distributions of main discharge parameters were used to study the wave dynamics in general and the structure of the wavefront and the plasma channel. Simulations have revealed that the total wave propagation time is comparable to the voltage rise-time and varies linearly from 10 to 30 ns for the considered rise-time range. It is further shown that for longer voltage rise-times, it is possible for the wavefront to reach the anode well before the applied voltage reaches the maximum, which can potentially result in suboptimal initiation of a fast capillary discharge with lower current rise rates and less effective energy input.

Influence of voltage pulse rise-time on initiation and propagation of fast ionization waves in extended capillaries

Creating stable and efficient compact X-ray sources based on fast capillary discharges that do not incorporate preliminary ionization circuits poses additional restrictions on parameters of voltage pulses and capillary geometry. Applying a voltage pulse with a rise rate of the order of 1 kV/ns results in gradual breakdown of non-ionized gas in the capillary which takes the form of an ionization wave that initiates at the powered electrode and propagates with typical velocities of 1 cm/ns. After the wave reaches the grounded electrode, a plasma channel with gradually increasing conductivity is formed. The current onset therefore appears only after a certain time delay after beginning of the voltage pulse. The ratio between the delay and the applied voltage rise-time will eventually influence the current rise rate that defines plasma heating and compression. It is therefore necessary to have the ability to estimate this delay time for a given capillary geometry and understand its dependence on the properties of a voltage pulse. In this work numerical simulations of fast ionization waves created in an extended Al2O3 capillary filled with nitrogen at 2 Torr were performed for cases of voltage pulses of negative polarity with rise-times varying in the range 10-50 ns. The numerical model was based on fluid approach with drift-diffusion approximation for charged particle fluxes. Influence of voltage rise-time on initiation and propagation of a fast ionization wave as well as on consequent rate of current rise is investigated.

Filamentary nanosecond surface dielectric barrier discharge. Experimental comparison of the streamer-to-filament transition for positive and negative polarities

Streamer-to-filament transition is a general feature of high pressure high voltage nanosecond surface dielectric barrier discharges (nSDBDs) for mixtures containing molecular gases. The transition is observed at high pressures and voltages in a single-shot experiment a few nanoseconds after the start of the discharge. A set of experimental results comparing streamer-to-filament transition and properties of plasma in the filaments for the identical high voltage pulses of negative and positive polarity is presented. The transition curves in voltage–pressure coordinates are obtained for N2:O2 mixtures with different content of molecular oxygen, from 0% to 20%, at the pressure range 1–12 bar. Continuous optical spectra are compared for both polarities in 6 bar synthetic air. Electron density is calculated from Stark broadening of Hα line at λ = 656.3 nm in the discharge and in early afterglow, 40 nanoseconds after the end of the high voltage pulse. Hydrodynamic perturbations are measured using schlieren imaging in 1–6 bar air for streamer and filamentary mode for both polarities. The review of common and distinctive features of the filamentary single-shot nSDBD for two polarities of the applied pulse is provided.

Ultrashort pulse decomposition in reflection symmetric meander line of four cascaded half-turns

The paper considers the protection of critical electronic equipment from an ultrashort pulse (USP) by means of modal filtering technology. Decomposition of the USP in a reflection symmetric meander line consisting of four cascaded half-turns is investigated for the first time. Quasi-static simulation of two half-turn connection diagrams has been performed. The output of the circuits shows the presence of negative polarity pulses, as well as additional pulses with time delay values non-multiple of per-unit-length time delays. It has been revealed that these values are linear combinations (arithmetic means) of per-unit-length time delays of the line modes.

Synaptic Weight Modulation By Controlling Metal Oxide Rram Switching

Device technologies developed to address specific needs for neuromorphic systems include non-volatile memory (NVM), which can serve as microelectronic "synapses" for low-power mobile computing applications. Desirable memory stack characteristics include, among others, scalability, fast programming, low voltage and power operations, high retention and endurance, multi-state operation and 3D integration capability. Metal-oxide RRAM meets most of these requirements [1]; however, the critical capabilities such as symmetric and gradual conductance update, and potentiation and depression in spike-timing-dependent-plasticity scheme have not been demonstrated. In this study, we show that hafnia-based filamental RRAM devices are capable of gradual multi-level switching at ultra-fast (ps) time scale at low operation voltages (< 1V). This enables high frequency/bandwidth operations with extremely low energy switching. RRAM devices are formed by crossbar 50x50 nm MIM capacitors fabricated with an ALD polycrystalline 5nm HfO2 film and TiN electrodes. An ultra-fast pulse setup (Fig. 1) provides precise control over the power delivered to the cell and removes the need for a current compliance control (1R configuration). Resistance is shown to stepwise increase or decrease depending on voltage value and polarity (Fig. 2). To evaluate how synaptic weight can be modulated by the time between the incoming signals, we investigated dependency of the RRAM resistance on time between the consequent pulses (Fig. 3). Reducing the time interval between pulses below 2 ns results in significant amplification of their effect on resistance. It’s consistent with theoretical expectations that, by briefly increasing local temperature [2], the first pulse magnifies the structural changes inducing by the subsequent pulse. To emulate the effect of time window between the pulses from pre- and post- synaptic neurons, a pair of pulses of positive (pre-synaptic) and negative (post-synaptic) polarity was applied to the same electrode (Fig. 4). In bi-polar switching, a negative polarity pulse induces resistance change in the same direction as a positive pulse applied to the opposite electrode. Potentiation or depression were observed depending on a positive pulse preceding or following the negative one, consistent with the reported brain-inspired operations. A very short time interval between incoming signals enabling potentiation/depression processes implies extremely fast neuromorphic system operations capabilities. G. Bersuker et al., J. Comput. Electron. 16, 1085, 2017 D. M. Nminibapiel et al., EDL 38, 326, 2017 Figure 1