Initial Stage Of Breakdown Research Articles

Luminosity with large amplitude pulses after the initial breakdown stage in intracloud lightning flashes

Optical data are presented for two intracloud flashes in which there were five events having large amplitude, bipolar electric field change (E-change) pulses and strong VHF emissions. These bright events occurred 18.98–67.33 ms after the initiating event and 10–59 ms after the end of the initial breakdown stage. The three largest events were coincident with WorldWide Lightning Location Network detections of the sort previously associated with terrestrial gamma-ray flashes (TGFs); these pulses had range-normalized E-change amplitudes of 8.73, 8.33, and 3.81 V/m and estimated peak current magnitudes of 262, 250, and 114 kA. The other two events were 3.20 and 1.62 V/m (96 and 49 kA). All five events have bright enhanced luminosity (>10% above background) for durations of 0.7–1.20 ms, similar to durations of their moderate-to-strong VHF emissions. Full-frame peak intensities are factors of 1.82–4.46 times the background level. Maximum cumulative intensity in the camera frame occurs 84–98 μs after the E-change peak in three cases and ranges from −10 μs (before) to +115 μs (after) for all cases. Notably, in all five events the luminosity starts increasing when the E-change and VHF sensors begin detecting oscillations, 135–550 μs before the E-change peak. Pulse locations of each event extend through 3.6 to 4.3 km depth, starting below and ending above the altitude of flash initiation, and the linear path-length of activity with these events is estimated at 8.9–11.3 km within radar reflectivities of 20–40 dBZ. Although these events are not initial breakdown pulses, their luminosity could be visible from satellite-borne instruments and they may be associated with TGFs.

Simultaneous Observations of EIP, TGF, Elve, and Optical Lightning

AbstractOn February 8, 2019, the Atmosphere‐Space Interaction Monitor observed a terrestrial gamma‐ray flash (TGF) and an Elve from a positive intracloud (+IC) lightning during the initial breakdown stage of a lightning flash north east of Puerto Rico. A second Elve produced by the return stroke (RS) of a negative cloud‐to‐ground (−CG) lightning was observed 456 ms later about 300 km south of the first one. Radio measurements show that a short (30 μs) and large (280 kA km) energetic in‐cloud pulse (EIP) produced the electromagnetic (EM) wave for the first Elve while the RS of the −CG was the EM source for the second Elve. Assuming that the EIP and the RS were the sources of the 777 nm emissions, both the delay relative to the ultra‐violet pulse and the shape and duration of the 777 nm emissions can be explained by scattering and absorption inside the clouds. The TGF produced by the +IC lightning had the same duration as the EIP (∼30 μs). Due to the ±80 μs timing uncertainty of the TGF, we can only state that TGF was produced just before or most likely simultaneously with the EIP. The large 777 nm pulse indicates that a large part of the EIP was produced by a current flowing in a hot channel, but it is likely that the TGF current also contributed significantly to the EIP.

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.

On the Transition From Initial Leader to Stepped Leader in Negative Cloud‐to‐Ground Lightning

AbstractHigh‐speed video and electric field change data are used to describe the first 5 ms of a negative cloud‐to‐ground flash. These observations reveal an evolution in character of the luminosity and electric field change pulses as two branches of the leader separately transition from initial leader to propagating as a negative stepped leader (SL). For the first time reported, there is evidence of weak luminosity coincident with the initiating event, a weak bipolar pulse 60 μs prior to the first initial breakdown (IB) pulse. During the IB stage, the initial leader advances intermittently at intervals of 100–280 μs, in separate light bursts that are bright for a few 20‐μs frames and are time coincident with IB pulses. In the intervals between IB pulses, the initial leader is dim or invisible during the earliest 1.8 ms. Within 2 ms, the leader propagation begins transitioning to an early SL phase, in which the leader tip advances at more regular intervals of 40–80 μs during relatively dim and brief steps which are coincident with SL pulses having short duration, small amplitude, and typically unipolar waveform. These data indicate that when the entire initial leader length behind the lower end begins to remain illuminated between bursts, the propagation mode changes from IB bursts to SL steps, and the IB stage ends. The results support a hypothesis that the early initial leader development occurs in the absence of a continuously hot channel, thus the initial leader propagation is physically unlike the self‐propagating SL advance.

Evaluation of the Electric Field Strength in a Pre-Breakdown Ionization Wave in a Long Discharge Tube from the Emission Spectrum

The paper presents the results of studying the characteristics of a slow ionization wave (IW) arising at the initial stage of breakdown in a long discharge tube under reduced pressure. The discharge tube is a Philips TUV-30W mercury lamp with an electrode spacing of 80 cm and an inner diameter of 23 mm. The tube is filled with argon at a pressure of 2–4 Torr (nominal data) and mercury vapor. One of the electrodes is grounded, and, to the second one, positive or negative voltage pulses with an amplitude of 2 kV, a leading edge duration of ≈0.5 μs, and a repetition frequency of 1 Hz are applied. The IW velocity and the time dependence of the intensities of the Ar, Ar+, and Hg lines in the IW at different distances from the high-voltage electrode are measured. It is shown that the velocity of a positive IW (3 × 107–5 × 107 cm/s) is higher than the velocity of a negative IW (1 × 107–1.8 × 107 cm/s). The magnitude of the electric field in the front of an IW is evaluated from the comparison of the measured and calculated intensity ratios of the Ar, Ar+, and Hg lines. It is shown that, in a positive IW, the magnitude of the reduced electric field (260–450 Td) is noticeably larger than in a negative one (120–165 Td).

Spatio-temporal dynamics of pulsed gas breakdown in microgaps

Microscale gas breakdown plays a critical role in microplasma generation for numerous applications and device lifetime for miniaturized electronics. This communication extends a previous investigation of pulsed breakdown morphology [G. Meng et al., Phys. Plasmas 25, 082116 (2018)] by providing further insight into the spatio-temporal dynamics of pulsed gas breakdown for different gap distances using an in-situ electrical-optical measurement method. Time-resolved sequential images and the corresponding photon number distributions are obtained to demonstrate the dynamic evolution of the breakdown channel morphology and the ionization intensity during breakdown development. For a 15 μm gap, breakdown transitions from a spot area on both electrode surfaces to a broad discharge region comprised of filamentary main breakdown channel (∼2.00 μm) and surrounding weak ionization area due to the local field enhancement. For a 2 μm gap, it transitions from a thin channel (∼1.09 μm) to a wider and uniform channel (∼2.14 μm) because the electric field is more uniform at smaller gaps. Interestingly, the main breakdown channel width at the instant of breakdown is independent of the gap width. For the 2 μm gap, field emission dominates the initial stage of breakdown and collision ionization (α process) dominates during breakdown development, while the Townsend avalanche dominates the breakdown process for the 15 μm gap. We apply a simple asymptotic theory to quantify the relative contribution of these phenomena and predict that breakdown will follow Paschen's law for gaps larger than 17.8 μm.

Observation and Investigation of “Reverse Breakdown” in a Discharge Tube

A discharge operating in a 80-cm-long discharge tube with an inner diameter of 15 mm, filled with a 3 : 1 neon–argon mixture at a pressure of 1 Torr, was investigated experimentally. Square voltage pulses with a period of 1 s were supplied to one of the tube electrodes, the second electrode being ungrounded. The initial stage of breakdown—the primary breakdown between the high-voltage (active) electrode and the tube wall, accompanied by the propagation of the prebreakdown ionization wave—was the same as in the conventional scheme with a grounded low-voltage electrode. Since the discharge gap was not closed, the discharge was not ignited. An essentially new effect was observed after the end of the voltage pulse. After a certain time interval, voltage spikes of opposite polarity, the amplitude and shape of which were close to those observed during the primary breakdown, appeared in the voltage and current waveforms of the active electrode. Simultaneously, a radiation pulse from the region adjacent to the active electrode was observed and an ionization wave began to propagate toward the second electrode. This work is dedicated to investigating this effect (which was named “reverse breakdown”) and analyzing its mechanism. A conclusion is made on the similarity of this phenomenon to the processes occurring in atmospheric-pressure dielectric barrier discharges.

Pre-breakdown phenomena and discharges in a gas-liquid system

In this paper, we investigate pre-breakdown and breakdown phenomena in gas-liquid systems. Cavitation void formation and breakdown in bubbles immersed in liquids are studied numerically, while complete breakdown of bubbled water is studied in experiments. It is shown that taking into account the dependence of water dielectric constant on electric field strength plays the same important role for cavitation void appearance under the action of electrostriction forces as the voltage rise time. It is also shown that the initial stage of breakdown in deformed bubbles immersed in liquid strongly depends on spatial orientation of the bubbles relative to the external electric field. The effect of immersed microbubbles, distributed in bulk water, on breakdown time and voltage is studied experimentally. At the breakdown voltage, the slow ‘thermal’ mechanism is changed by the fast ‘streamer-leader’ showing a decrease in breakdown time by two orders of magnitude by introducing microbubbles (0.1% of volumetric gas content) into the water. In addition, the plasma channel is found to pass between nearby microbubbles, exhibiting some ‘guidance’ effect.

Study of the prebreakdown stage of a gas discharge in a diode with point cathode by laser probing

The prebreakdown stage of a gas discharge in a diode with strongly overloaded cathode is studied by laser methods (by simultaneous use of multiframe interferometry and shadow and schlieren photographing) at atmospheric pressure. The spatial resolution of the methods is about 20 μm. A probing pulse of a laser (LS-2151 Nd: YAG laser with a half amplitude duration of 70 ps and a pulse energy of up to 40 mJ) is synchronized with a voltage pulse with accuracy of about 1 ns. High field strength at the cathode is achieved due to the use of thin individual metal tips on the electrodes. It is shown that the initial stage of breakdown of a discharge gap is accompanied by the emergence of a dense plasma cloud at the end of a tip with electron density of about 5 × 1019 cm–3 with a size of tens of microns, as well as by a sharp increase in the total current through the diode. After the emergence of a dense plasma cloud at the end of a cathode tip, a similar cloud is formed on the surface of the anode; sometime later, these clouds join together and form a tubular current channel. The dynamics of the breakdown, as well as the parameters of the plasma are studied by the abovementioned techniques in three independent optical channels.

The slip extrusion test: A novel method to characterise bolus properties.

The test could be used to measure bolus properties from the initial stages of breakdown to the point of swallow for all types of food. The ability to measure the changes in bolus properties through all stages of breakdown using the same instrument is a significant development. The resistance to deformation and slip are quantitative measurements that could potentially be used to further develop the Swallow Safe model by providing numerical limits to the identified properties. This could be of interest to the development of foods for dysphagia sufferers.

Experimental studies of breakdown characteristics in pulse-modulated radio-frequency atmospheric discharge

The influences of the pulse off-time on the breakdown voltage of the first pulse and the stable pulse discharge (having repeatedly undergone a process of ignition, maintenance, and extinction) are experimentally investigated in a pulse-modulated radio-frequency atmospheric pressure argon discharge. The experimental results show that the first pulse discharge breakdown voltage decreases, but the stable pulse discharge breakdown voltage increases with increasing the pulse off-time. In a large region of the pulse off-time, the luminescence property of the initial breakdown stage is studied using a high speed camera. The captured images at different pulse off-times demonstrate that the gas breakdown exhibits five key characteristics: single-point random breakdown, multi-point random breakdown, stable uniform breakdown, stable glow mixed with pattern breakdown, and stable nonuniform pattern breakdown. The physical reasons for these results are discussed.

Luminosity in the initial breakdown stage of cloud‐to‐ground and intracloud lightning

AbstractWe present simultaneously measured luminosity and electric field data from the initial breakdown (IB) stage in seven cloud‐to‐ground (CG) and eight intracloud (IC) lightning discharges along with, in three cases, radar and Lightning Mapping Array (LMA) data. The data were taken in north‐central Florida in 2013 and 2014. Seven CG discharges had an arithmetic mean (standard deviation, SD) IB stage luminosity pulse train duration of 3 ms (2 ms), and within the seven CG discharges, 30 luminosity pulses had the following means (SD): 10% to 90% risetime 25 µs (16 µs), full width at half maximum 68 µs (21 µs), and delay between onset of the electric field pulse and associated luminosity pulse 8 µs (8 µs). Eight IC discharges had a mean (SD) IB stage luminosity pulse train duration of 11 ms (4 ms), and within the 8 IC discharges, 37 luminosity pulses exhibited mean risetimes, widths, and delays of 59 µs (36 µs), 176 µs (70 µs), and 34 µs (20 µs), all significantly greater than in the CG case. The roughly 10 LMA sources associated with each of the three IB stages in 2014 are grouped horizontally within about 1 km2. The mean altitude (SD) of the LMA points during two CG IB stages is 5 km (600 m) and 4.3 km (250 m) and during one IC discharge is 6.2 km (550 m). We discuss the role of optical scattering in delaying and distorting the observed luminosity waveforms.

Zero-Dimensional Theoretical Model of Subnanosecond High-Pressure Gas Discharge

This paper deals with the results of the breakdown process simulation in strongly overvoltage gaps under high pressures. The presented 0-D model of discharge enables the estimation of characteristic parameters of the initial stage of the breakdown: current rise rate, time of voltage decay across the gap, and current pulse duration of runaway electrons. The discharge model takes into account 0-D growth kinetics of the plasma density in the gap and the impact of the external power supply circuit of the discharge, including the interelectrode capacitance. The model enables estimation of the number and energy range of runaway electrons generated at the initial stage of high-pressure gas breakdown. As an example, computations were conducted for discharge in nitrogen. A comparison of the simulation results with the experimental data enables estimation of the level of the critical field in which we can expect the generation of runaway electrons in the gas discharge.

Leader observations during the initial breakdown stage of a lightning flash

AbstractThis paper describes luminosity and leader propagation during the initial breakdown (IB) stage of a cloud‐to‐ground flash, beginning at 6.06 km altitude and 31.86 ms before the return stroke (RS). High‐speed video (50,000 frames per s) and time‐correlated electric field change (E‐change) data show multiple branch ends advance concurrently in the first 6 ms of the flash; each branch begins with IB pulses and propagates first via bursts as an initial leader. Burst luminosity (pixel intensity) is directly related to IB pulse amplitude. Some initial leader branches transition to advancing as stepped leaders after a few milliseconds. Each initial leader branch end makes the transition to a stepped leader branch end at a different time, resulting in a complex E‐change waveform including relatively narrow step‐type pulses during the IB stage and no apparent Intermediate stage prior to the Leader stage. There is no visible evidence of an upward propagating leader end prior to the RS nor of any light above the earliest visible IB luminosity prior to or during the RS. During the RS, the topmost visible portion of the channel that developed as an initial leader (above 5.1 km) behaves differently from the channel below, indicating it is less conductive. Radar and time‐of‐arrival lightning source data indicate that the IB luminosity visible to the camera comes from about 6 km inside the thundercloud echo. The 1276 m long initial leader transitions to a stepped leader at 4.9–5.0 km, near the altitude of the radar bright band.

Initial stage of breakdown of a point-plane gap filled with high-pressure nitrogen and SF6

The initial stage of breakdown of sulfur hexafluoride (SF6) and nitrogen in a nonuniform electric field at high pressures is studied. High voltage pulses with amplitudes of up to 350 kV have been applied across the discharge gap with the “point-plane” geometry of the electrodes. Experimental results on the dynamics of light emission from different zones of the discharge gap can be explained by generation of an ionization wave, which started from a potential electrode with a small radius of curvature. It is found that the speed of the ionization wave front in nitrogen and SF6 is higher in the second half of the discharge gap as compared to the first one. It is shown that the speed of ionization wave front decreases as SF6 and nitrogen pressure increases. The average speed of ionization wave front is shown to be ∼2 cm/ns in SF6 under a pressure of 0.25 MPa and ∼3.6 cm/ns in nitrogen under a pressure of 0.3 MPa under the negative polarity of high-voltage pulses when bridging a discharge gap of about 13 mm.

Shadowgraph of Pulse CO<sub>2</sub> Laser Induced Breakdown in Different Pressure Air

The expansion property of an infrared CO2 laser produced air plasma is characterized using a high-speed imaging shadowgraph technique. The shadowgraphs were taken by a time-gated intensified charge-coupled device at various delay times after single pulses induced gas breakdown. We examined five incident laser energy of 180, 240, 345, 420 and 600 mJ induced air breakdown at the pressure of atmospheric and 104 Pa. A shock wave produced by laser induced breakdown was also observed and its speed was measured as a function delay time between the breakdown and the shadow imaging under different air pressure. The experimental results indicated that the radial and axial shock wave front evolutions showed similar behavior, which increased fast with delay time at early stage and slowly at later stage. The propagation speed of the wavefront was about 2 cm/μs at the initial stage of breakdown, and then decreased very quickly. The propagation speed under low air pressure was higher than that of gases under high pressure and the spark sustained less time at lower pressure. The size of laser induced air spark increased with incident laser energy but not simple linear relationships.

Initial breakdown pulses in intracloud lightning flashes and their relation to terrestrial gamma ray flashes

AbstractThe initial breakdown stage of 10 intracloud lightning flashes that may have produced terrestrial gamma ray flashes (TGFs) is studied with wideband E‐change, multiband B‐change, and VHF lightning mapping data; these flashes fit published criteria known to be associated with TGFs. The (x, y, z, t) locations of fast initial breakdown pulses (IBPs) were determined with E‐change data using a time‐of‐arrival (TOA) technique. Each IBP includes one or more fast‐rising subpulses. Previous research has shown that a typical intracloud flash initiates just above the main negative cloud charge (MNCC), then an initial negative leader propagates upward in 1–20 ms to the bottom of the upper positive cloud charge (UPCC), thereby establishing a conducting path between the MNCC and UPCC. TOA locations indicate that IBPs are directly related to the initial negative leader. The IBPs primarily occur in short (<750 µs) bursts of two to five pulses, and each burst produces a slow, monotonic E‐change. Typically, one to three IBP bursts are needed to span the vertical gap from the MNCC to the UPCC, with successive bursts separated by 1–5 ms. In the B‐change data, each IBP burst has an associated ULF pulse and several LF pulses, and these are caused by the same physical events that produce the slow, monotonic E‐change and fast‐rising IBP subpulses, respectively. Based on similarities with known TGF‐associated signals, we speculate that a relativistic electron avalanche causes each LF pulse/IBP subpulse pair; thus, each pair has the potential to cause a TGF.

Luminosity of initial breakdown in lightning

Time correlated high‐speed video and electromagnetic data for 15 cloud‐to‐ground and intracloud lightning flashes reveal bursts of light, bright enough to be seen through intervening cloud, during the initial breakdown (IB) stage and within the first 3 ms after flash initiation. Each sudden increase in luminosity is coincident with a CG type (12 cases) or an IC type (3 cases) IB pulse in fast electric field change records. The E‐change data for 217 flashes indicate that all CG and IC flashes have IB pulses. The luminosity bursts of 14 negative CG flashes occur 11–340 ms before the first return stroke, at altitudes of 4–8 km, and at 4–41 km range from the camera. In seven cases, linear segments visibly advance away from the first light burst for 55–200 µs, then the entire length dims, then the luminosity sequence repeats along the same path. These visible initial leaders or streamers lengthen intermittently to about 300–1500 m. Their estimated 2‐D speeds are 4–18 × 105 m s−1 over the first few hundred microseconds and decrease by about 50% over the first 2 ms. In other cases, only a bright spot or a broad area of diffuse light, presumably scattered by intervening cloud, is visible. The bright area grows larger over 20–60 µs before the luminosity fades in about 100 µs, then this sequence may repeat several times. In several flashes, a 1–2 ms period of little or no luminosity and small E‐change is observed following the IB stage prior to stepped leader development.

Reactive Molecular Dynamics Study of Chloride Ion Interaction with Copper Oxide Surfaces in Aqueous Media

Using reactive force-field (ReaxFF) and molecular dynamics simulation, we study atomistic scale chloride ion adsorption and transport through copper oxide thin films under aqueous conditions. The surface condition of passive oxide film plays a key role in chloride ion adsorption and facilitates initial adsorption when surface corrosion resistance is low. Using implemented surface defects, the structural evolution of the copper oxide film from thinning to breakdown is investigated. In addition to chemical thinning of passive film, extended defects in the metal substrate are observed, at high concentration of adsorbed chloride ions. The initial stage of breakdown is associated with rapid depletion of adjacent chloride ions, which creates a locally deficient environment of chloride ions in the solution. The dissolved copper cations gain higher charge upon interaction with chloride ions. Owing to the increased Coulomb interactions resulted from dissolved copper ions and locally low density of chloride ions, far-field chloride ions would diffuse into the local corrosion sites, thereby promoting further corrosion.

On the plasma mechanism of developing the early stages of breakdown in gases

The time evolution of the initial stages of breakdown in high-pressure gases is theoretically investigated. The following three stages are considered: initial: the origination and development of an ionization avalanche, the change of this avalanche to a plasma avalanche, and the change of the plasma avalanche to a plasma streamer. The streamer ionizes the gas via its radiation, causes new avalanches propagating in an enhanced discharge field, and bridges the discharge gap. Simple formulas are derived for the times of transition between various breakdown stages and for the ionization front velocity at separate stages.