Study of the formation and propagation of a leader channel in air

Results are presented from calculations of the initial stage of leader channel formation in air. It is shown that the channel forms in two stages: one occurring at an essentially constant gas density on time scales much shorter than the characteristic gas-dynamic time and another in which gas-dynamic rarefaction of the channel becomes important and thermal-ionizational instability develops. The leader propagation velocity is largely determined by the time of channel contraction. The dependence of the leader velocity on the current and initial pressure is calculated. The results of calculations agree with experiment for relatively low leader currents (I _L = 0.5–5 A) and for leader currents above tens of amperes. A comparative analysis of the leader propagation velocity in air at different initial pressures is given. In the current range I _L = 0.5–30 A, the leader velocity depends weakly on the pressure; however, for currents as high as I _L ≥ 50 A, it increases appreciably with pressure.     

Experimental study of the processes accompanying argon breakdown in a long discharge tube at a reduced pressure

Results are presented from experimental studies of the breakdown stage of a low-pressure discharge (1 and 5 Torr) in a glass tube the length of which (75 cm) is much larger than its diameter (2.8 cm). Breakdowns occurred under the action of positive voltage pulses with an amplitude of up to 9.4 kV and a characteristic rise time of 2–50 μs. The discharge current in the steady-state mode was 10–120 mA. The electrode voltage, discharge current, and radiation from the discharge gap were detected simultaneously. The dynamic breakdown voltage was measured, the prebreakdown ionization wave was recorded, and its velocity was determined. The dependence of the discharge parameters on the time interval between voltage pulses (the socalled “memory effect”) was analyzed. The memory effect manifests itself in a decrease or an increase in the breakdown voltage and a substantial decrease in its statistical scatter. The time interval between pulses in this case can reach 0.5 s. The effect of illumination of the discharge tube with a light source on the breakdown was studied. It is found that the irradiation of the anode region of the tube by radiation with wavelengths of ≤500 nm substantially reduces the dynamic breakdown voltage. Qualitative explanations of the obtained results are offered.     

Hydrodynamic model of a dielectric-barrier discharge in pure chlorine

A one-dimensional hydrodynamic model of a dielectric-barrier discharge (DBD) in pure chlorine is developed, and the properties of the discharge are modeled. The discharge is excited in an 8-mm-long discharge gap between 2-mm-thick dielectric quartz layers covering metal electrodes. The DBD spatiotemporal characteristics at gas pressures of 15–100 Torr are modeled for the case in which a 100-kHz harmonic voltage with an amplitude of 8 kV is applied to the electrodes. The average power density deposited in the discharge over one voltage period is 2.5–5.8 W/cm³. It is shown that ions and electrons absorb about 95 and 5% of the discharge power, respectively. In this case, from 67 to 97% of the power absorbed by electrons is spent on the dissociation and ionization of Cl₂ molecules. Two phases can be distinguished in the discharge dynamics: the active (multispike) phase, which follows the breakdown of the discharge gap, and the passive phase. The active phase is characterized by the presence of multiple current spikes, a relatively high current, small surface charge density on the dielectrics, and large voltage drop across the discharge gap. The passive phase (with no current spikes) is characterized by a low current, large surface charge density on the dielectrics, and small voltage drop across the discharge gap. The peak current density in the spikes at all pressures is about 4 mA/cm². In the multispike phase, there are distinct space charge sheaths with thicknesses of 1.5–1.8 mm and a mean electron energy of 4.3–7 eV and the central region of quasineutral plasma with a weak electric field and a mean electron energy of 0.8–3 eV. The degree of ionization of chlorine molecules in the discharge is ~0.02% at a pressure of 15 Torr and ~0.01% at 100 Torr. The DBD plasma is electronegative due to the fast attachment of electrons to chlorine atoms: e + Cl₂ → Cl + Cl^–. The most abundant charged particles are Cl ₂ ⁺ and Cl^? ions, and the degree of ionization during current spikes in the active phase is (4.1–5.5) × 10^–7. The mechanism of discharge sustainment is analyzed. The appearance of a series of current spikes in the active phase of the discharge is explained.     

Dynamics of the optical emission from a high-voltage diffuse discharge in a rod-plane electrode system in atmospheric-pressure air

Results are presented from experimental investigations of the dynamics of optical emission from a nanosecond diffuse discharge in a rod-plane electrode system. A study was made of discharges in a 10-cm-long interelectrode gap in atmospheric-pressure air (the cathode being a 1-cm-diameter rod with a bullet-shaped end). The voltage across the discharge gap was 220 kV and the voltage pulse duration was 180 ns, the voltage rise time being 10 ns. In experiments, the discharges were observed to evolve through two stages: the bridging stage and the conduction stage. The bridging stage begins with intense optical emission from the cathode region, the onset of the emission being delayed with respect to the beginning of the voltage pulse. Simultaneously with the onset of optical emission, a displacement current corresponding to the motion of charged particles begins to be generated in the cathode region. The duration of this current corresponds to the time the emission front takes to bridge the gap. As the emission front reaches the anode region, the current increases abruptly, indicating the beginning of the conduction stage. It was found that the time delay of optical emission relative to the beginning of the voltage pulse largely governs the discharge parameters: as the time delay becomes longer, the emission front velocity in the bridging stage increases from 0.6 to 1.5 cm/ns, the probability of realizing a multichannel structure of the discharge becomes higher, and the discharge current and the intensity of X-ray emission from the discharge grow.     

Numerical simulations of spark channels propagating along the ground surface: Comparison with high-current experiment

A numerical model of a spark discharge propagating along the ground surface from the point at which an ∼100-kA current pulse is input into the ground has been developed based on experiments in which the velocity of a long leader was measured as a function of the leader current. The results of numerical simulations are in good agreement with the measured characteristics of creeping discharges excited in field experiments by using a high-power explosive magnetic generator. The reason why the length of a spark discharge depends weakly on the number of simultaneously developing channels is found. Analysis of the influence of the temporal characteristics of the current pulse on the parameters of the creeping spark discharge shows that actual lighting may exhibit similar behavior.     

Radial Distribution of the Nanosecond Dielectric Barrier Discharge Current in Atmospheric-Pressure Air

Experimental results on the radial distribution of the nanosecond dielectric barrier discharge (DBD) current in flat millimeter air gaps under atmospheric pressure and natural humidity of 40–60% at a voltage rise rate at the electrodes of 250 V/ns are presented. The time delay of the appearance of discharge currents was observed to increase from the center to the periphery of the air gap at discharge gap heights above 3 mm, which correlated with the appearance of constricted channels against the background of the volume DBD plasma. Based on the criterion of the avalanche-streamer transition, it is found out that the development of a nanosecond DBD in air gaps of 1–3 mm occurs by the streamer mechanism.     

Development of Discharge in a Saline Solution at Near-Threshold Voltages

The development of a discharge in a point?plane gap filled with a saline solution with a salt content of 3% was studied experimentally. The duration of the voltage pulse applied to the gap was about 2 ms. Data are presented on the formation dynamics of gas microcavities at near-threshold voltages at which gas-discharge plasma appears in some microcavities. The cavities are conglomerates of microbubbles with a typical size of ≈100 μm. At the threshold voltage (≈750 V), the active electrode is covered with a gas layer and the gap voltage is in fact applied to this layer, which leads to the development of discharges in individual microbubbles. In this case, the discharge operates in the form of short current pulses. The number of microcavities filled with plasma increases as the voltage grows above the threshold value. At the plasma boundary, new microbubbles are formed, in which discharges are ignited. As a result, the plasma front propagates from the active electrode into the gap with a characteristic velocity of 10³ cm/s.     

Influence of a Nitrogen Admixture on the Anomalous Memory Effect in the Breakdown of Low-Pressure Argon in a Long Discharge Tube

The memory effect (the dependence of the dynamic breakdown voltage U _b on the time interval τ between voltage pulses) in pulse-periodic discharges in pure argon and the Ar + 1%N₂ mixture was studied experimentally. The discharge was ignited in a 2.8-cm-diameter tube with an interelectrode distance of 75 cm. The measurements were performed at gas pressures of P = 1, 2, and 5 Torr and discharge currents in a steady stage of the discharge of I = 20 and 56 mA. Breakdown was produced by applying positive-polarity voltage pulses, the time interval between pulses being in the range of τ = 0.5–40 ms. In this range of τ values, a local maximum (the anomalous memory effect) was observed in the dependence U _b (τ). It is shown that addition of nitrogen to argon substantially narrows the range of τ values at which this effect takes place. To analyze the measurement results, the plasma parameters in a steady-state discharge (in both pure argon and the Ar + 1%N₂ mixture) and its afterglow were calculated for the given experimental conditions. Analysis of the experimental data shows that the influence of the nitrogen admixture on the shape of the dependence U _b (τ) is, to a large extent, caused by the change in the decay rate of the argon afterglow plasma in the presence of a nitrogen admixture.     

Drift model of the cathode region of a glow discharge

A one-dimensional drift model of the cathode region of a glow discharge with allowance for both electron-impact ionization and charged particle loss is proposed. An exact solution to the model equations is obtained for the case of similar power-law dependences of the ion and electron drift velocities on the electric field strength. It is shown that, even in the drift approximation, a relatively wide transition layer in which the ion-to-electron current ratio approaches a constant value typical of the positive column of a glow discharge should occur between the thin space-charge sheath and the quasineutral plasma, the voltage drop across the space-charge sheath being comparable to that across the transition layer. The calculated parameters of the normal and anomalous glow discharges are in good agreement with available experimental data.     

Phase resetting of the rhythmic activity of embryonic heart cell aggregates. Experiment and theory.

Injection of a current pulse of brief duration into an aggregate of spontaneously beating chick embryonic heart cells resets the phase of the activity by either advancing or delaying the time of occurrence of the spontaneous beat subsequent to current injection. This effect depends upon the polarity, amplitude, and duration of the current pulse, as well as on the time of injection of the pulse. The transition from prolongation to shortening of the interbeat interval appears experimentally to be discontinuous for some stimulus conditions. These observations are analyzed by numerical investigation of a model of the ionic currents that underlie spontaneous activity in these preparations. The model consists of: Ix, which underlies the repolarization phase of the action potential, IK2, a time-dependent potassium ion pacemaker current, Ibg, a background or time-independent current, and INa, an inward sodium ion current that underlies the upstroke of the action potential. The steady state amplitude of the sum of these currents is an N-shaped function of potential. Slight shifts in the position of this current-voltage relation along the current axis can produce either one, two, or three intersections with the voltage axis. The number of these equilibrium points and the voltage dependence of INa contribute to apparent discontinuities of phase resetting. A current-voltage relation with three equilibrium points has a saddle point in the pacemaker voltage range. Certain combinations of current-pulse parameters and timing of injection can shift the state point near this saddle point and lead to an interbeat interval that is unbounded . Activation of INa is steeply voltage dependent. This results in apparently discontinuous phase resetting behavior for sufficiently large pulse amplitudes regardless of the number of equilibrium points. However, phase resetting is fundamentally a continuous function of the time of pulse injection for these conditions. These results demonstrate the ionic basis of phase resetting and provide a framework for topological analysis of this phenomenon in chick embryonic heart cell aggregates.     

Pulsed submicrosecond multichannel sliding discharges of opposite polarities: Filling of the discharge gap with spark channels

Results are presented from measurements of the discharge current and the factor of the discharge gap filling with spark channels during pulsed sliding discharges of opposite polarities in Ne, Ar, and Xe on an aluminum oxide ceramic surface. The measurements were performed in the regime of single pulses of submi-crosecond duration at discharge voltages of 0?C12 kV with two discharge chambers with different thicknesses of the ceramic plate (0.4 and 0.17 cm) and different electrode gap lengths (4 and 10.3 cm) at gas pressures of 30 and 100 kPa. The results obtained for discharges of opposite polarities are compared with one another, and common features of discharges in three gases are revealed. It is shown that the filling of the discharge gap with spark channels in the gases under study is more efficient in the case of the positive polarity of the discharge voltage, except Xe at a pressure of 100 kPa in the electrode gap of length 10.3 cm. The quasi-homogeneous regime of discharge in each of the three gases is attained easier at lower gas pressures. Comparison of the data on the filling factors of the discharge gap and the peak currents of opposite-polarity discharges for each gas at a given pressure indicates that the higher the discharge current, the more densely the discharge gap is filled with spark channels.     

Effect of the vibrational excitation of CO molecules on the parameters of an RF discharge

A one-dimensional model of an RF discharge in CO-containing gas mixtures is developed. The model takes into account the effect of the degree of vibrational excitation of CO molecules on the structure of the discharge and on its parameters. Experimental data are presented from measurements of the voltage-power characteristics of RF discharges in gas mixtures with different CO contents in the pressure range of 10–100 torr. The model developed is used to calculate the dependence of the root-mean-square discharge voltage on the specific power deposition in an RF discharge under our experimental conditions. The experimental data are compared to the results of numerical simulations. For working gas pressures of about 100 torr, which are typical of the operation of slab CO lasers, the calculated voltage-power characteristics of an RF discharge agree satisfactorily with those obtained experimentally. The theoretical model predicts that the vibrational excitation of CO molecules leads to a redistribution of the RF field in the discharge gap and to an increase in the laser efficiency.     

The voltage-activated hydrogen ion conductance in rat alveolar epithelial cells is determined by the pH gradient

Voltage-activated H+ currents were studied in rat alveolar epithelial cells using tight-seal whole-cell voltage clamp recording and highly buffered, EGTA-containing solutions. Under these conditions, the tail current reversal potential, Vrev, was close to the Nernst potential, EH, varying 52 mV/U pH over four delta pH units (delta pH = pHo - pHi). This result indicates that H+ channels are extremely selective, PH/PTMA > 10(7), and that both internal and external pH, pHi, and pHo, were well controlled. The H+ current amplitude was practically constant at any fixed delta pH, in spite of up to 100-fold symmetrical changes in H+ concentration. Thus, the rate-limiting step in H+ permeation is pH independent, must be localized to the channel (entry, permeation, or exit), and is not bulk diffusion limitation. The instantaneous current- voltage relationship exhibited distinct outward rectification at symmetrical pH, suggesting asymmetry in the permeation pathway. Sigmoid activation kinetics and biexponential decay of tail currents near threshold potentials indicate that H+ channels pass through at least two closed states before opening. The steady state H+ conductance, gH, as well as activation and deactivation kinetic parameters were all shifted along the voltage axis by approximately 40 mV/U pH by changes in pHi or pHo, with the exception of the fast component of tail currents which was shifted less if at all. The threshold potential at which H+ currents were detectably activated can be described empirically as approximately 20-40(pHo-pHi) mV. If internal and external protons regulate the voltage dependence of gH gating at separate sites, then they must be equally effective. A simpler interpretation is that gating is controlled by the pH gradient, delta pH. We propose a simple general model to account for the observed delta pH dependence. Protonation at an externally accessible site stabilizes the closed channel conformation. Deprotonation of this site permits a conformational change resulting in the appearance of a protonation site, possibly the same one, which is accessible via the internal solution. Protonation of the internal site stabilizes the open conformation of the channel. In summary, within the physiological range of pH, the voltage dependence of H+ channel gating depends on delta pH and not on the absolute pH.     

Limiting Parameters of the Plasma Opening Switch

Implementing programs for nuclear fusion research and X-ray generation requires the creation of superpower generators based on plasma opening switches (POSs) capable of commutating currents as high as several tens of megaamperes at output voltages of up to 5 MV and higher. The physical mechanisms limiting the POS voltage are investigated. It is shown that, as the generator voltage U _g increases, the voltage multiplication factor k = U_POS/U_g (where U_POS is the POS voltage) decreases. An explanation for such a dependence is proposed, and the maximum value of the POS voltage is estimated. A POS design that enables operating in the above current and voltage ranges is considered. The design is based on applying an external magnetic field to the POS interelectrode gap, increasing the initial generator voltage, and decreasing the linear (along the perimeter of the outer electrode) density of the charge passing through the POS during the conduction phase.     

A theoretical study on the sucrose gap technique as applied to multicellular muscle preparations. II. Methodical errors in the determination of outward currents.

This paper presents a mathematical model for analyzing systematic errors associated with the membrane conductance of multicellular muscle preparations as determined in a sucrose gap apparatus. The errors arise because of the interdiffusion of sucrose and saline in the interstitial fluid spaces, which results (a) in spatial variations of equilibrium potentials, membrane conductance, and solution conductivity, and (b) in the existence of a liquid junction potential. The model was applied to simulate the measurement of outward currents predominantly carried by potassium ions; time variations were not considered. Output current/voltage (I/V) curves were computed and compared with the membrane I/V relationship used in the computation. The output curves look very much like experimental results but are distorted considerably from the membrane I/V relationship: (a) under favorable conditions (negligible shunt current), the membrane current is overestimated over the entire range of membrane potential, (b) regions with negative slope conductance of I/V relations with "anomalous rectifier" properties are found to be less pronounced or even absent, and (c) resting potentials may be either increased or reduced. The origin of these errors is related to currents emerging from the sucrose compartment (local circuit as well as externally applied currents). Their dependence on several experimental parameters is discussed.     

Mechanism of Ba2+ block of M-like K channels of rod photoreceptors of tiger salamanders

IKx is a voltage-dependent K+ current in the inner segment of rod photoreceptors that shows many similarities to M-current. The depression of IKx by external Ba2+ was studied with whole-cell voltage clamp. Ba2+ reduced the conductance and voltage sensitivity of IKx tail currents and shifted the voltage range over which they appeared to more positive potentials. These effects showed different sensitivities to Ba2+: conductance was the least sensitive (K0.5 = 7.6 mM), voltage dependence intermediate (K0.5 = 2.4 mM) and voltage sensitivity the most sensitive (K0.5 = 0.2 mM). Ca2+, Co2+, Mn2+, Sr2+, and Zn2+ did not have actions comparable to Ba2+ on the voltage dependence or the voltage sensitivity of IKx tail currents. In high K+ (100 mM), the voltage range of activation of IKx was shifted 20 mV negative, as was the tau-voltage relation. High K+ did not prevent the effect of Ba2+ on conductance, but abolished its ability to affect voltage dependence and voltage sensitivity. Ba2+ also altered the apparent time-course of activation and deactivation of IKx. Low Ba2+ (0.2 mM) slowed both deactivation and activation, with most effect on deactivation; at higher concentrations (1-25 mM), deactivation and activation time courses were equally affected, and at the highest concentrations, 5 and 25 mM Ba2+, the time course became faster than control. Rapid application of 5 mM Ba2+ suggested that the time dependent currents in Ba2+ reflect in part the slow voltage-dependent block and unblock of IKx channels by Ba2+. This blocking action of Ba2+ was steeply voltage- dependent with an apparent electrical distance of 1.07. Ba2+ appears to interact with IKx channels at multiple sites. A model which assumes that Ba2+ has a voltage-independent and a voltage-dependent blocking action on open or closed IKx channels reproduced many aspects of the data; the voltage-dependent component could account for both the Ba(2+)- induced shift in voltage dependence and reduction in voltage sensitivity of IKx tail currents.     

Beam discharge excited by distributed virtual cathode

A new type of beam discharge, i.e., beam discharge with a distributed virtual cathode (VC) is proposed and considered by numerical simulation. The discharge is established during counter motion of high-current electron beams in a gas-filled equipotential cavity and is characterized by a state of hot dense electron plasma of primary electrons. The discharge temporal dynamics is studied. It is shown that the VC lifetime depends linearly from this sum in a wide range of the sum of beam currents, from the boundary current of two-beam instability to the critical current of Pierce instability. Generation of nonlinear electrostatic structures shaped as phase bubbles in the discharge is detected, and their dynamics is studied. The parameters are determined, at which the multiple coexistence of phase bubbles and their coalescence during collisions is observed.     

Abnormal and subnormal regimes of a spherical glow discharge in the drift-diffusion approximation

A spherical glow discharge with a pointlike anode is considered in a self-consistent drift-diffusion approximation. The model includes the time-dependent continuity equations for ions and electrons in the drift-diffusion approximation and Poisson’s equation for the radial electric field. In finding steady-state distributions, Ohm’s law is used to relate the discharge voltage and discharge current. Steady-state distributions of the plasma parameters across the discharge gap, current-voltage characteristics, and cathode characteristics for an abnormal spherical discharge in molecular nitrogen are obtained. In a subnormal glow-discharge regime, oscillations in the conduction current, potential, and other discharge parameters are revealed. Similar regimes are also observed in conventional discharges in tubes.     

Effect of the scheme of plasmachemical processes on the calculated characteristics of a barrier discharge in xenon

The dynamics of a repetitive barrier discharge in xenon at a pressure of 400 Torr is simulated using a one-dimensional drift-diffusion model. The thicknesses of identical barriers with a dielectric constant of 4 are 2 mm, and the gap length is 4 mm. The discharge is fed with an 8-kV ac voltage at a frequency of 25 or 50 kHz. The development of the ionization wave and the breakdown and afterglow phases of a barrier discharge are analyzed using two different kinetic schemes of elementary processes in a xenon plasma. It is shown that the calculated waveforms of the discharge voltage and current, the instant of breakdown, and the number of breakdowns per voltage half-period depend substantially on the properties of the kinetic scheme of plasmachemical processes.     

Effect of the Air Flow Velocity on the Characteristics of a Pulsating Discharge Produced by a DC Power Source

Results are presented from experimental studies of a pulsating discharge produced by a dc power source in subsonic and supersonic cold (T = 150–300 K) air flows at static air pressures in the flow of 40–760 Torr. Two modes of pulsating discharge were implemented experimentally: without and with (from one to five) intermediate breakdowns. The discharge pulsation frequency, the maximum attainable voltage across the discharge gap, the length of the plasma channel, and the electric field in the discharge plasma were studied as functions of the air flow velocity and discharge current.