Formation of Wide Streamers during a Subnanosecond Discharge in Atmospheric-Pressure Air

Results are presented from experimental and computational studies of a subnanosecond breakdown of atmospheric-pressure air in a nonuniform electric field. It is shown that the ionization waves (streamers) formed in the prebreakdown stage have a nearly spherical or conical shape. The diameter of the streamer in its widest part is found to increase with increasing voltage and discharge gap length. For a rise time of the voltage pulse of ≈0.5 ns and its amplitude of ≈250 kV, streamers about 8 cm in diameter were observed in a 7-cm-long gap.     

Memory effect in the ignition of a low-pressure glow discharge in nitrogen in a long discharge tube

Results are presented from experimental studies of breakdown and the initial stage of a discharge in a long tube (with an interelectrode distance of 40 cm and a diameter of 2.8 cm) at a pressure of ～1 Torr and pulse discharge current of ～10 mA. Breakdown was produced by positive voltage pulses with a linearly growing leading edge with a steepness of dU/dt ～ 10⁶–10⁸ V/s. The time interval between pulses was varied from τ = 0.5 ms to 1 s, the pulse duration being 10 ms. The work was aimed at studying the memory effect of the discharge gap, namely, the influence of the previous pulse on the breakdown characteristics of the next one. In the experiments, the breakdown voltage was measured at different values of dU/dt and τ. It was found that the memory effect was absent at τ ～ 1 s. At the same time, an increase in the breakdown voltage with increasing dU/dt was observed. In the range of τ ≈ 50–200 ms, the breakdown voltage also did not depend on τ, but the memory effect took place. The memory effect in this case consisted in that the breakdown voltage decreased with increasing dU/dt, so that, at dU/dt ～ 10⁷ V/s, the breakdown voltage was two times lower than in the case of τ ～ 1 s. For τ ～ 1–10 ms, the memory effect manifested itself in that the breakdown voltage depended on τ: it could either decrease (the “normal” effect) or increase (the “anomalous” effect) with increasing τ. Breakdown of the discharge gap was preceded by the propagation of an ionization wave, except for the case of small τ values in the domain of existence of the anomalous effect. Estimates allowing one to qualitatively explain the experimental results are made.     

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.     

Current-to-voltage convertor for measurement of oxygen

A highly sensitive, miniature, inexpensive circuit for the measurement of PO2 in vivo has been described. The circuit is constructed from a current-to-voltage convertor, clamping circuit, differential amplifier, and reverse voltage and overvoltage protector. The design of the circuit allows us to apply voltage bias to the measuring electrode while grounding the preparation. The clamping circuit holds the selected bias voltage constant while the differential amplifier subtracts this bias potential from the PO2 signal yielding an output voltage that is proportional to the current sensed by the oxygen electrode. The circuit is protected from reverse voltage and overvoltage.     

Initiation of Positive Streamers near Uncharged Ice Hydrometeors in the Thundercloud Field

Since the threshold electric field required for breakdown of air is much higher than the maximum field strength measured in thunderstorm clouds, the problem of lightning initiation still remains unsolved. According to the popular hypothesis, lightning can be initiated by a streamer discharge in the field enhanced near a hydrometeor. To verify the adequacy of this hypothesis, the development of a positive streamer propagating along the thunderstorm electric field in the vicinity of an ice needle at an air pressure corresponding to an altitude of 5 km (which is typical of the lightning initiation conditions) was simulated numerically. The hydrometeor dimensions are determined at which streamers can be initiated at different strengths of the thunderstorm electric field.     

Two-component structure of the current pulse of a ranaway electron beam generated during electric breakdown of elevated-pressure nitrogen

Conditions are investigated at which two current pulses of ranaway electron beams are generated in elevated-pressure nitrogen during one voltage pulse. It is shown that the regime with two runaway electron beam current pulses takes place at decreased values of the electric field strength E in the gap (or decreased values of the parameter E/p, where p is the gas pressure). The regime with two runaway electron beam current pulses is observed both at high (1500?C3000 Torr) and low (below 100 Torr) pressures. It is shown that, for the second runaway electron beam current pulse to form, the voltage across the gap should be partially reduced during the first pulse. At low nitrogen pressures (~10 Torr), the regime in which two runaway electron beams are generated can be implemented by increasing the breakdown strength of the gap and/or increasing the value of E/p. In experiments carried out in atmospheric-pressure air with a picosecond time resolution, a rather complicated structure of the beam current pulse is observed at a voltage rise time of ~300 ps.     

Subnanosecond processes in the stage of breakdown formation in gas at a high pressure

Results are presented from experimental studies of the prebreakdown stage of a discharge in nitrogen at pressures of a few tens of atmospheres, gap voltages higher than 140 kV, and a voltage rise time of about 1 ns. Breakdown occurs at the front of the voltage pulse; i.e., the time of breakdown formation is shorter than the front duration. It is shown that, in gaps with a nonuniform electric field, the breakdown formation time is mainly determined by the time of avalanche development to the critical number of charge carriers. The subsequent stages of breakdown (the development of the ionization wave and the buildup of the conductivity in the weakly conducting channel bridging the gap) turn out to be shorter than this time or comparable to it.     

Specific features of a single-pulse sliding discharge in neon near the threshold for spark breakdown

Experimental data on the spatial structure of a single-pulse sliding discharge in neon at voltages below, equal to, and above the threshold for spark breakdown are discussed. The experiments were carried at gas pressures of 30 and 100 kPa and different polarities of the discharge voltage. Photographs of the plasma structure in two discharge chambers with different dimensions of the discharge zone and different thicknesses of an alumina dielectric plate on the surface of which the discharge develops are inspected. Common features of the prebreakdown discharge and its specific features depending on the voltage polarity and gas pressure are analyzed. It is shown that, at voltages below the threshold for spark breakdown, a low-current glow discharge with cathode and anode spots develops in the electrode gap. Above the breakdown threshold, regardless of the voltage polarity, spark channels directed from the cathode to the anode develop against the background of a low-current discharge.     

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.     

Alternating nonsteady gas-discharge modes in an atmospheric-pressure air flow blown through a point-plane gap

The electric and spectral characteristics of a nonsteady discharge in an atmospheric air flow blown through a point-plane interelectrode gap were investigated experimentally. The discharge was produced by applying a constant positive voltage to the point electrode, the amplitude of the applied voltage being much higher than the corona ignition voltage. The nonsteady character of the discharge is due to the spontaneously repeating streamer-spark breakdown, followed by the formation of either a diffuse ultracorona or a filamentary glow discharge. In the latter case, the length of the plasma column increases progressively, being blown off by the gas flow from the discharge gap. The extinction of a filamentary discharge is unrelated to the break of the current channel: the discharge decays abruptly when the filament length reaches its critical value. The distribution of active particles (O, OH, and N*₂) carried out from the discharge gap is determined from the data of spectral measurements.     

Spark channel propagation in a microbubble liquid

Experimental study on the development of the spark channel from the anode needle under pulsed electrical breakdown of isopropyl alcohol solution in water with air microbubbles has been performed. The presence of the microbubbles increases the velocity of the spark channel propagation and increases the current in the discharge gap circuit. The observed rate of spark channel propagation in microbubble liquid ranges from 4 to 12 m/s, indicating the thermal mechanism of the spark channel development in a microbubble liquid.     

Parameters of a supershort avalanche electron beam generated in atmospheric-pressure air

Conditions under which the number of runaway electrons in atmospheric-pressure air reaches ∼5 × 10¹⁰ are determined. Recommendations for creating runaway electron accelerators are given. Methods for measuring the parameters of a supershort avalanche electron beam and X-ray pulses from gas-filled diodes, as well as the discharge current and gap voltage, are described. A technique for determining the instant of runaway electron generation with respect to the voltage pulse is proposed. It is shown that the reduction in the gap voltage and the decrease in the beam current coincide in time. The mechanism of intense electron beam generation in gas-filled diodes is analyzed. It is confirmed experimentally that, in optimal regimes, the number of electrons generated in atmospheric-pressure air with energies T > eU _m , where U _m is the maximum gap voltage, is relatively small.     

Picosecond runaway electron beams in air

Experimental data on the generation of picosecond runaway electron beams in an air gap with an inhomogeneous electric field at a cathode voltage of up to 500 kV are presented. The methods and equipment developed for these experiments made it possible to measure the beam characteristics with a time resolution of better than 10⁻¹¹ s, determine the voltage range and the beam formation time in the breakdown delay stage, and demonstrate the influence of the state of the cathode surface on the stability of runaway electron generation. It is demonstrated that the critical electron runaway field in air agrees with the classical concepts and that the accelerated beam can be compressed to ∼20 ps. It is unlikely that, under these conditions, the beam duration is limited due to the transition of field emission from the cathode to a microexplosion of inhomogeneities. The maximum energy acquired by runaway electrons in the course of acceleration does not exceed the value corresponding to the electrode voltage.     

Direct‐Current Triboelectric Nanogenerator Realized by Air Breakdown Induced Ionized Air Channel

The air breakdown phenomenon is generally considered as a negative effect in previous research on triboelectric nanogenerators (TENGs), which is always accompanied by air ionization. Here, by utilizing the air breakdown induced ionized air channel, a direct‐current triboelectric nanogenerator (DC‐TENG) is designed for harvesting contact‐separation mechanical energy. During working process, the charges first transfer from bottom to top electrodes through an external circuit in contact state, then flow back via the ionized air channel created by air breakdown in the separation process. So a unidirectional flow of electrical charges can be observed in the external circuit. With repeating contact‐separation cycles, continuous pulsed DC output through the external circuit can be realized. This working mechanism was verified by real‐time electrode potential monitoring, photocurrent signal detection, and controllable discharging observation. The DC‐TENG can be used for directly and continuously charging an energy storage unit and/or driving electronic devices without using a bridge rectifier. Owing to its simplicity in structure, the mechanism is further applied to fabricate the first flexible DC‐TENG. This research provides a significant fundamental study for DC‐TENG technology and may expand its application in flexible electronics and flexible self‐charging power systems.     

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.     

Conditions for electron runaway under leader breakdown of long gaps

An original hydrodynamic model in which inelastic collisions in the equations of motion and energy balance play a decisive role is developed and applied to simulate electron avalanches in strong electric fields. The mean energy and drift velocity of electrons, as well as the ionization coefficient and electric field in a wide range of mean electron energies, are determined for helium and xenon. A criterion is derived for the runaway of the average electron in discharges with ionization multiplication. It is shown that runaway can take place at any value of E/p, provided that the momentum mean free path exceeds the gap length. The voltage corresponding to electron runaway is found for helium, xenon, and air as a function of the electric field, the electron mean energy, and the parameter pd. Conditions for the formation of a precursor in electronegative gases are analyzed. It is shown that the presence of a precursor with a high electric conductance is necessary for the formation of a new leader step. The voltage and time ranges corresponding to efficient electron runaway and X-ray generation during leader breakdown in air are determined.     

Investigation of lightning ignition characteristics based on an impulse current generator

Lightning strike is an important ignition source of forest fires. Artificial lightning discharge is a method for studying lightning fires. However, there is not enough data on the ignition of combustible materials caused by artificial lightning discharge. Previous studies on lightning ignition have focused on the heating and ignition effects of long continuing current (LCC), but the function of the impulse current that occurs before the LCC has not been taken into account. In this paper, an impulse current generator of 8/20 μs was used to simulate the ignition effect of impulse current on conifer needle beds. Different current waveforms have different ignition characteristics. We compared five kinds of conifer needle beds. The average of the current needed to ignite the needle bed of Larix gmelinii (Ruprecht) Kuzeneva was the smallest, and the average of the breakdown voltage was the smallest for the needle bed of Pinus massoniana Lamb. The total energy input to the conifer needle beds was fitted as a multiple log‐linear regression model. The heating energy proportion value varies with different bulk densities, current amplitudes, and moisture contents. Based on this data, the heating energy of the impulse current transferred to the needles can be predicted. This information in conjunction with previous research on LCC was used to derive a lightning ignition prediction model of the full waveform for conifer needle beds.     

Comparison of the AC barrier corona with DC positive and negative coronas and barrier discharge

Results are presented from experimental studies of ac corona discharges between a point electrode and a dielectric-coated plate in nitrogen, argon, helium, and air in the voltage frequency range f=50 Hz–50 kHz. The characteristic features of this type of discharge are compared with the well-known features of dc positive and negative coronas and a barrier discharge between plane electrodes. It is shown that the presence of a dielectric barrier on the plane electrode significantly changes the electric characteristics and spatial structure of the corona, whereas the main phases of the discharge evolution remain unchanged as the voltage increases. With a point electrode, the breakdown voltage of the barrier corona decreases substantially as compared to the breakdown voltage of a barrier discharge with plane electrodes. This leads to softer conditions for the streamer formation in a barrier corona, which becomes more stable against spark generation.     

Study of Cold-Cathode Thyratron Triggering Stability at High Anode Voltages

The delay time to breakdown of a cold-cathode thyratron with a trigger unit based on an auxiliary steady-state low-current glow discharge was studied experimentally. The device was connected into the electric circuit according to the circuit of a classical thyratron. The main experiments were carried out at low working gas (deuterium) pressures and high anode voltages of about 40 kV. It is found that, during the triggering current pulse or commutation of the main discharge current, the auxiliary discharge in the trigger unit passes from the stable segment of the current-voltage characteristic into the regime with a reduced operating voltage. Spontaneous reverse transitions from this regime are also possible. On the other hand, the initial conditions of the auxiliary discharge affect the delay time to thyratron breakdown and lead to the jitter in the total delay time to breakdown relative to the triggering pulse. The total delay time amounts to 100 ns, the jitter in delay time being within 15 ns.