The mechanism of anode-initiated breakdown in liquid organic dielectrics with long molecular chains is proposed on the basis of the experimental data on high velocities of the breakdown channel propagation in organosilicon and organofluorine liquids (~10^{7} cm/s), which is comparable to that obtained earlier in crystals in the same conditions. The high velocities of the anode-initiated breakdown channel are satisfactory explained within the model of the cascade Auger transitions, developed for the crystalline materials. According to this model, velocity of the breakdown channel propagation is proportional to the electrical field strength. The time delay in breakdown channel formation relative to the voltage pulse front does not exceed ~5·10^{-10} s within the margin of error.

Particle flow from vacuum flashover on surface of PTFE, PMMA, and PE initiated by 60-kV nanosecond pulses in self-breakdown mode is studied in this work in the range of discharge currents of 2–6 kA. The feature of this work is changing the discharge current at the constant working voltage (~60 kV). The discharge current is varied by changing a ballast resistor. The time lag between voltage application and current rise is ~12 ns for all the values of the discharge current. This lag corresponds to the stage of conductive channel formation (high-voltage stage) and depends only on working voltage of the generator. In contrast to the low-voltage experiments (~1–5 kV) with external ignition, in our case, plasma flow characteristics depend only on intrinsic properties of the breakdown. The novelty of the proposed approach is that all the measurements are carried out at high repetition rate of pulse application (30–100 Hz). We obtained the dependence of particle flow thrust, mass loss, and share of ion component on the value of the discharge current. The results show that there is a share of the ion beam, intensity of which does not depend on the discharge current. Presumably, these ions are generated at the high-voltage stage of the flashover process. The thrust is directly proportional to the charge passed through the discharge channel. It indicates that the key factor in raise of particle flow intensity is the widening of the discharge channel at higher currents. Some of the results are of practical interest for the development of high-voltage pulsed plasma thrusters.

The mechanism of anode-initiated breakdown in liquid organic dielectrics with long molecular chains is proposed on the basis of the experimental data on high velocities of the breakdown channel propagation in organosilicon and organofluorine liquids (~107 cm/s), which are comparable to that obtained earlier in crystals in the same conditions. The high velocities of the anode-initiated breakdown channel are satisfactory explained within the model of the cascade Auger transitions, developed for the crystalline materials. According to this model, velocity of the breakdown channel propagation is proportional to the electrical field strength. The time delay in breakdown channel formation relative to the voltage pulse rise time does not exceed ~5·10-10 s within the margin of error Keywords: electrical breakdown in liquid, nanosecond breakdown, breakdown mechanism.

In this work, we present the results on thrust performance of 0.5 kg sub-joule pulsed plasma thruster prototype based on a high-voltage transformer with magnetic storage capable of work at frequency of 400 Hz. The discharge unit is made of ferroelectric ceramics with an option for utilizing liquid propellant. In case of vacuum oil as a propellant, we obtained values of thrust of ~ 80 nN·s per discharge and 33 μN·s for 400 pulses in 1 second.

In this work we investigated nanosecond vacuum surface flashover of Al 2 O 3 , BaTiO 3 -based and (Sr, Bi)TiO√-based ceramic materials. We measured ion current, mass loss and thrust of the particle beam initiated by the discharge at voltage of 70 kV and current of 4 kA with pulse duration of 15 ns in matched-load mode. We obtained average beam velocities of ~9 km/s for Al 2 O 3 and ~3 km/s for ferroelectric ceramics. Average ion velocities are in range of 140-170 km/s for all the materials. Ionization degree in the discharge plasma is higher (~3-9%) than previously measured for polymers. We theoretically estimated the thrust of the beam assuming that the discharge plasma is van der Waals gas in critical state.

In this work, we measure directional patterns of charge flow of ions and mass flow of neutrals in particle beam generated by vacuum surface flashover in linear configuration at voltage of 70 kV. We used a generator, which provide a discharge current of 2.8 kA. The samples used are polymethylmethacrylate, polytetrafluorethylene and polyethylene. We found that the directional patterns of mass and charge flows in linear configuration have distinct axial asymmetry. The corresponding angular distributions are significantly wider in the plane which is normal to the discharge than in the plane which contains the discharge. Angular distributions of charge flow are wider than distributions of neutral mass flow in the same planes. Besides, the asymmetry in ion current amplitudes at the cathode side and at the anode side is observed. We found that the share of low-velocity ions in ion current drops drastically in the plane which contains the discharge at the angles of 15 degrees and larger for all the tested materials.

The vacuum arc discharge is intensively explored for a long time. It acts as a source of multiply charged plasma. The results of the special type plasma gun (5–10 kA, 12 μs) ion flow study with high temporal resolution and the electrode erosion dependences on the amplitude of the current pulse are presented in the research. The ion flow intensity had good reproducibility from series to series, while the values of total mass erosion differ significantly for different series of experiments under the same conditions. The ion erosion was measured to be significantly higher than that for arc sources with currents up to 1 kA.

Abstract In this work we study plasma beams generated by vacuum surface flashover discharge. We measured ionic current, velocity of ions at values of discharge current 2.8 kA and 5.5 kA for polymethylmethacrylate (PMMA), polytetrafluorethylene (PTFE) and polyethylene (PE). The aim of the work is to compare parameters of particle flows at different values of discharge current and unchanged voltage. We used generators with coaxial and disk-shaped glycerol-filled pulse-forming lines. Maximum voltage is 70 kV; voltage pulse duration is 20 ns; width of a current pulse first half wave is 30 ns. Average velocity of the ions for all materials is within the range ∼ 100−150 km/s and doesn’t change significantly as the current increases from 2.8 kA to 5.5 kA. Ion current of the beam increase approximately linearly at higher discharge current. We estimate the density of electrons in plasma as a function of distance from the discharge. The applicability of this type of discharge for creating plasma for neutralizing high-power ion beams in inertial confinement fusion facilities is discussed.

In this paper, we studied the process of vacuum surface flashover of several polymeric materials initiated by 80-kV, 15-ns pulses with peak discharge current 3 kA. We measured breakdown propagation speed, energy input to the discharge, full ion charge and directional pattern of the plasma flow, the full energy of the particle flow, propellant mass loss, and thrust for polytetrafluorethylene (PTFE), polyethylene (PE), and polymethyl metacrylate (PMMA). In the case of PTFE, we obtained the maximum absolute values of thrust (4.7 mu N.s) and the full energy (66 mJ). The values of average mass velocity for all materials lie in the range of 3-7 km/s; values of the average velocity of ions lie in the range of 60-85 km/s. Directional patterns of charge and mass are similar and have distinct symmetry in equatorial and meridional planes. We estimated the temperature of the dielectric surface within a model of energy transfer from the discharge channel to the surface due to thermal conductivity. We showed that at this power level (similar to 10(7) W) and short duration of high-current stage of the discharge (similar to 10(-8) s), the propellant is consumed mainly during the stage of plasma channel formation.

Porous ceramics can be used to supply liquid dielectric to a discharge area in liquid propellant pulsed plasma thrusters [1]. It is interesting to use perfluorinated liquids as propellants, because they have relatively low specific plasma formation energy. Also, it is an attractive idea to use the liquid both as an insulator and propellant. Under nanosecond pulsed conditions, the combination of porous insulating media and liquid dielectric may demonstrate unexpected behavior [2]. In this work, we present the results of measurements of the anode-initiated electrical breakdown front velocity in porous alumina ceramics saturated with perfluorinated esters, alkanes and other liquids. The experimental setup used comprises of a nanosecond pulse generator, breakdown cell and digital oscilloscope. The generator is a Tesla transformer, built into a coaxial forming line with a stored energy of 0.8 J, controlled by a gas-filled spark gap. The generator impedance is 50 $\Omega$, and voltage under no-load is 140 kV. The pulse duration is 8 ns under load-matched conditions, and the rise time is less than 0.5 ns. The configuration of electrodes is point-to-plane with positive point. Samples were made by uniaxial quasistatic compression of alumina nanopowder with specific surface area of 30 m2/g and annealed in air at a temperature of 1900 K [3]. The density of the samples was 66% of that of monolithic alumina. We used liquids that have relatively a high breakdown channel velocity in the single-pulse mode of voltage application $(\text{up}\ \text{to}\ 1.5\cdot 10^{5}\mathrm{m}/\mathrm{s})$. Ceramics saturated with propyl perfluoropentanoate demonstrates a significant decrease in breakdown channel speed (3–4 times) when the samples are thicker than those broken within a single pulse. This behavior may be attributed to the multi-pulse voltage application, and to the properties of the heterogeneous insulating media. Discharge cannot be initiated in a liquid within pores as easily as it can be in a free liquid, nor does it go as quickly as in monolithic alumina or sapphire. Pores with liquid between the grains of powder serve as barriers, which significantly reduce the mobility of charge carriers. Therefore, a combination of porous ceramics and liquid dielectric may be used as an effective insulator in the multipulse mode even in the case of easy-to-breakdown liquids.

A non-thermal atmospheric pressure plasma (NTP) can initiate physical and chemical processes in gas without significant heating. One of the important applications of NTP is the purification of air and exhaust gases by removing toxic substances, such as volatile organic compounds (VOCs) as well as some inorganic compounds (NOx, SO2, CS2 etc.). Due to the extreme variety of research objects and methods of exposure, there is a significant problem of correct comparison of energy the efficiencies of NTP-based methods. In [1], [2] we proposed to use a mixture of VOCs with different reactivities to estimate the parameters of the NTP generated by a pulsed corona discharge. Components of the mixture react with plasma components at different rates, so their concentration dependencies give information on the qualitative and quantitative parameters of the plasma. On the other hand, NTP also is a mixture comprised of active particles of various types (ions, electrons, radicals, excited molecules, etc.) which react with the mixture components of at different rates. Mathematical modeling of these reactions is extremely time-consuming because the values of the reaction rate constants are unknown. To clarify the main characteristics of the plasma, we propose to use two groups of vapor mixtures: a mixture of poly-functional compounds and a mixture of halogen-containing compounds. These mixtures may be considered as standard for estimation of the energy efficiency of NTP-based methods. In this work, the characteristics of plasma generated by pulsed corona discharge and pulsed electron beam irradiation are compared using this standard mixtures method. It is shown that these methods of plasma generation are different in terms of energy efficiency, whereas the basic plasma and chemical processes they initiate are similar. The method proposed here can be used to study the characteristics of various types of discharges and develop new technological applications.

High voltage vacuum surface flashover is a source of fast particles with velocities of up to $5.10^{5}$ m/s which makes it attractive for use in highly efficient pulsed plasma thrusters (PPT). Directional patterns of mass, charge and velocity of particles in plasma bunches are important parameters by which the efficiency of PPT is characterized. In this work we measured directional patterns of plasma bunches generated on the surfaces of PTFE, PMMA, and polyethylene. We used nanosecond pulse generator with a stored energy of 2 J, output voltage of 90 kV, and current pulse duration of 30 ns. The discharge unit has a linear electrode geometry with a 20-mm gap. The ion current of the plasma bunch was registered by a Faraday cup positioned in two mutually perpendicular planes. The angular distributions of ion charge for all materials have a peculiar axial asymmetry which is typical for the spatial distribution of particulate mass [1]. The angular distributions of average ion velocity are also axially asymmetric but not similar. The directional patterns of ion velocities in the plane perpendicular to the sample surface and in which a discharge path lies are almost the same for all three materials, whereas the patterns in the plane normal to the discharge path differ noticeably. We show that for PTFE samples the average ion velocity demonstrates a slow linear decrease from 110 to 100 km/s as the angle changes from 0 to 75 degrees, respectively. By contrast, the ion velocities for PMMA and polyethylene in the normal direction are lower than for the others growing almost linearly from 150 km/s for the normal direction to 180 km/s for angle of 60 degrees. Analysis of the physical mechanism of this effect allows for an increase in the efficiency of PPTs.

High-voltage electric discharge in water initiated by a pulse with nanosecond front may serve as an effective method for microorganism inactivation [1]. We present results on the effect of strong impulse electric fields and discharges on the inactivation degree of E. coli bacteria in drinking and waste water. We use a Marx generator with maximum output voltage of ~ 1 00 kV as an energy source. The pulse duration $(0.3 < \tau < 3\mu \mathrm{s})$ and pulse front (~ 20 ns) were adjusted by varying the capacity of the discharge circuit and shortening the pulses in the output of the generator [2]. We used a pin-plane electrode system, with pin electrodes of both polarities. We find that high voltage pulses of positive polarity are a more effective pulsed mode, because of a higher rate of physical processes and rapid breakdown of water gap. Also, shortening the pulse front facilitates the penetration of the high-frequency electric field directly into the nuclei of biological cells. Microbiological analysis shows that the concentration of E. coli bacteria in treated water decreases from ~10 8 CFU/cm 3 to ~10 3 CFU/cm 3 after 1000 discharges at a voltage of ~45 kV and positive polarity of the pin electrode. The results of the experiments and calculations confirm the chosen functional dependence of energy released into water on parameters of electrode system. Application of the offered optimized high-voltage, short- front pulsed technology of water treatment seems to be more efficient and environmentally friendly rather than chemical water treatment.

Получена картина тонкой структуры канала пробоя с положительного электрода в монокристалле KCl в режиме многоимпульсного воздействия при напряжении до 140 kV. Определены размеры и форма пробойных структур в зависимости от действующего напряжения. Рассчитаны скорости распространения вершины трещины, формирующей пробойную структуру, а также давление в канале пробоя. Показано, что в определенных условиях структура механических разрушений вблизи канала пробоя сохраняется даже после воздействия нескольких десятков импульсов. Работа поддержана Российским фондом фундаментальных исследований (грант N 11-08-01003). DOI: 10.21883/FTT.2017.08.44755.336

The fine structure of a breakdown channel from a positive electrode in KCl single crystal is studied in the multipulse exposure at a voltage of 140 kV. The dimensions and the shapes of breakdown structures are established as the functions of the applied voltage. The velocity of propagation of a crack vertex forming the breakdown structure, as well as the pressure in the breakdown channel, are estimated. It is shown that under certain conditions the mechanical destruction structure near the breakdown channel is retained even after several dozen pulses are applied.

This work presents the results of the experimental study of a method of liquid propellant supply by the means of a porous ceramic diffusion element placed in a coaxial discharge unit. Diffusion pump oil is used as a propellant. Supply rate of liquid to the discharge area is controlled by the pressure at the back side of the porous element. Plasma flow is generated by discharge initiated by 30 ns pulses of 3.5 kA current at initial voltage of 90 kV. Measured parameters are weight loss and ion current of the plasma bunch. We show that the discharge in such experimental conditions is localized within a layer of liquid on a surface of porous ceramics. Plasma formation efficiency depends on thickness of the liquid layer, being higher in case of thinner layers.

In this work, we present the results of measurements of the electrical breakdown velocity in perfluorinated liquids of several chemical classes. Breakdown is initiated from point anode by 8-ns pulses with a rise time less than 0.5 ns at voltage of 140 kV. We show that perfluorinated esters have close values of breakdown velocity over a wide range of gaps, and demonstrate relatively low jitter in gaps for which time to breakdown is comparable to pulse duration.

Ion concentration and velocity spectra of ions generated by nanosecond pulsed high-voltage discharge over polyethylene and lithium fluoride in vacuum are measured in the work. Suggestions on ion acceleration mechanism have been made.

Summary form only given. In our previous works [1, 3] we measured ion velocity spectra of a plasma bunch generated by vacuum flashover. In this work the parameters of plasma bunches ejecting from channels of nanosecond breakdown in KCl are measured. Comparison of parameters of plasma bunches generated by these different methods may advance our understanding of basic mechanisms of both processes. Experimental setup comprised nanosecond generator, vacuum chamber, and digital oscilloscope. The generator was a Tesla transformer built in coaxial forming line with gas-filled spark discharger. Pulses of voltage have amplitude of 140 kV under no load, pulse duration is 8 ns, rise times are less than 0.5 ns. Samples of crystalline KCl were put into a cell with needle anode and diaphragm-like cathode. Test cell was in the vacuum chamber and was connected to the generator. Faraday cup was used to measure current of ion component of plasma bunch. Permanent magnets near the cup effectively suppress the electron component of plasma bunch. Time-of-flight base varied from 10 to 50 cm. Velocities of the ions are derived from the measured delays of ion currents. The ions might be grouped as `fast' (100 km/s) and `slow' (30 km/s). The amplitudes These results let us conclude that bulk breakdown might serve as a source of dense non-ideal plasma as well as vacuum flashover. Earlier [2] a sequentially constricted breakdown channel which was as a result of an action of train of voltage pulses was observed in KCl. The ion spectrum of plasma ejecting from the sequentially constricted channel is also measured in this work.

A technique for estimating the resistance of the electrical breakdown channel in ionic crystals is proposed. This technique is based on measuring the channel velocity in a sample when a ballast resistor is connected to the circuit of a needle anode and on using the theoretical dependence of the channel velocity on the channel conductivity. The breakdown channel resistance at a voltage of 140 kV is about 6.5 kΩ in KCl and about 6.1 kΩ in KBr. These resistances are shown to characterize a gas phase. The gas-phase resistance is found to be nonuniform along the breakdown channel. The head part ∼1 mm long has the maximum resistance. This head region is concluded to contain dielectric substance clusters, which then decompose into metal and halogen ions. The cluster lifetime is ∼10 −9 s.