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The paper is devoted to the study of a pulsed streamer discharge, similar to that in the Earth’s upper atmosphere. An experimental setup providing the formation of two ionization waves propagating in opposite directions from a region filled with a plasma formed by the capacitive discharge in low-pressure atmospheric air was created. In a physical experiment, the process of propagation of red ionization waves (streamers) was simulated. It was established that the average propagation velocities of their fronts correspond to those of red sprites. This was shown as a result of spectral studies that at air pressures of 0.4-3 Torr, the radiation color radiation observed visually and captured on an integral photograph from the region of passage of ionization waves is determined by the spectral transitions of the first positive system (FPS) of nitrogen molecules, similar to what occurs for red sprites. In this case, the spectral energy density of radiation in the most intense band of the second positive system (SPS) of the nitrogen molecule with a wavelength of 337.13 nm is an order of magnitude or higher than that in the most intense band of the FPS with the wavelength of 775.32 nm. Using the emission spectra and methods of optical emission spectroscopy (OES), the main parameters of the discharge plasma are estimated. Thus, the created setup makes it possible to simulate the process of the formation of red sprites propagating in opposite directions under laboratory conditions.

Features of the nanosecond discharge development in a non-uniform electric field are studied experimentally. High spatial resolution imaging showed that thin luminous tracks of great length with a cross-section of a few microns are observed against the background of discharge glow in air and argon. It has been established that the detected tracks are adjacent to brightly luminous white spots on the electrodes or in the vicinity of these spots, and are associated with the flight of small particles. It is shown that the tracks have various shapes and change from pulse to pulse. The particle tracks may look like curvy or straight lines. In some photos, they can change their direction of movement to the opposite. It was found that the particle’s track abruptly breaks and a bright flash is visible at the break point. The color of the tracks differs from that of the spark leaders, while the bands of the second positive nitrogen system dominate in the plasma emission spectra during the existence of a diffuse discharge. Areas of blue light are visible near the electrodes as well. The development of glow and thin luminous tracks in the gap during its breakdown is revealed using an ICCD camera. Physical reasons for the observed phenomena are discussed.

Features of nanosecond discharge development in a non-uniform electric field are studied experimentally. High spatial resolution imaging showed that thin luminous tracks of great length with a cross-section of a few microns are observed against the background of discharge glow in air and argon. It has been established that the detected tracks are adjacent to brightly luminous white spots on the electrodes or to the vicinity of these spots, and are associated with the flight of small particles. It is shown that the traces have various shapes and change from pulse to pulse. The particle tracks may look like curvy or straight lines. In some photos, they can change the direction of movement to the opposite. The particle trace was found to abruptly cut off and a bright flash is visible at the trail break point. The color of the tracks differs from that of the spark leaders, while the bands of the second positive nitrogen system dominate in the gap spectra during the existence of a diffuse discharge. Areas of blue light are evident near the electrodes, as well. Development of glow in the gap during its breakdown is revealed using an ICCD camera. Physical reasons for the observed phenomena are discussed.

Xenon emission in the spectral range of 120 – 800 nm upon excitation by subnanosecond voltage pulses is investigated. Excitation is performed using diffuse and spark discharges in an inhomogeneous electric field at a xenon pressure ranging from 0.3 to 3 atm. It is shown that xenon excitation by a series of successive 0.7-ns voltage pulses with a pause of 30 ns leads to a decrease in the second-continuum radiation intensity by the second and subsequent voltage pulses and an increase in the first-pulse afterglow intensity. It is confirmed that in the case of a pulsed diffuse discharge, the second continuum of xenon dimers, which is used to generate laser and spontaneous radiation in the VUV spectral region, makes the largest contribution to the radiation energy. Broadband radiation in the visible range (λ > 400 nm) is found to occur when the discharge is constricted. The obtained results indicate that the influence of the dynamic displacement current on the pin-diode caused a measurement error in the papers by V.I. Baryshnikov et al., who did not record the second-continuum radiation from xenon excited by a short homogeneous discharge.

The parameters of a discharge in xenon formed by voltage pulses with a duration at half-maximum of 0.7 ns were studied. It has been confirmed that at the stage of the formation of the diffuse discharge, the most intense radiation in the 120–800-nm region is concentrated in the band with a maximum at the wavelength of 172 nm (the second continuum of xenon dimers). It was found that recombination continua appear after the discharge contraction, which occurs faster with a decrease in the interelectrode distance or an increase in the input energy. It is shown that the Xe $_{2}^\ast $ radiation pulse duration at gas pressures of 0.3–3 atm significantly exceeds the duration of the incident voltage pulse. In their articles, Baryshnikov and Paperny claim that there is no radiation of the second continuum in a diffuse discharge plasma of xenon. Thus, the results obtained indicate that the data given in these articles require additional verification of the measuring techniques of VUV radiation, as well as, the voltage across the gap and discharge current.

We describe the recently discovered phenomenon of the formation of an extended luminous structure at the bend of the channel of a high-voltage pulse discharge. It is called the apokamp (from the Greek words απó — ‘from’ and κάμπη — ‘bend’), and the discharge as a whole is referred to as apokampic. In the course of experimental and theoretical studies, it is shown that the apokamp is a narrow streamer channel that propagates at a characteristic speed of tens to hundreds of km/s, depending on the applied voltage, pressure, and gas type. The necessary conditions for apokamp formation are established. The apokamp discharge is used for laboratory studies of conditions for the formation of blue jets and red sprites — large-scale transient luminous events (transients) observed in Earth’s atmosphere above areas with thunderstorm activity. The revealed signs of similarity between apokamps and blue jets are described. The proposed experimental setup can also be used for meaningful testing of hypotheses about the conditions of formation of transients in Earth’s atmosphere and in the atmospheres of other planets.

The use of stationary solvers instead of approximate solution methods or time-dependent solvers, which are standard tools in gas discharge modeling, allows one to develop a very fast and robust numerical model for studying the time-averaged characteristics of dc corona discharges. Such an approach is applied to dc corona discharges in point-plane gaps in ambient air. A wide range of currents of both voltage polarities and various gap lengths are investigated, and the simulation results are validated by comparing the computed current–voltage characteristics and spatial distributions of the radiation intensity with experimental results. Specific features of the numerical and experimental results at both polarities are discussed.

When various materials are used as radiators for Cherenkov detectors registering electron fluxes with energies of tens to hundreds of keV, cathodoluminescence negatively affects the useful signal. Therefore, in the process of creating Cherenkov detectors, the search for materials with a low level of the cathodoluminescence is important. In this paper, the spectral and amplitude-time characteristics of the yttrium sesquioxide ceramics glow under irradiation with an electron beam with electron energy up to ~ 350 keV are experimentally studied. The experimental spectrum of the ceramics glow is compared with the calculated spectrum of Cherenkov radiation. It is shown that the main part of the ceramics radiation energy belongs to Cherenkov radiation, and the cathodoluminescence level is low. The conclusion on the suitability of yttrium sesquioxide ceramics as a material for radiators of Cherenkov detectors is made.

The parameters of stimulated lasing in diffuse discharges formed in mixtures of helium and fluorine in a strongly inhomogeneous electric field are investigated. Lasing is obtained in the visible and VUV spectral regions on the transitions of fluorine atoms and molecules. It is shown that lasing in He – F2 mixtures at a wavelength of 157 nm continues for several half-periods of the discharge current. Due to the homogeneity of the diffuse discharge, the maximum lasing efficiency of the F2 laser is 0.15 %, which corresponds to the efficiency of this type of lasers pumped by pre-ionised transverse volume discharges.

Consideration is given to the formation of conditions and parameters of diffuse and volume discharges, which are widely used for laser generation in VUV, UV, visible and IR spectral ranges in different high-pressure gases and gas mixtures. It was found that spectral parameters of volume and diffuse discharges are similar. The difference between these discharge modes is found to be associated with the conditions of their formation. The diffuse discharges are formed due to runaway electrons, for which one or two electrodes with a small radius of curvature are used. To form volume discharges, it is necessary to carry out preionization of the discharge gap using an additional source, as well as to ensure a uniform electric field between the electrodes.

The principle of operation and technical parameters of excilamps with a small-diameter output window from a series of portable narrow-band ultraviolet and vacuum ultraviolet radiation sources are described. An example of using a KrCl excilamp to excite photoluminescence of crystalline materials is given.

It is ascertained that heating of volcanic material from the Etna volcano (Italy) by an apokampic discharge reduces the voltage at which a positive streamer (apokamp) starts from the discharge channel and increases its propagation speed. The luminescence spectra show these processes to be accompanied by the emission of easily ionizable K and Na, which is consistent with data on the elemental composition of Etna samples. A hypothesis is suggested about an increase in the appearance probability of blue jets and starters over sites of high volcanic activity at altitudes of 10–18 km (at the level of the troposphere).

—This paper describes the design and parameters of a nanosecond accelerator with an additional transmission line that has variable wave impedance and a gas-filled diode. The possibility of controlling the beam current parameters by changing the air pressure in the diode is shown. Behind the anode foil, at a half-maximum pulse width of ≈1.3 ns and an electron energy of up to 350 keV, the beam current amplitude was ≈700 A. Vavilov–Cherenkov radiation spectra and oscillograms were recorded when were excited by the electron beam of this accelerator. quartz, sapphire, and synthetic diamond were excited by the electron beam of this accelerator.

The paper studies the amplitude and spectral-kinetic behavior of pulsed X-ray and cathodoluminescence of non-alloyed zinc selenide (ZnSe) single crystals and those alloyed by gallium, tellurium and samarium. At 300 K, the spectra of pulsed cathodoluminescence of pure ZnSe single crystals show a 490 nm illumination at ~20 ns pulse time. Two spectral bands are observed in alloyed ZnSe single crystals, band edge at ~477 nm and with maximum at ~600 nm at a pulse time of ~20 ns and ~5 μs, respectively. The intensity ratio of these peaks depends on the electron-beam energy density and the geometry of spectrum measurements. X-ray luminescence spectra of the alloyed ZnSe single crystals show an ~600 nm illumination, the intensity of which depends on the type of impurity. The possibility of applying the pure and alloyed ZnSe single crystals is discussed herein for the development of amplitude-time characteristic sensors of electron beams and X-ray radiation energy ranging from tens to hundreds of kiloelectronvolts.

The formation of a negative streamer in air is investigated at different pressures in a sharply nonuniform electric field under the conditions of nanosecond and subnanosecond breakdowns. The investigations are performed with a four-channel ICCD-camera and an ultrafast streak-camera. In a separate series of experiments, the generation of runaway electrons is investigated in the course of streamer formation by measuring the dynamic displacement current. It is shown that the streamer velocity changes during its motion from the electrode with a small curvature radius towards the opposite flat electrode. The data are obtained on the instantaneous streamer velocity. It is shown that runaway electrons are generated at the start of the streamer.

The velocities of the positive and negative streamers in the air at atmospheric pressure are experimentally, theoretically, and computationally studied at the discharge of the needle–plane gap by the nanosecond voltage pulses. The data on variation in the streamer’s velocity are obtained, including the data at various amplitudes of the voltage pulses. It is established that the streamer’s velocity varies along its motion and is maximum at the electrode with a small curvature radius and at the planar electrode. It is demonstrated that the character of variation in the computational values of the streamer’s velocity over the gap agrees with the experimental data.

The results of the study of the radiation spectra of the Earth’s atmosphere in the ultraviolet (UV) and visible spectral regions are presented. The input window of the spectrometer was directed to the Sun or located at different angles to the axis through the solar disk center. The Vavilov–Cherenkov (VCh) radiation spectrum was recorded with a standard spectrometer in the region 200–300 nm for the first time. The threshold electron energy for VCh radiation excitation in air and the maximal angle of VCh radiation propagation with respect to the electron motion direction are calculated. The VCh radiation is assumed to be excited when high-energy solar wind particles are braking in the Earth’s atmosphere.

The results of experimental investigations of formation of diffuse nanosecond discharges in a nonuniform electric field in air and other gases at the pressures within 12.5–400 kPa are presented. The experiments were performed using a four-channel ICCD-camera, an ultra-fast streak-camera, and wideband oscilloscopes. It is found out that spherical streamers are formed in a sharply nonuniform electrical fields under high voltages in a pre-breakdown stage of the discharge, irrespective of the gas used (air, nitrogen, hydrogen, methane, neon, or helium). The data are obtained on the instantaneous streamer velocity in air at different voltages and on the displacement current caused by the electric field re-distribution in the gap during the streamer formation. Beams of runaway electrons are recorded. The mechanism of formation of anode- and cathode-directed streamers at high voltages is discussed.

It is shown that the channel of apokampic discharge is a source of streamers at low air pressure and low frequencies (7–16 Hz) of high-voltage positive-polarity pulses and voltage amplitudes from 26 to 35 kV. The maximal starting speed of laboratory streamers was 560 km/s. A hypothesis about blue jet origination in nature is suggested based on experimental data. For their start, high frequencies of voltage pulses are not required, breakdowns in the Hz range are sufficient, but the discharge channel should bend and the field intensity should be high at the bending point.

A review of the results of investigations of the Cherenkov radiation (CR) and pulsed cathodoluminescence during irradiation of different crystals and polymethylmethacrylate (PMMA) with subnanosecond and nanosecond beams is made. The calculations demonstrating the influence of the electron energy and refractive index of a material on the CR intensity and spectrum, as well as its spatial characteristics are performed. The experimental data on the CR observation at the electron energy E up to 400 keV are presented. It is shown that an increase in E allows detecting the CR in diamond, leucosapphire, and quartz KU-1 crystals using a standard spectrometer. It is found out that is more difficult to register the CR in PMMA because of the radiation absorption in the region of wavelengths shorter than 300–350 nm and an internal breakdown of the specimens due to the accumulation of electrons in them at high beam current densities or (and) pulse durations, even with a monochromator and a PMT. Some recommendations are given for designing detectors of runaway electrons in tokomak-type facilities.

The experimental and theoretical investigations of a new form of burning of a repetitively-pulsed discharge – apokamp – are performed. It is shown that an apokampic discharge represents a narrow streamer channel, growing from the point of plasma channel bending, having a characteristic propagation velocity of tens and hundreds of meters per second, depending on the applied voltage and gas pressure and type. The conditions necessary for an apokamp to evolve are formed in a comparatively weak macroscopic electric field. The channel bend ensures a local field enhancement, which prescribes the starting orientation of the growing streamer channel. The pulse-frequency power supply regime of the discharge reproduces the streamer channel in every pulse, but the preceding framework of the ion-ion plasma ensures reproduction of the channel shape from pulse to pulse.

The ratio between energies of Vavilov–Cherenkov radiation and cathodoluminescence excited by an electron beam in diamond samples is determined taking into account electron scattering in these samples, energy distribution of beam electrons, ionization losses of the electron energy, and dispersion of the diamond refractive index. Experimental results on measuring spectral characteristics of the glow of natural and synthetic diamond samples under the action of a subnanosecond electron beam with electron energy of up to 200 keV are presented. It is shown that most of the radiation of the diamond samples in the region of 240–750 nm under the action of an electron beam with electron energy of up to 200 keV belongs to cathodoluminescence.

It is observed that the formation of an apokamp is preceded by the appearance of streamer coronas starting from the channel of a repetitively pulsed discharge. It is shown that the streamer coronas under experimental conditions alleviate the development of an apokamp, acting as a preionization source.

A corona discharge is experimentally studied in atmospheric air on a needle electrode with the curvature radius 40 μm. For both electrode voltage polarities, but different discharge burning modes, repetitive current pulses with the FWHM ~200 ns are registered. It is shown that in a wide range of negative-polarity voltages the area of the glow registered in the vicinity of the needle has a spherical shape, and in the case of a positive polarity of the needle tip, starting from a certain voltage value, cylindrical streamers are formed from the spherical glow area, whose length increases with the voltage. It is found out that the size of the glowing spherical formations near the needle tip, given the same voltage value, is larger in the case of negative polarity than in the positive one. It is validated that the radiation of the second positive system of nitrogen in the UVspectral region dominates in the corona discharge emission spectrum before its transition into a spark discharge.

Laboratory simulation of the conditions for the occurrence of blue jets in the Earth’s atmosphere has been carried out. For this purpose, the diffuse formation of jets and plasma buildup in corona and apokampic discharges in air has been studied. It is shown that the jets are formed due to streamer breakdown and their color depends on air pressure. At atmospheric air pressures of 30–120 Torr, streamers starting from different parts of a repetitively pulsed discharge are recorded in a nonuniform electric field. It has been ascertained that a spherical corona discharge is formed before a breakdown between pointed electrodes, near a metal high-voltage electrode of positive polarity, from which streamer coronas start as the voltage increases. Data on the streamer head size and streamer propagation speed in the corona and apokampic discharges are presented.

Cherenkov radiation and pulsed cathodoluminescence spectra in the ultraviolet, visible, and near infrared regions generated by a 6-MeV electron beam passing through GE-014 and KU1 quartz plates have been experimentally studied. The Cherenkov radiation spectrum has been recorded in the ultraviolet and visible regions and its change is shown at the variation of the angle between the plane of the plate and the direction of the electron beam. The data on the conditions of recording of the Cherenkov radiation spectra at the excitation of plane-parallel plates of quartz, sapphire, and diamond have been presented. It has been shown that the total internal reflection in the crystals with a high refractive index imposes certain conditions on the geometry of the output of Cherenkov radiation into vacuum at electron energies above 500 kev.

In this paper, we experimentally studied pulsed electron beams with a high local density. The conditions in which the energy density cumulation is observed during the interaction of electrons with the anode are shown to develop in vacuum and gas diodes at nanosecond and subnanosecond durations of a beam current pulse and a decrease in the interelectrode gap. The average electron energy in filamentation and self-focusing of an electron beam in a vacuum diode of an accelerator at a current of ~2 kA and a no-load voltage of ~400 kV was established to be 50–100 keV while the energy density was 109–1010 J/cm3. It is confirmed that the beam current density in a gas diode can exceed 1 kA/cm2. It is hypothesized that superdense electron beams in vacuum and gas diodes are formed as a result of avalanche multiplication of runaway electrons in the cathode–anode gap plasma.

The mode of a pulsed discharge in a nonuniform electric field is investigated at which bright plasma bunches with a beaded structure are generated in atmospheric-pressure air. Using an ICCD camera, it is found that, at centimeter gap lengths and a voltage pulse duration of ≈300 ns, the beaded structure can be observed with a probability close to 100% within time intervals from a few nanoseconds to several tens of nanoseconds. The beaded structure can also be observed in the time-integrated photographs of the discharge gap, but with a low probability. It is shown that individual beads arise in the point-to-plane gap after the diffuse stage of the discharge and start from the electrode with a small curvature radius. It is established that the spark channel bridges the gap by passing through the formed beads. The glowing beads are again observed in the final stage of the discharge, when the discharge current and, accordingly, the intensity of spark emission decrease.

Data that explain the generation of runaway electrons after the breakdown of a gap with a sharply inhomogeneous distribution of the electric field strength are presented. Using a special technique for measuring a displacement current caused by the appearance and propagation of a streamer, the voltage and current waveforms of runaway electrons have been synchronized to each other, as well as to the dynamics of the streamer formation, which has been recorded using a four-channel ICCD camera. It is shown that the first runaway electron beam is generated near the pointed cathode at the time when the streamer appears. The second runaway electron beam is generated at the time when a backward ionization wave reaches the pointed cathode. It is assumed that the generation of the second runaway electron beam occurs in the cathode layer. This is confirmed by the fact that the second runaway electron beam does not appear in those discharge implementations at which the cathode spot is observed before the gap is bridged by the plasma.

Persistent pharmaceutical pollutants (PPPs) have been identified as potential endocrine disruptors that mimic growth hormones when consumed at nanogram per litre to microgram per litre concentrations. Their occurrence in potable water remains a great threat to human health. Different conventional technologies developed for their removal from wastewater have failed to achieve complete mineralisation. Advanced oxidation technologies such as dielectric barrier discharges (DBDs) based on free radical mechanisms have been identified to completely decompose PPPs. Due to the existence of pharmaceuticals as mixtures in wastewater and the recalcitrance of their degradation intermediate by-products, no single advanced oxidation technology has been able to eliminate pharmaceutical xenobiotics. This review paper provides an update on the sources, occurrence, and types of pharmaceuticals in wastewater by emphasising different DBD configurations previously and currently utilised for pharmaceuticals degradation under different experimental conditions. The performance of the DBD geometries was evaluated considering various factors including treatment time, initial concentration, half-life time, degradation efficiency and the energy yield (G50) required to degrade half of the pollutant concentration. The review showed that the efficacy of the DBD systems on the removal of pharmaceutical compounds depends not only on these parameters but also on the nature/type of the pollutant.

The paper presents research data on positive and negative coronas in atmospheric pressure air in a highly inhomogeneous electric field. The data show that irrespective of the polarity of pointed electrodes placed in a high electric field (>200 kV/cm), this type of discharge develops via spherical streamers even if the gap voltage rises slowly (£0.2 kV/ms). The start voltage of first positive streamers, compared to negative ones, is higher and the amplitude and the frequency of their current pulses are much lower: about two times and more than two orders of magnitude, respectively. The higher frequency of current pulses from negative spherical streamers provides higher average currents and larger luminous regions of negative coronas compared to positive ones. Positive and negative cylindrical streamers from a pointed to a plane electrode are detected and successive discharge transitions at both polarities are identified.

This article presents the experimental results of the initiation of a corona discharge from point electrodes with a small radius of curvature. It is shown that short current pulses with a half-long duration of 200 ns flow through the gap during corona ignition. This mode of initiation of a corona discharge is observed not only with a negative polarity of the point electrode, but also with a positive one. It is assumed that the appearance of the first pulses of the discharge current is associated with the formation of streamers that have a ball shape. It was found that the front propagation velocity of the first anode-directed streamer in atmospheric pressure air is at least ∼0.02 mm/ns, and the cathode-directed streamer one ∼0.1 mm/ns.

The autographic method was used to study the morphology of the damage occurring on the surface of metals and in polymethylmethacrylate (PMMA) after a single exposure to an electron beam formed in the vacuum diode of a pulsed electron accelerator in the modes of filamentation and self-focusing. It was found that in the zones of interaction between the filamentous beam and PMMA at a depth of $40-80\ \mu\mathrm{m}$ , micro-fractures of a round and more complex shape with a diameter of $10-50\ \mu\mathrm{m}$ form. The mechanism of local microdestruction formation is analyzed.

Dielectric barrier discharge XeCl-excilamps can be used for detecting of surface defects in case of the luminescent liquid penetrant inspection method (LLPI). In case of LLPI, surface defects are detected by brightly glowing indications formed on the developing coating (developing agent) in the locations of discontinuities. This method differs from the color one that the inspection is performed under darkening conditions and requires ultraviolet irradiation, so it is more sensitive, since the visual luminescence of the display is detected better than the color contrast. This article experimentally compares the results of LLPI using different UV-light sources (two barrier discharge excilamps (portable and front-mounted), a mercury UV lamp and an led UV lantern). It is shown that the XeCl-excilamp gives a good detection and fixation of indicator traces in comparison with other sources of UV radiation.

In this article the initiation of a corona discharge from a negatively charged point with a small radius of curvature is investigated experimentally and as a result of modeling. Modeling showed that the periodicity of the discharge current in our model is observed only in the case of adding electron detachment reactions to the plasma-chemical scheme. The reason for the pulse-periodic mode of current flow is the presence of negative O2− ions. It was shown experimentally that the first streamers at a minimum voltage have a ball shape.