The paper reports on a study of a supershort avalanche electron beam (SAEB) formed in atmospheric pressure air in a diode with dielectric films. The study shows that the number of electrons downstream of an Al foil anode increases gradually as a dielectric film 50 mu m thick is shifted from cathode to anode. With a film 100 mu m thick placed at 3 mm from the cathode, the number of electrons decreases similar to 4 times, whereas their number with the same foil placed at 1 and 11 mm decreases by no more than 30 %. It is supposed that the dielectric at 3 mm from the cathode substantially influences the formation of an ionization wave and that increasing its thickness causes a rapid decrease in SAEB amplitude.

The parameters of X-ray radiation and runaway electron beams (RAEBs) generated at long-pulse discharges in atmospheric-pressure air were investigated. In the experiments, high-voltage pulses with the rise times of 500 and 50 ns were applied to an interelectrode gap. The gap geometry provided non-uniform distribution of the electric field strength. It was founded that at the voltage pulse rise time of 500 ns and the maximum breakdown voltage Um for 1 cm-length gap, a duration [full width at half maximum (FWHM)] of a RAEB current pulse shrinks to 0.1 ns. A decrease in the breakdown voltage under conditions of a diffuse discharge leads to an increase in the FWHM duration of the electron beam current pulse up to several nanoseconds. It was shown that when the rise time of the voltage pulse is of 500 ns and the diffuse discharge occurs in the gap, the FWHM duration of the X-ray radiation pulse can reach ≈100 ns. It was established that at a pulse-periodic diffuse discharge fed by high-voltage pulses with the rise time of 50 ns, an energy of X-ray quanta and their number increase with increasing breakdown voltage. Wherein the parameter Um/pd is saved.

Nanosecond discharges in air, SF 6 , and helium at pressures of units–tens of Torr are studied. Spatial inhomogeneities of diffuse jets and autographs of runaway electron (RAE) beams are recorded in all three gases during a discharge in a nonuniform electric field. It is shown that diffuse jets change their shape and their lengths increase and change from pulse to pulse in the discharge gap as the pressure decreases; further, it is confirmed that the RAE beam amplitude increases as the gas pressure decreases. It is assumed that inhomogeneities observed in the diffuse jets and RAE beams can be associated with transient luminous events in the Earth’s atmosphere that have sizes of tens of kilometers and occur at high altitudes at low pressures under high thunderstorm activity.

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.

In the paper, we study the conditions for the generation of backward runaway electrons through a grounded grid cathode in atmospheric pressure air at high-voltage pulses with a full width at half maximum of 1 ns and risetime of 0.3 ns applied to the gap from a SLEP-150 pulser. The study confirms that backward runaway electrons and X-rays do arise near grid cathodes in atmospheric pressure air. It is shown that the current of the backward beam and the X-rays from the gas diode depend differently on the interelectrode distance. The average X-ray exposure dose in a pulse is more than 3.5 mR.

Results are presented from experiments on the generation of runaway electron beams and X-ray emission in atmospheric-pressure air by using voltage pulses with an ∼0.5-μs front duration. It is shown that the use of small-curvature-radius spherical cathodes (or other cathodes with small curvature radii) decreases the intensity of the runaway electron beam and X-ray emission. It is found that, at sufficiently high voltages at the electrode gap (U m ∼ 100 kV), the gap breakdown, the formation of a spark channel, and the generation of a runaway electron beam occur over less than 10 ns. At high values of U m behind the anode that were reached by increasing the cathode size and the electrode gap length, a supershort avalanche electron beam with a full width at half-maximum (FWHM) of up to ∼100 ps was detected. At voltages of ∼50 kV, the second breakdown regime was revealed in which a runaway electron beam with an FWHM of ∼2 ns was generated, whereas the FWHM of the X-ray pulse increased to ∼100 ns. It is established that the energy of the bulk of runaway electrons decreases with increasing voltage front duration and is ⩽30 keV in the first regime and ⩽10 keV in the second regime.

The generation of runaway electron beams in gases at atmospheric pressure has been studied with a real picosecond accuracy. Their main parameters have been determined. It has been found that three groups of electrons can be separated at a subnanosecond voltage pulse in a runaway electron beam generated in air at atmospheric pressure. It has been proven that the duration of a beam pulse in air at atmospheric pressure behind an anode foil is ~100 ps.

The aim of the work was to take measurements during subnanosecond breakdowns with a time resolution no worse than 100 ps. The current through gaps filled with atmospheric pressure air was investigated during the generation of runaway electrons and X-rays. Three generators, when ensuring the formation of a runaway electrons preionized diffuse discharge (REP DD), were used. It is confirmed that during a subnanosecond voltage rise time, the amplitude of the dynamic displacement current can be higher than 4 kA. It is demonstrated that during the generation of supershort avalanches electron beams with amplitudes of 10 A and more. The conductivity in the air gap at the breakdown stage is ensured by the ionization wave, whose front propagates from the electrode of small curvature radius, and by the dynamic displacement current between the ionization wave front and the plane electrode.

The influence of the cathode design on the energy of the main group of electrons generated during a subnanosecond breakdown in atmospheric-pressure air was studied experimentally. The electron energy was measured using a time-of-flight spectrometer with a picosecond time resolution. It is shown that the energy of the main group of electrons increases with increasing cathode curvature radius. It is established using 400- to 650-μm-thick aluminum foils that the electron energy reaches its maximum value in voltage pulses with abrupt trailing edges and amplitudes below the maximum amplitude. Electrons with maximum energies are generated with a stronger spatial and amplitude scatter than those with average energies.

The evolution of an electric discharge in the active self-terminating metal atom laser medium is examined. Electrodes in the gas discharge tube are placed in cold buffer zones at a distance of several centimeters from the thermally insulated gas discharge channel. It is shown that an abnormal glow discharge is initiated in the cold buffer zones, as capacitive components of the discharge circuit charge from a storage capacitor. In this case, the current-voltage characteristic of the abnormal glow discharge in the cold buffer zones exhibiting a steep current growth and sharp voltage drop is illustrated in the right-hand branch of the Pashcen curve. These processes cause the discharge to pinch. As the capacitive components charge from the storage capacitor for the electrodes in the gas discharge tube placed in the cold buffer zones at a distance of ≤1–3 mm from the thermally insulated gas discharge channel, an obstructed discharge is formed in the cold zones. On ignition of the discharge shown in the right-hand branch of the Pashcen curve the current accompanied by gas heating eliminates the contraction of the discharge in the cold buffer zones and initiates a high-frequency discharge in the active medium since the instant the breakdown (pinch) occurs. In this case, the current-voltage characteristic is demonstrated in the left-hand branch of the Pashcen curve.

Formation and decay of diffuse “channels” of a corona discharge are studied in atmospheric pressure air, as well as optical and X radiation. Modulated (∼290 kHz) high voltage (∼250 kV) pulses of 10 ms were used. Soft X rays were recorded from a corona discharge in atmospheric pressure air. Bright glowing points were observed in the region of diffuse “channels” and plasma formations, at a distance from the channels. It was ascertained that wriggles appear on the diffuse “channels” of a corona discharge, the length of which increases towards the end of a voltage pulse.

In recent years, the generation of beams of runaway electrons has been the subject of increasing interest; see review [1] and references in [1]. It was shown that in atmospheric pressure air, the FWHM of the beam current pulse is 0.1 ns at a generator voltage ranging to over 100 kV, and the amplitude of the beam current downstream of the gas diode foil can reach 100 A [2]. The number of fast electrons under these conditions was at a level of about 6·1010 electron/pulse and up.

In this work, an experiment to examine the conditions under which a runaway electron beam moves not only toward the anode but also backward (to the space downstream of the cathode) was performed. For recording a backward runaway electron beam, a positive voltage pulse was applied to the high-voltage electrode of the gas diode. The breakdown of atmospheric pressure air gaps with a special cathode design at a rate of voltage rise of 1014 V/s was studied with subnanosecond and picosecond time resolution. In the space downstream of the cathode, which was made of thin wires arranged parallel to a thin flat foil, a runaway electron beam (fast electrons) was detected. The current of the fast electron beam downstream of the cathode depended strongly on the anode material. With the Ta anode, the number of electrons in the backward beam increased 4 times compared to that with the Al anode. Measurements of the X-ray exposure dose downstream of the cathode show that for the Ta anode, it was also 4 times higher.

Diffuse discharges were formed in a highly nonuniform electric field by point-plane gaps in atmospheric pressure air, nitrogen, CH 4 , SF 6 , xenon, krypton, argon and helium by two nanosecond-pulse generators. The rise time of generators were 0.3 (SLEP-150) and 1 ns (RADAN-220), and a full width at half maximum (FWHM) were 1 and 2 ns, respectively. The variables affecting the discharge characteristics, including the gap spacing d and applied pulse parameters, were investigated. With both generators supershort avalanches electron beam (SAEB) and X-rays were obtained. The characteristics of measured SAEB and X-rays on the all generators were studied. With Generator #1 in atmospheric pressure air it was obtained that FWHM of the total SAEB current pulse from the entire foil area is ̃ 100 ps and the amplitude of SAEB ̃ 100 A. With Generator #2 in all gases at the pressure of one atmosphere SAEB was registered.

This paper presents new experimental data on the generation of runaway electron beams and X-rays in repetitively pulsed discharges in helium and air with an inhomogeneous electric field. Both negative and positive voltage pulses of duration 1-5 ns and amplitude in the incident wave up to 18 kV were applied to a "tube-to-plane" gap at a pulse repetition frequency of up to 1 kHz. It is shown that at a helium pressure of several and tens of torrs, whatever the polarity of the main voltage pulse, an electron beam is generated on arrival of reflected negative voltage pulses in the gap. X-rays were detected over a wide pressure range of helium and air, including atmospheric pressure. The breakdown of atmospheric-pressure air gaps with a special cathode design at a rate of voltage rise of 1014 V/s was studied with subnanosecond and picosecond time resolution. At a voltage amplitude higher than 100 kV, a fast electron beam in atmospheric-pressure air was detected in the space downstream of the cathode, which was made of thin wires arranged parallel to a thin flat foil. The current of the fast electron beam downstream of the cathode depended strongly on the anode material.

This paper describes experimental studies of the dependence of the X-ray intensity on the anode material in nanosecond high-voltage discharges. The discharges were generated by two nanosecond-pulse generators in atmospheric air with a highly inhomogeneous electric field by a tube-plate gap. The output pulse of the first generator (repetitive pulse generator) has a rise time of about 15 ns and a full width at half maximum of 30–40 ns. The output of the second generator (single pulse generator) has a rise time of about 0.3 ns and a full width at half maximum of 1 ns. The electrical characteristics and the X-ray emission of nanosecond-pulse discharge in atmospheric air are studied by the measurement of voltage-current waveforms, discharge images, X-ray count and dose. Our experimental results showed that the anode material rarely affects electrical characteristics, but it can significantly affect the X-ray density. Comparing the density of X-rays, it was shown that the highest x-rays density occurred in the diffuse discharge in repetitive pulse mode, then the spark discharge with a small air gap, and then the corona discharge with a large air gap, in which the X-ray density was the lowest. Therefore, it could be confirmed that the bremsstrahlung at the anode contributes to the X-ray emission from nanosecond-pulse discharges.

The dependences of the electron beam intensity and X-ray dose on the thickness of metal foils (Al, Cu) in a nanosecond discharge initiated in atmospheric-pressure air are studied theoretically and experimentally. Calculated curves of electron beam attenuation in aluminum and X-ray dose attenuation in copper agree well with experimental data. It is found that the amplitude of a super-short avalanche electron beam and the X-ray exposure dose reach maximal values at different values of the interelectrode gap. When the length of the cathode’s edge with a small radius of curvature increases, an interelectrode gap maximizing the amplitude of the runaway electron current shrinks.

The results of experimental studies of the parameters (amplitude and duration) of a supershort avalanche electron beam (SAEB) generated in air at atmospheric pressure are presented. It is shown that the pulse duration of the beam current behind the foil from the entire area of the anode foil is larger than from small areas and depends on the cathode design. The number of electrons that are detected behind the 10-μm-thick Al foil is ∼6 × 10 10 electrons, which corresponds to a SAEB amplitude of ∼100 A at a FWHM of the current pulse of ∼100 ps. An X-ray exposure dose per pulse of ∼1.8 mR was obtained using a 20-μm-thick copper foil. It was confirmed that the FWHM of a SAEB pulse is within ∼50 ps from small foil areas (with diameters of ∼7 mm or smaller).