Dmitri V. Rybka

Supershort Avalanche Electron Beams and X-rays in Atmospheric-Pressure Air

The conditions for the generation of runaway electron beams with maximum amplitudes and soft X-rays with maximum exposure doses in a nanosecond discharge in atmospheric-pressure air were determined. A supershort avalanche electron beam (SAEB) with a current of amplitude ~50 A, a current pulse of full-width at half-maximum (FWHM) ~ 100 ps, and a current density up to 20 A/cm2 was recorded downstream of the gas diode foil. It is shown that the maximum of the SAEB current amplitude shifts in time relative to the voltage pulse rise as a collector is displaced over the foil surface. A source of soft X-rays with an FWHM of less than 200 ps and an exposure doze of ~3 mR per pulse was designed based on a SLEP-150 pulser (maximum voltage amplitude ~140 kV, FWHM ~1 ns, and pulse rise time ~0.3 ns). It is demonstrated that X-ray quanta with an effective energy of ~9 keV make a major contribution to the exposure dose.

Runaway electrons and x-rays from a corona discharge in atmospheric pressure air

The characteristics of a corona discharge in atmospheric pressure air are studied using pulsed power generators that produce voltage pulses of different durations, polarities and shapes. The characteristics are measured in the single pulse, batch, and repetitively pulsed modes. It is shown that no matter what the voltage pulse polarity is, a corona discharge starts developing as a conical diffuse discharge near the electrode tip with a voltage rate of increase of 10 15 Vs 1 across an electrode of small curvature radius. With lower voltage rate of increase ( 10 13 Vs 1 or lower), one or several diffuse jets develop from this electrode. The diameter of the jets at their front is less than 1mm and depends on many factors (voltage pulse amplitude and increase, inter- electrode gap width, pulse repetition rate, etc). It is found that at long voltage pulse durations, the radiation spectrum of the corona discharge changes, and the bands and lines of the material of the electrode appear in the UV region at 200-300nm. It is demonstrated that a runaway electron beam in a corona discharge is generated and detected at a distance several times greater than the brightly glowing plasma region of the corona discharge. It is shown that x-rays are generated from a corona discharge at high pulse repetition rates of up to 1kHz.

Corona discharge in atmospheric pressure air under a modulated voltage pulse of 10 ms

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.

Generation of Runaway Electrons and X-rays in Repetitive Nanosecond Pulse Corona Discharge in Atmospheric Pressure Air

A pulsed corona discharge in an inhomogeneous electric field was studied in atmospheric air. A runaway electron beam from the corona discharge was detected with a collector at nanosecond-pulse duration. It is shown that with a nanosecond-pulse voltage of 300 kV, the full width at half maximum of the beam current during the pulsed corona discharge is about 100 ps. It is demonstrated that with a pulse voltage of 90 kV, a full width at half maximum of 40 ns, and pulse repetition frequencies of up to 1 kHz, the corona discharge is an X-ray source.

Radiation from a diffuse corona discharge in atmospheric-pressure air

Optical and X-radiation from a corona discharge in atmospheric-pressure air is investigated. Spectra of the optical radiation in the range 200–850 nm are obtained under various parameters of the voltage pulse. It was shown that an increase in the voltage pulse changes the corona discharge mode so that the discharge becomes a source of UV radiation, not only from nitrogen 2+ bands, but also from the cathode material. It was also shown that the formation of diffuse corona discharges in a non-uniform electric field under high pressure is conditioned by fast electrons and X-ray generation. It was determined that fast electrons originating from discharges in air under atmospheric pressure generate 525-eV photons from the K-shell of oxygen. Calculations have shown that the photons can effectively initiate new electrons near areas of strong fields. This process explains the formation of types of diffuse discharge for a positive-polarity electrode with a small radius of curvature at atmospheric pressure, and of a fast-moving cathode streamer.

High-Pressure Diffuse and Spark Discharge in Nitrogen and Air in a Spatially Nonuniform Electric Field of High Intensity

Runaway-electron-preionized (REP) diffuse discharges are very attractive in many applications. It was found that, in the case of a cathode with a small radius of curvature and a flat anode, a diffuse corona arises within several hundreds of picoseconds at the early stage of the REP discharge. Both diffuse and spark discharges in dependence on the cathode-anode distance, type, and pressure of gas were observed. Images of REP diffuse and spark discharge plasma glowing in air, nitrogen, and its mixture with methane at different pressures are presented.

Gradual tuning of the current pulse width within 1–0.03 ns in gas-filled diodes of nanosecond electron accelerators

The objective of the work is to study the beam current amplitude and pulse width at a voltage pulse rise time of -0.3 ns for interelectrode gaps of differing width in a diode pumped down to a pressure of about 0.01 Torr and filled with different gases (air, nitrogen, and helium). SLEP-150 generator with a tubular cathode was used. The conducted studies show that with subnanosecond voltage pulse rise times, it is possible to gradually vary the FWHM of the beam current pulse over a wide range. The FWHM and shape of the beam current pulse was varied by varying the pressure and gas kind in the diode. It is shown that in the vacuum diode mode, the beam current rise time decreases from 0.5 to 0.2 ns as the interelectrode gap is decreased from 12 to 2 mm. It is found that with a 3-mm interelectrode gap, increasing the air pressure in the diode of the SLEP-150 generator from 0.1 to 6 Torr decreases the FWHM of the current pulse from 1 to 0.18 ns, with the beam current amplitude greater than 400 A and the beam current rise time less delayed with respect to the voltage pulse rise time. The attained beam current density at a beam current pulse width of 0.18-1 and 0.1 ns is 500 and 100 A/cm2, respectively.

The spectra of electron beams produced in air-filled diodes at atmospheric pressure

It is shown that the breakdown of a gap with a small-curvature-radius cathode by a voltage pulse with an amplitude of >100 kV and a rise time of about 0.3 ns is accompanied by the generation of runaway electrons with energies from a few keV to several hundred keV. In the electron spectrum, three groups of electrons with different energies can be distinguished. It is shown that the energy spectrum of the beam electrons depends on the voltage rise time, voltage amplitude, and cathode design.

Fast electrons downstream of a plane grid cathode in a nanosecond discharge in atmospheric pressure air

In this work, we performed experiments 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). For recording a backward runaway electron beam, a positive voltage pulse was applied to the highvoltage electrode of the gas diode. The breakdown of atmospheric pressure air gaps with a special cathode design at a rate of voltage rise up to 1 MV/ns 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, the fast electrons were 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.

Pulsed UV and VUV excilamps

Emission characteristics of a nanosecond discharge in nitrogen, inert gases and its halogenides without preionization of the gap from an auxiliary source have been investigated. A volume discharge, initiated by an avalanche electron beam (VDIAEB) was realized at pressures up to 12 atm. It has been shown that at VDIAEB excitation no less than 90% energy in the 120-850 nm range is emitted by Xe, Kr, Ar dimers. Xenon spectra in the range 120-850 nm and time-amplitude characteristics have been recorded and analyzed for various excitation regimes. In xenon at pressure of 1.2 atm, the energy of spontaneous radiation in the full solid angle was ~ 45 mJ/cm3, and the FWHM of a radiation pulse was ~ 110 ns. The spontaneous radiation power rise in xenon was observed at pressures up to 12 atm. Pulsed power densities of radiation of inert gases halogenides excited by VDIAEB was ~ 4.5 kW/cm2 at efficiency up to 5.5 %.