M. V. Erofeev

Diffuse discharge produced by repetitive nanosecond pulses in open air, nitrogen, and helium

Atmospheric-pressure gas discharge driven by high voltage pulses with fast rise-time and short duration has attracted significant attention for various plasma applications. In this paper, discharges were generated in a highly non-uniform electric field by point-plane gaps in open air by four repetitive nanosecond-pulse generators with repetition rate up to 1 kHz. The rise time of generators was 25 (generator #1), 15 (generator #2), 3 (generator #3), and 0.2 ns (generator #4) and a full width at half maximum was 40, 30-40, 5, and 1 ns, respectively. The experimental results show that there were typical discharge fashions, i.e., corona, diffuse, spark, or arc modes. The variables affecting the discharge characteristics, including the gap spacing and applied pulse parameters, were investigated. Especially, the diffuse discharges were investigated and discussed. With generator #1 at voltage 70-120 kV, characteristics of measured x-rays on the discharge modes were studied, and it indicates that counts of x-rays in a diffuse discharge are up to a peak value under the experimental conditions. With amplitude of voltage pulses in incident wave up to 18 (generator #3) and 12.5 kV (generator #4), runaway electron beam in low pressure helium, nitrogen, and air in a pulse-periodic mode of discharge with repetition rate up to 1 kHz was obtained. Electron beam was registered behind a thin foil in a pressure range from several to tens of Torr. X-ray radiation was obtained in a wide range of pressures, as well as at atmospheric pressure of helium, nitrogen, and air. Voltage pulses of positive and negative polarities were used. Generation of runaway electrons with pulses of positive polarity appeared because of reflected voltage pulses of reverse polarity.

Capacitive and barrier discharge excilamps and their applications (Review)

The results of studies, development works, and tests of barrier-and capacitive-discharge excilamps radiating in the UV and VUV spectrum regions are presented. The main information on the designs of radiators and generators is presented. The spectral, temporal, and energy characteristics of excilamps based on the emission of KrCl*, XeCl*, XeI*, XeBr*, KrBr*, Xe*2, Br*2, and Cl*2 molecules and the I* atomic line are described. It is shown that the created generators and sealed-off radiators have long service lives. Examples of specific applications of excilamps are presented.

Nanosecond discharge in sulfur hexafluoride and the generation of an ultrashort avalanche electron beam

A discharge in the presence of a nonuniform electric field and the generation of an ultrashort avalanche electron beam (UAEB) are studied in the insulating gas SF6 at the pressures 0.01–2.50 atm. High-voltage nanosecond pulses (about 150 and 250 kV) and the voltage pulses with an amplitude of 25 kV and a duration of tens of nanoseconds are applied across the gap. An electron beam is obtained behind the AlBe foil with a thickness of 45 μm at a sulfur hexafluoride pressure in a gas-filled diode of up to 2 atm. It is demonstrated that, at relatively high pressures (greater than 1 atm) and in the presence of high-voltage nanosecond pulses across the gap, the UAEB pulse FWHM increases. The spectra of the diffuse and contracted discharges in sulfur hexafluoride are measured.

Generation of runaway electrons in a nonuniform electric field by applying nanosecond voltage pulses with a frequency of 100–1000 Hz

Generation of runaway electrons and X-ray radiation in helium and air under the action of a pulsed-periodic discharge in a nonuniform electric field is studied. Positive and negative voltage pulses with a repetition rate of up to 1 kHz, a duration on the order of 1 ns, and an incident wave amplitude of 12.5 kV are applied to a needle-plane gap. For both polarities of the main voltage pulse and a helium pressure from several Torr to several tens of Torr, the arrival of negative reflected voltage pulses at the gap is shown to be accompanied by an electron beam generation. X-ray radiation is detected in a wide range of pressure, including under normal pressure of helium and air.

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 (1500–3000 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.

Pulsed discharge in nitrogen and argon under an elevated pressure in a nonuniform electric field

The characteristics of a discharge and radiation in nitrogen and argon under pressures of 10–760 Torr and the discharge formation without pre-ionization of the gap from an auxiliary source are considered. A peak is detected on the pressure dependence of the radiation power of the second positive system of nitrogen for E0/p ∼ 270 V/cm Torr and nitrogen pressure p ∼ 70 Torr. In the pressure range 10–760 Torr and for a voltage pulse leading front duration of ∼ 10 ns, an electron beam is formed behind the grid anode with various half-amplitude pulse durations. It is shown that, under the given conditions, the electron beam is formed at the voltage pulse front both in the case of a discharge gap breakdown and in the absence of a clearly manifested breakdown, as well as for a 10-ns delay of breakdown at the leading front of a discharge current pulse.

Pulse-periodic generation of supershort avalanche electron beams and X-ray emission

Pulse-periodic generation of supershort avalanche electron beams (SAEBs) and X-ray emission in nitrogen, as well as the transition from a single-pulse mode to a pulse-periodic mode with a high repetition frequency, was studied experimentally. It is shown that, in the pulse-periodic mode, the full width at halfmaximum of the SAEB is larger and the decrease rate of the gap voltage is lower than those in the single-pulse mode. It is found that, when the front duration of the voltage pulse at a nitrogen pressure of 90 Torr decreases from 2.5 to 0.3 ns, the X-ray exposure dose in the pulse-periodic mode increases by more than one order of magnitude and the number of SAEB electrons also increases. It is shown that, in the pulse-periodic mode of a diffuse discharge, gas heating in the discharge gap results in a severalfold increase in the SAEB amplitude (the number of electrons in the beam). At a generator voltage of 25 kV, nitrogen pressure of 90 Torr, and pulse repetition frequency of 3.5 kHz, a runaway electron beam was detected behind the anode foil.

Effect of a transverse magnetic field on the generation of electron beams in the gas-filled diode

The effect of a transverse magnetic field (0.080 and 0.016 T) on generation of an electron beam in the gas-filled diode is experimentally investigated. It is shown that, at voltage U = 25 kV across the diode and a low helium pressure (45 Torr), the transverse magnetic field influences the beam current amplitude behind a foil and its distribution over the foil cross section. At elevated pressures and under the conditions of ultrashort avalanche electron beam formation in helium, nitrogen, and air, the transverse magnetic field (0.080 and 0.016 T) has a minor effect on the amplitude and duration of the beam behind the foil. It is established that, when the voltage of the pulse generator reaches several hundreds of kilovolts, some runaway electrons (including the electrons from the discharge plasma near the cathode) are incident on the side walls of the diode.

Capacitive Discharge Excilamps

Investigation was made ofthe characteristics ofXeCl (λ ~ 308 nm), KrC1 (λ ~ 222 nm) and Xci (λ ~ 253 nm) capacitive discharge excilamps. High efficiency of exciplex molecules and simple design have been obtained under capacitive HF discharge excitation. Cylindrical excilamps with radiation output through side surface ofthe cylinder and through one or two windows placed on the tube ends have been developed. High UV radiation power and electrical power deposition to fluorescence conversion resulted in efficiencies of up to 12%. The study of XeC1, KrCl and XeI excilamps have shown, that it is possible to create sealed-off samples with lifetime more than 1000 hours. The stability of output parameters ofthe capacitive discharge excilamps is studied and the mechanism of chlorine losses in low pressure halogencontaimng excilamps made of quartz was determined. The possibility of creation of capacitive discharge excilamps with short pulse duration was studied. In capacitive discharge cylindrical KrClexcilamp, at λ~222 nm the radiation pulse power up to 2.5 kW was obtained. Powerful radiation pulses 50 ns in duration were obtained at pulse repetition rate of 1 kHz.

New bactericidal UV light sources: excilamps

A reliable bactericidal effect on Escherichia coli cells irradiation by excilamps has been established. Both on primary and secondary irradiation there exists the reciprocally proportional dependence between irradiation doze (or exposure time) and survived cells number. The microorganisms survived after primary irradiation are shown to have not changed sensitivity to excilamps irradiation. The best results have been obtained during XeBr-excilamp irradiation. Owing to their technical parameters, the excilamps are promising systems for UV-sterilization. Comparison of capacitive discharge excilamp characteristics with other conventional UV light sources in presented. A comparative study on UV doze effect of a barrier discharge KrCl-excilamp (λ = 222 nm) on Staphylococcus aureus and Escherichia coli inactivation was carried out. KrCl-excilamp emission power is 65 W, and emitting area is 0.1 m2. It has been demonstrated that Staphylococcus aureus cell sensitivity to UV radiation at this wavelength is higher than that of Escherichia coli.