The impulse dielectric behavior of N2 gas in sphere-plane gap

The focus of this work has been on the pre-breakdown phenomena and the breakdown characteristics of N₂ gas in a sphere-plane gap under various impulse voltages. Both electrical and optical experimental investigation methods were used. Following parameters were considered: gas pressure range from 0.2 to 0.6 MPa, electric field utilization factor of the electrode configuration 71%, positive and negative impulse waveforms with the rise time of 500 ns, 1.2 μs and 180 μs. The observed discharge processes before the breakdown through the light emission images by the ICCD camera are in good agreement with the streamer mechanism. Under both polarity stresses, discharges are initially concentrated around the tip of the sphere and later pointing towards the earth electrode. However, negative streamers are thinner and more diffuse. As expected, the breakdown voltages for negative polarity are lower than those for positive polarity regardless of the gas pressure and shape of the applied impulse voltage. The breakdown voltage is increased with shortening the rise time of pulse waveforms. As a substitute for SF₆, N₂ gas under pressures above 0.3 MPa can reach the standard rated withstand voltage for 24 kV C-GIS.     

A magnetically stabilized coaxial laser discharge

A new magnetic discharge stabilization technique for coaxial laser systems is described. The approach utilizes crosses electric and magnetic fields to create and maintain a large and rapidly rotating plasma volume which does not experience glow-to-arc transitions. Very high cw specific discharge power loadings have been achieved even without the benefit of external gas cooling or circulation.Performance is insensitive to gas composition and pressure such that high power coaxial discharges have been run in CO₂ laser gas mixtures up to several hundred torr. Stable cw discharges have also been obtained in mixtures containing several torr of SF₆.The technique appears to be readily scalable to give very large excited volumes in systems with comparatively small overall physical dimensions.     

Novel long-pulse TE CO2 laser excited by pulser–sustainer discharge

A novel long-pulse TE CO₂ laser with UV-preionization is presented. With an active volume of 1.17 l and gas pressure of 30 kPa, the laser can discharge stably with low pulser energy and high sustainer energy. Various long-pulse discharges such as 12, 20 or 25 μs are demonstrated. At discharge pulse width of 23.9 μs, maximum output laser energy of 6.8 J is obtained at an efficiency of 9.0%.     

Self-sustaining discharges in needle-to-plane geometry with hundreds of microns electrode gaps

Atmospheric pressure needle-to-plane discharges have been explored experimentally in electrode gaps from 100 μm to 400 μm. These discharges can be self-sustained and follow the form of existing empirical formulae describing the current-voltage characteristics of corona discharge. The discharge can also be self-sustained by its lower sustaining voltage applied between the two electrodes once it is ignited by the initial high output voltage from power supply. The experiments of charging aerosol particles by the self-sustaining discharge operating with a lowered power have shown that for particles with a diameter of 46 nm, the charging efficiency attained 43.6%.     

Hollow cathode fall field strength measured by Doppler-free two-photon optogalvanic spectroscopy via Stark splitting of the 2S level of hydrogen

It has recently been demonstrated that Doppler-free two-photon optogalvanic spectroscopy is very well suited to measure the strong electric field strength (0.4 kV/cm to 4 kV/cm) present in the cathode fall of hollow cathode discharges, via the Stark splitting of the 2S level of atomic hydrogen and its isotopes. Based on an improved reliability and precision of the measurements, the aim of the present study is to analyze more deeply the dependence of the cathode fall behaviour for a hydrogen discharge on the usual discharge parameters like gas pressure and discharge current; and for changes of the discharge geometry using two different cathode diameters of 10 mm and 15 mm.     

Measurement of atomic hydrogen density in non-thermal H<Subscript>2</Subscript> plasmas via threshold ionisation-molecular beam mass spectrometry

Determinations of radical density are essential to investigate the physical-chemical processes in plasmas and setup the related theoretical models. This paper presents the experimental measurement of atomic hydrogen near grounded electrode in dielectric barrier discharge medium-pressure hydrogen plasma via threshold ionisation-molecular beam mass spectrometry. After investigating the possible influences from parent molecules in excited states, background component and space-charge, evolution of atomic hydrogen density as functions of discharge parameters are investigated utilising the signal of H₂ molecule beam as the reference. At fixed gas pressure of 6.0 torr and a discharge voltage of 24 kV, atomic hydrogen density increases monotonously from 1.1×10¹⁴ to 2.0×10¹⁵ cm^-3 as the discharge frequency increases from 9 to 26 kHz. Similarly the rising discharge voltage also lead to enhancement of atomic hydrogen density.     

Threshold for a cumulative resonant microwave streamer discharge in a high-pressure gas

Results are reported from experiments aimed at investigating how the structure of a microwave streamer discharge depends on the gas pressure. The formation of a bright core in the streamer channel is shown to be of a threshold nature: in discharges initiated in the field of a standing electromagnetic wave of an open two-mirror cavity, a bright core forms in air and hydrogen in the pressure ranges p ₀≥540±50 torr and p ₀≥740±70 torr, respectively. Estimates are presented, according to which the appearance of a bright core can be attributed to the onset of a local microwave pinch effect.     

A New Method for Measuring Breakdown and Maintenance Fields in Microwave Discharges

A new experimental approach for comparative measurements of breakdown and maintenance electric fields in microwave discharges is described. The method is based on the analysis of the RF pulses transmitted to a matched load, in absence and in presence of the discharge, by the applicator employed for coupling the RF power to the gas under investigation. Results on breakdown and maintenance field in microwave discharges in nitrogen, at a frequency of 2.45 GHz, for flow and non-flow conditions, in the pressure range 0.5 to 6 torr, are reported.     

Effects of nonlocal electron kinetics and transition from αto γ regime in an RF capacitive discharge in nitrogen

The electron kinetics and regime of operation of asymmetrically coupled RF (27-MHz) nitrogen discharges in two vessels with different interelectrode gaps over the range of 0.20-0.35 torr gas pressure are studied in terms of the electron energy distribution function (EEDF). The latter is measured by means of a computer-controlled data acquisition system, a Langmuir probe with cross-modulation second derivative technique applied. The experiments are performed with an axial resolution along the RF electric field and accompanied with a measurement of electrical discharge characteristics. The effects of local and nonlocal plasma response are considered. The transition between α and γ discharge regimes is registered by measuring the EEDF and its moments' changes with the increase of the RF discharge current density     

Surface charge measurement in surface dielectric barrier discharge by laser polarimetry

Measurements of surface charge in a surface dielectric barrier discharge driven in atmospheric air were successfully demonstrated by a laser polarimetry. AC voltage at a frequency of 2 kHz generated the discharge between an exposed electrode and the dielectric barrier over a buried electrode. Although the discharge behaviors varied depending on the polarity of the exposed electrode, there were no differences in the tendency and amplitude of the surface charge accumulation except the polarity. With higher applied voltage, the amount of surface charge became larger and the charge was distributed farther from the exposed electrode.     

Ignition of a cloud of dry powder using super brush discharges

Ignition of a cloud of dry powder is a major concern in the field of industrial process safety. The different types of discharges are already defined (spark discharges, brush discharges, propagating discharges, cone discharges, corona discharges) such as their ignition properties in a gas or a dust atmosphere. For example, it is known that a classic brush discharge cannot ignite a cloud of dry flammable dust [6,13]. Glor and Schwenzfeuer performed direct ignition tests using brush discharges and defined that even if the energy released by this kind of discharge equaled the one of a spark, the power released by the brush discharge is too low to trigger an ignition.However, some doubts remained for super brush discharges. A brush discharge as a super brush discharge occurs between a charged insulating object and a conductive electrode. The main difference lies in the surface charge density reached on the insulator that is much higher for a super brush discharge than for a brush discharge. A high charge density can be reached for example using pipes of polyethylene individually charged by tribo-charging piled one above another. Such a configuration was evocated by Lüttgens [12] and tested by Larsen [11] who performed direct ignition tests in oxygen enriched atmospheres.This study is relevant with the actual safety problems since pharmaceutical and chemical powders are well known to generate electrostatic charges during their transport or handling and since the same configuration of independent polyethylene fibers can be found in flexible bulk containers that are one of the most common solutions to package this kind of powder.This paper presents the experimental set-up and the results of direct ignition tests performed with a polyethylene wax whose MIE is lower than 1  mJ at ambient conditions. The electric field reached at 1 m and the charge transfer were also registered and are described. Finally, numerical simulations are carried out to define the original surface charge density in order to help to understand the phenomenology of this discharge and its frequency of occurrence in industry.     

Optical emission spectroscopy of point-plane corona and back-corona discharges in air

Results of spectroscopic investigations and current-voltage characteristics of corona discharge and back discharge on fly-ash layer, generated in point-plane electrode geometry in air at atmospheric pressure are presented in the paper. The characteristics of both discharges are similar but differ in the current and voltage ranges of all the discharge forms distinguished during the experiments. Three forms of back discharge, for positive and negative polarity, were investigated: glow, streamer and low-current back-arc. In order to characterize ionisation and excitation processes in back discharge, the emission spectra were measured and compared with those obtained for normal corona discharge generated in the same electrode configuration but with fly ash layer removed. The emission spectra were measured in two discharge zones: near the tip of needle electrode and near the plate. Visual forms of the discharge were recorded with digital camera and referred to current-voltage characteristics and emission spectra. The measurements have shown that spectral lines emitted by back discharge depend on the form of discharge and the discharge current. From the comparison of the spectral lines of back and normal discharges an effect of fly ash layer on the discharge morphology can be determined. The recorded emission spectra formed by ionised gas and plasma near the needle electrode and fly ash layer are different. It should be noted that in back arc emission, spectral lines of fly ash layer components can be distinguished. On the other hand, in needle zone, the emission of high intensity N₂ second positive system and NO _γ lines can be noticed. Regardless of these gaseous lines, also atomic lines of dust layer were present in the spectrum. The differences in spectra of back discharge for positive and negative polarities of the needle electrode have been explained by considering the kind of ions generated in the crater in fly ash layer. The aim of these studies is to better understand the discharge processes encountered in electrostatic precipitators.     

On the operating conditions of the electrical blowing module for a periodic-pulse laser based on fluorine atoms and molecules

The results of an investigation of the ignition conditions and characteristics of a multiple-electrode corona discharge, intended to be used in the module which performs electrical blowing of the working media of high pressure lasers operating on fluorine atoms and molecules, in a tip-grid system are reported. The electrical and optical characteristics of corona discharges of positive and negative polarity, which were distributed along the length of a transverse discharge of lasers operating on the transitions F(3s-3p) and F₂(D′-A′), were studied. The characteristics of corona discharges were studied in a mixture He/F₂ at atmospheric pressures. Zh. Tekh. Fiz. 68, 84–87 (September 1998)     

Temperatures of Tesla discharges from OH A → X intensity measurements

The OH A²Σ⁺ → X²Π system is used as a diagnostic tool for temperature measurements of Tesla discharges in Ar and He seeded with 0.5 torr H₂O. From relative intensities of low-N′ lines in the 0-0 band we obtain temperatures which are independent of pressure for the range 100–650 torr and which are only a few degrees higher than the estimated wall temperatures. A check on the theoretical line strengths for OH A → X shows 1% agreement for main-branch transitions but only 5% agreement for satellite lines. Self-absorption problems are encountered and corrected for in the He discharge work; from the ≈10% magnitude of the correction, there are significant amounts (8 mtorr) of ground-state OH produced in the He Tesla discharge.     

Corona discharges in sub-millimeter electrode gaps

Atmospheric pressure wire-to-plane positive corona discharges have been explored experimentally in electrode gaps from 0.3 mm to 5 mm. Reductions in the gap distance and wire diameter resulted in a significant increase in discharge currents and lowered onset potentials, and the current–voltage characteristics followed the typical Townsend’s relationship. The experimentally observed onset potentials were higher than Peek’s prediction although they still showed the same logarithmic behavior with gap distance. Additionally, a theoretical relationship for the current–voltage discharge in a wire-to-plane configuration was derived to obtain the dependence on the geometric parameters, and it compared well with the experimental results.     

Carbon fiber formation in electrical discharges in hydrocarbons

The process of synthesis of carbon fiber from hydrocarbon vapours in low-current electrical-discharge plasma was investigated in the paper. The carbon fibers were effectively synthesised in discharge of positive polarity generated between a stainless steel needle and a plate made of nickel alloy, for the discharge current ranged from 1 mA up to 3 mA. The experiments were carried out at normal pressure in cyclohexane vapours with argon as carrier gas. The diameter of produced fibers varied from about 20 to 70 μm. The growth rate of the fiber was about 0.25 mm/s.     

Experimental investigation on the measurement deviations in recording brush discharges from insulating surfaces by Coulombmeters

The total charge on a charged sheet before and after a provoked brush discharge, their difference “C”, the induced charge “A” on different probes immediately before the discharge and their transferred charge “B” measured by commercially available Coulombmeters have been measured for probes from 2 mm to 25 mm in diameter including human thumb and shielded probe. The brush discharges were provoked on PTFE discs 10 cm–20 cm. The results obtained can be used for correct estimation of the incendivity of brush discharges between 10 nC and 90 nC.     

Low temperature,atmospheric pressure,direct current microplasma jet operated in air,nitrogen and oxygen

Micro-plasma jets in atmospheric pressure molecular gases (nitrogen, oxygen, air) were generated by blowing these gases through direct current microhollow cathode discharges (MHCDs). The tapered discharge channel, drilled through two 100 to 200 μm thick molybdenum electrodes separated by a 200 μm thick alumina layer, is 150 to 450 μm in diameter in the cathode and has an opening of 100 to 300 μm in diameter in the anode. Sustaining voltages are 400 to 600 V, the maximum current is 25 mA. The gas temperature of the microplasma inside the microhollow cathode varies between ~2000 K and ~1000 K depending on current, gas, and flow rate. Outside the discharge channel the temperature in the jet can be reduced by manipulating the discharge current and the gas flow to achieve values close to room temperature. This cold microplasma jet can be used for surface treatment of heat sensitive substances, and for sterilization of contaminated areas.     

Investigation of a single barrier discharge in submillimeter air gaps. Nonuniform field

Pulse characteristics of single barrier discharges as well as parameters of charges accumulated on the surface of a dielectric under the atmospheric pressure in the “needle-(0.1–2.0)-mm air gap-polymer barrier-plane” system are investigated. It is found experimentally that for the positive polarity of the needle, the voltage for the discharge initiation is higher than in the case of the negative polarity by ～25–35%. The reversal of the needle polarity from negative to positive increases the amplitude of the discharge current and the accumulated surface charge by ～1.5–3 times. For the positive polarity of the needle, the discharge is governed by a streamer mechanism, while for the negative polarity, the discharge is initiated by the formation of a single Trichel pulse. The single pulse regime is observed for the discharge current up to a certain electrode gap d _CR. For the positive needle and for air gap width d _air > d _CR ≈ 1.5 mm, a multipulse burst corona is formed, while for the negative needle and d _air > d _CR ≈ 0.9 mm, a damped sequence of Trichel pulses evolves in the system.