Low-current Townsend Discharge Research Articles

Effective ionization coefficients for low current dc discharges in alcohol vapours at low pressure

This paper presents results for effective ionisation coefficients (alpha _{mathrm {eff}}/N, N—gas density) obtained from the breakdown voltage and emission profile measurements in low-pressure dc discharges in vapours of alcohols: methanol, ethanol, isopropanol, and n-butanol. Our results for alpha _{mathrm {eff}}/N are determined from the axial emission profiles in low-current Townsend discharge and lay in the interval of reduced electric field E/N (E—electric field, N—gas density), from 1 kTd to 8.8 kTd. We also give a comparison of our experimental results with those from the available literature. Our data cover the high E/N range of the standard operating conditions and in the region where other data are available we have a good agreement.Graphic abstract

Study of GaAs oxidation in the low-current Townsend discharge

An anodic oxidation of GaAs substrates in a non-self-sustained low-current dc Townsend discharge is investigated. The process is carried out at the room temperature in a three-electrode microreactor filled by 98%Ar + 2%O2 gas mixture. Investigation of the oxidation kinetics indicates that the formed oxide is characterized by a high resistivity, ρ ∼ 1011 Ω·cm. It is demonstrated the application of the method for formation of oxide films with dimensions of tens of microns, which thickness is on the nanometer scale. Examination of the “GaAs substrate - oxide layer” interface using the high resolution transmission electron microscopy has revealed that the oxide is characterized by the amorphous structure.

Comparison between Trichel pulse in negative corona and self-pulsing in other configurations

We present here a comparison study on self-pulsing phenomena in negative corona, hollow cathode discharges (HCD) and parallel-plate discharge in air. The voltage-current (V-I) curve, the waveforms of self-pulsed currents, and the time-resolved images of the pulsed discharge are measured under various operating conditions. It is experimentally evidenced that the Trichel pulse in a negative corona and the self-pulsing in HCD and/or parallel-plate discharge have similar features as well as spatial-temporal developing process. It is suggested that they should have a similar mechanism that the pulsing reflects the mode transition of discharge between the low-current Townsend and the high-current normal glow. The pulse rising corresponds to the breakdown and formation of temporal glow discharge in a background of low-current Townsend discharge, while the decay edge relates to the transition back to Townsend discharge. The pulse interval is the re-building process of the space charge layer of high density to ensure the glow breakdown.

Comparative study of noise in low-current Townsend discharge in nitrogen and argon.

Noise in a low-current Townsend discharge in argon and nitrogen is studied. Experiments were performed in a "semiconductor-gas-discharge" structure constituted by a short plane-parallel discharge gap and a high-resistivity photosensitive semiconductor electrode. The noise of optical emission from gas excited by the discharge was investigated with a photomultiplier. Experiments were done at the room temperature of the discharge structure. According to the obtained data, the noise of discharge glow in Ar is astonishingly low-similar to that earlier found for the cryogenic discharge in this gas. At equal time-average photon fluxes emitted by discharges in both gases, the noise of gas glow in nitrogen substantially exceeds the corresponding value for argon. A statistical processing of signals of glow brightness for the two gases revealed the origin of this difference: While the power spectral density of noise in nitrogen is specified by a band of frequencies with the increased density, in argon it is nearly constant in an extended frequency range. At the same time, intensity of noise is practically the same for both gases in the ranges of low and high frequencies. The relationship of the increased noise in nitrogen with a low critical current density for oscillatory instability of the Townsend discharge in this gas is considered.

Time-resolved analysis and optical diagnostics of Trichel corona in atmospheric air

Trichel pulses of negative corona discharge in atmospheric air are investigated in a needle-to-plate configuration. Time-resolved images of the pulsed discharge are recorded by using an ICCD camera. Light emission spectra of N2 molecules and ions during the discharge are measured by optical diagnostics. The distributions of the positive ions and the reduced electric field along the discharge channel are analyzed accordingly. The results show that the positive ions that can accumulate in space and distort the electric field play an important role on the formation of the Trichel pulse. The Trichel pulse is a mode transition between low-current Townsend discharge and high-current glow discharge. The rising time of the pulse corresponds to the breakdown and formation of glow discharge, and the decay time is related to destroy the process of positive ion clouds and the quenching of glow discharge. The time interval between the pulses is determined by the re-accumulation of positive ions near the cathode in the low-current Townsend discharge to form the positive ion clouds as well as a strongly-distorted electric field.

Townsend discharge in nitrogen at low temperatures: enhanced noise and instability due to electrode phenomena

The temperature dependence of the properties of the low-current Townsend discharge in nitrogen is studied. The experiments are carried out on a device with a high-resistivity semiconductor electrode in the temperature range 85–292 K at a current density not exceeding 25 μA cm−2. The discharge gap of the device is 0.85 mm, while the nitrogen density corresponds to the gas pressure Pa (49.5 Torr) at T = 292 K. The earlier increase found in the discharge sustaining voltage in time at a fixed current is quantitatively investigated for the cryogenic discharge. The relaxation is accompanied by transitions to new discharge states with an increased noise intensity. The characteristic time of the relaxation is hundreds of seconds. The effect is especially pronounced in the range of 100–150 K and is relatively weak at room temperature. The interpretation of the experimental data is based on the hypothesis that considers the formation of neutral aggregates of nitrogen in the form [(N2)n]− in the discharge volume. The condensation of the clusters on the cathode leads to a change in its emission properties, which changes the properties of the discharge.

Breakdown and dc discharge in low-pressure water vapour

In this paper we report studies of basic properties of breakdown, low-current Townsend discharge and high-current discharge regimes in water vapour. Paschen curves and the corresponding distributions of emission intensities at low current were recorded in the range of pd (pressure x electrode gap) from 0.1 to 10 Torrcm covering the region of Paschen minimum. From the experimental profiles we obtained effective ionization coefficient of water vapour for the E/N range 650 Td–7 kTd and fitted the results by using the extended Townsend analytical formula. Using the obtained ionization coefficient, we calculated the effective yield of secondary electrons from the copper cathode. Results of the measurements of Volt-Ampere characteristics in water vapour were presented together with the images of the axial structure of the discharge in a wide range of discharge currents for two pd values. Recorded profiles showed development of the spatial structure of the discharge in different operating regimes. We were able to identify conditions where processes induced by heavy particles, probably fast hydrogen atoms, are dominant in inducing emission from the discharge. Finally, standard scaling laws were tested for low current and glow discharges in water vapour.

DC Townsend Discharge in Nitrogen: Temperature‐Dependent Phenomena

AbstractLow‐current Townsend discharge in nitrogen has been studied in the temperature range of T = 100–300 K in a semiconductor‐gas‐discharge structure. It was found that the sustaining voltage US increases with time when a current is passed through the structure at low T. This effect was not observed at room temperature. A hypothesis is put forward that a film of a neutral phase of nitrogen is formed on the electrodes under cryogenic discharge conditions. The presence of the condensed thin‐film phase leads to a decrease in the secondary electron emission from the electrode and to a corresponding increase in US. A possible mechanism of the phenomenon is associated with the formation of large neutral aggregates in the form of [N+2(N2)n]– in the gas discharge volume. The condensation of these aggregates seems to yield a phase that is comparatively stable at cryogenic temperatures (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On Anisotropy and Spatial Dependence of Doppler-Induced Broadening Due to Heavy-Particle Excitation

We study the anisotropy of anomalously broadened <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{H}\alpha$</tex> </formula> lines in a low-current Townsend discharge in hydrogen. Monte Carlo simulation based on the best available data was used to predict angular distributions of heavy particles and spatial dependence of line profiles due to heavy-particle excitation at 10 kTd.

Modification of GaAs surface by low-current Townsend discharge

The influence of stationary spatially homogeneous Townsend discharge on the (1 0 0) surface of semi-insulating GaAs samples is studied. Samples exposed to both electrons and ions in a nitrogen discharge at a current density j = 60 µA cm−2 are studied by means of x-ray photoelectron spectroscopy, ellipsometry and atomic force microscopy. It is shown that an exposure to low-energy ions (<1 eV) changes the crystal structure of the semiconductor for a depth of up to 10–20 nm, although the stoichiometric composition does not change. The exposure to low-energy electrons (<10 eV) forms an oxide layer, which is 5–10 nm thick. Atomic force microscopy demonstrates that the change in the surface potential of the samples may exceed 100 mV, for both discharge polarities, while the surface roughness does not increase.

Space–time development of low-pressure gas breakdown

In this paper we present an analysis of time- and space-resolved development of several regimes of low-pressure dc discharges in argon. These regimes include low current Townsend discharge, oscillations and constrictions of discharge and high current diffuse glow discharges. Our work is based on ICCD recordings of the discharge structure, synchronized with current and voltage measurements. Measurements were made at three different pressure × electrode gap parameters (pd = 250, 150, 45 Pa cm), at three gaps (d = 1.1, 2.1, 3.1 cm). Special attention is given to radial effects and the influence of dielectric walls during the development of the glow discharge structure. We were able to show how the space time-resolved structure of the discharge can lead to an understanding of the physics of the initial stages of gas breakdown and formation of different regimes of low-pressure discharge. The results also give the basis for interpreting formative time delays associated with gas breakdown.

Spatiotemporal Development of Low-Pressure Gas Discharges

We present an analysis of time and space resolved development of all typical regimes of low-pressure DC discharge in argon - low current Townsend discharge, oscillations and constrictions of discharge and high current glow discharges. Our work is based on time-resolved ICCD recordings of the discharge structure, synchronized with current and voltage measurements. Special care is given to radial effects and influence of dielectric walls during development of the glow discharge structure.

Negative Coaxial Cylindrical Corona Discharges in Pure N2, O2, and Mixtures Thereof

Direct-current corona studies were made on coaxial cylinders with inner cylinder at negative potentials in pure N2, pure O2, and mixtures of 1 percent 10 percent O2 in N2, and on clean dry air from 27-mm to 760-mm pressure. Observations of current potential relations from field intensified ionization currents to as near breakdown as possible were made noting thresholds for current transitions, pulsed discharges and other oscillographic as well as visual appearances. Negative wire corona does not exist as such in truly pure N2. Unless a high series resistance was in the line once the critical region of potential was reached, the gap broke down to an arc. With current limiting resistor bombardment of the filament with positive ions from the low order pre-discharge released enough gaseous impurity so that after six minutes the resistor could be removed and the internal resistance of the negative ion space charge near the anode held the current at 2000 μa. The behavior indicated that breakdown was preceded by a low current Townsend discharge that cleaned up or conditioned the filament. In all the negative wire studies the onset thresholds vary with the past history of the wire, and the threshold potential at higher pressures overshoots the minimum operating potential of the discharge once it has set in. Studies with 1 percent O2 indicate that the threshold discharge with clean gas is a continuous current with a diffuse glow which in time contracts to a cathode spot with Trichel pulses. Similar behavior applies to air at lower pressure. The negative wire in O2 shows very heavy predischarge currents with relatively small current jumps when random Trichel pulses appear, and further small current jumps when the fixed spot with regular Trichel pulses appears. There is evidence for a continuous heavy background of Townsend discharge along the whole wire until the visible spot and heavy pulsed discharge appears. The duration of Trichel pulses in pure O2 is two to ten times those in air with point to plane geometry and two to three times those in air with coaxial cylindrical geometry. Lower pressures in pure O2 show many current transitions and transitions between discharge forms.