Spark Channel Research Articles

液中放電による圧力波の発生と伝ぱに関する研究 (第1報)

In order to investigate the generation and propagation of pressure wave by spark discharge in liquid systematically, the voltage across the electrode gap and the discharge current are measured with an accuracy required in the above-mentioned analysis as the first step. Using the results, the resistance of the electrode gap and the electrical energy supplied to spark channel are discussed. The kinds of liquid used in the experiment are tap water, O. 1% and 1. 0% NaCl solutions. The results obtained are as follows : (1) Regardless of the kinds of liquid used, the resistance of the electrode gap decreases abruptly from the intrinsic resistance of the liquid to O.1-0.3 ohms after the breakdown of the liquid. After that, it varies with the cyclic variation of the discharge current. Also it has a tendency to increase with an increase of the electrode gap distance in all kinds of liquids.(2) Regardless of the kinds of liquid used, as the first minimum value increases in the cyclic variation of the resistance after the breakdown of the liquid, the electrical energy of a first-wave in the electric power wave becomes larger and the electrical energy supplied to spark channel is centered at the first-wave in the electric power wave.

Pulse discharges in gases under conditions of strong ionization by electrons

A review is given of investigations of pulsed high-pressure volume discharges excited by fast-electron beams. The following topics are discussed: classification of discharges; methods for calculating the current-voltage characteristics; analysis of the optimal ways of depositing energy in the gas in the volume stage of the discharge; discharge instability mechanisms and the corresponding experimental observations; applications of discharges. The results are given of calculations of the electric field in the cathode and anode regions, and also in the discharge column in the case of a spatially inhomogeneous ionication of the gap. It is shown that a stable volume flow of the current in molecular gases in which the specific deposited energy is 0.1–1 J/cm3 may be attained in a nonself-sustaining discharge and in a discharge with ionization multiplication. In both cases a spark channel appears in two stages: formation of sparkinitiating centers in the form of plasma regions with a higher density near the electrodes is followed by growth of the spark channel from such initiating centers. In some cases the spark channel growth can be described by the available mathematical models. Discharges in mixtures of rare gases with halogencontaining compounds, when electrons are lost mainly by capture by complex molecules, are considered separately. Applications of volume discharges in laser pumping, switching of pulsed currents, plasma chemistry, etc., are described.

Development of discharge in a system of electrodes situated on interface of liquid and solid dielectrics

The development of a discharge in the case where the electrodes are situated at the interface of solid and liquid dielectrics was investigated with the aid of an apparatus based on two photomultipliers and an electrolytic tank. In these conditions the discharge develops along a single pathway, which runs along the interface for part of its length and then passes into the solid dielectric. Possible factors that predetermine the pathway of the spark channel in the considered system of electrodes and dielectrics are examined.

Spark channel formation in helium-neon mixtures

Spark channel formation in helium-neon mixtures

Schlieren studies of impulse breakdown in air gaps

The initial expansion of the leader channel and its subsequent cooling have been investigated in 10-30cm rod-plane gaps in air using a high-speed schlieren technique. The channel was found to expand to a maximum value of 0.6 mm and a shock wave, associated with the initial rapid expansion, was observed. When breakdown occurred, the main spark channel always followed the track of the leader. Thermal time-constant calculations indicate that the neutral gas density along the track of the leader would be reduced for periods of ~100μs. As reillumination intervals in the case of the stepped leader are shorter than this, it is suggested that enhanced ionisation in the hot column would have to be included in any explanation of the mechanism governing the propagation of the stepped-leader discharge.

Spark Discharge: Application Multielement Spectrochemical Analysis

Spark discharge is shown to be a cyclic process of energy dissipation, with one spark in a time-connected train influenced by its relation to predecessor sparks. Spectroscopic instruments having temporal, spatial, and spectral resolution indicate that the light emission is highly ordered with cylindrical symmetry about the current-conducting spark channel. The favored spatial coincidence is between the channel and the most highly ionized and most excited species sampled from the cathode, with less ionized and less excited species emitting farther outward. Light absorption occurs to such an extent that there are full line reversals in excited states of magnesium ions, distant from the channel. Schlieren data indicate a toroidal structure in the postdischarge environment. Charge transfer, Penning ionization, and sensitized fluorescence are thought to be the chemical mechanisms responsible for the spectroscopic topography. Experiments in spectrochemical analysis based on the topography and designed for increased sensitivity, reduced matrix effects, and simpler spectra are discussed.

Spark channel formation in helium-neon mixtures

An image convertor is used to study the development of breakdown in a uniform field produced between two 5 cm plane electrodes. The discharge is simultaneously photographed with streak and still cameras. The formation and propagation of the ionization fronts are seen to move towards the cathode and the anode. Special attention is paid to the secondary avalanches that originate from the cathode and move towards the cathode ionizing wave. The densitometry of recordings gives rise to the characterization of the isodensity lines through which the authors obtain a detailed analysis of the spatio-temporal growth of light emission from the discharge.

Method of controlling the growth and formation of a system of parallel sliding spark channels in air at atmospheric pressure

A method of controlling the growth process of a sliding spark along the surface of a film dielectric by introducing emission centers from chemical compounds with a low electron work function into the surface is presented. It is shown that it is possible, by using this method, to produce a spark channel with sharp corners, such as a Z-shaped channel, and an ordered system of parallel channels, as well as a strictly rectilinear channel up to 2 m long. It is established that the rate of growth of the sliding spark is nonuniform. The mean rate is heavily dependent on the overvoltage, which is related to a reduction in the pauses in its growth.

Breakdown Phenomena in Point-to-Point Gaps

Measurements with multiple techniques delineating the complete sequence of events from the primary streamer to the formation of the spark channel for relatively small point-to-point gaps are reported. The pulsed potential with 0.4 µs rise time and 1800 µs decay time was applied to the point-to-point gap, of which length was changed from 6 to 14 cm. The light pulses were observed by 5 photomultipliers as well as current and potential wave. It is shown that the spark channel can be materialized through the following three processes: 1) initial process, in which the primary, the secondary, and the tertiary streamers develop from both point electrodes into gap, and the ionizing wave bridges the gap; 2) intermediate process, in which the dark period proceeds with ion and electron movement if the applied potential is around the mean breakdown potential, or in the other case, the ``leader'' develops from the anode with the help of ionizing waves; and 3) final process, in which the channel is highly ionized at first by the ionizing wave, then gas heating, to lead the formation of a spark channel. On the bases of the measurements of streamer speed and current, the ion and electron density were calculated. It is estimated that starting with the primary streamer with an electron density of 1012 cm-3, the electron density in the streamer channel is augmented by the ionizing wave up to 1012cm -3 when the gap is bridged by the ionizing wave.

High Speed Streak Techniques for Measuring the Growth of Spark Channels

AbstractThe manner of expansion of an electric spark channel is of interest in the under tanding of such phenomena as lighting. The development of a camera suitable for photography of this cxpansion is described, with some results for hydrogen.

Laser initiated spark development in an air gap

Spark development is studied by 20-nsec image converter photography. A diffuse and transparent prechannel bridges the gap from the top of the metal vapor jet, which has counterelectrode potential. The prechannel cuts off the development of the cone shaped jet with increasing gap voltage. The final breakdown is initiated by a z-axis, laser induced filament, which expands into the prechannel volume within less, similar10 nsec. This interval represents the final high current thermalization phase of the breakdown. Thermal expansion of the initial spark channel (Braginskii) follows.

Effects of sparks in TEA lasers

A mass spectrometer was used to determine the effect of spark discharges on sealed CO2 TEA laser mixtures. The carbon dioxide in the spark channel was completely dissociated, but the overall dissociation coefficient of the CO2 was less than that in a comparable distributed discharge. No oxides of nitrogen were found in concentrations which would lead to instability in the main discharge.

Breakdown process in rod-to-plane gaps with negative switching impulses

This paper reports with multiple techniques delineating complete sequence of events from the primary corona to the formation of the spark channel for large rod-to-plane air gaps with negative switching impulses. It is shown that starting with the primary corona the breakdown proceeds through the secondary corona and the intermittent appearance of the light flash to the final spark formation, qualitatively independent of the front time of applied impulses. The detailed analysis were made to each event, especially to the intermittent flash, with extremely high time-resolution techniques.

Untriggered Water Switching

Recent experiments indicate that synchronous untriggered multichannel switching in water will permit the development of relatively simple, ultra-low impedance, short pulse, relativistic electron beam (REB) accelerators. These experiments resulted in the delivery of a 1.5 MV, 0.75 MA, 15 ns pulse into a 2 ? line with a current risetime of 2 x 1014 A/sec. The apparatus consisted of a 3 MV Marx generator and a series of three 112 cm wide strip water lines separated by 2 edge-plane water-gap switches. The Marx generator charged the first line in < 400 ns. The first switch then formed 5 or more channels. The second line was charged in 60 ns and broke down with 10 to 25 channels at a mean field of 1.6 MV/cm. The closure time of each spark channel along both switches was measured with a streak camera and showed low jitter. The resulting fast pulse line construction is simpler and should provide considerable cost savings from previous designs. Multiples of these low impedance lines in parallel can be employed to obtain power levels in the 1014 W range for REB fusion studies.

1343. A hypersonic interpretation of the development of the spark channel is gases: M M Kekez and P Savic, J Phys D: Appl Phys, 7 (4), 1974, 620–628

1343. A hypersonic interpretation of the development of the spark channel is gases: M M Kekez and P Savic, J Phys D: Appl Phys, 7 (4), 1974, 620–628

A hypersonic interpretation of the development of the spark channel in gases

A new mechanism for the development of the spark channel in gases is proposed, in which it is assumed that the shock wave, known to exist after the channel is complete (Freeman and Craggs 1969), actually develops while the channel is still in formation. The paraboloidal shock is thought to derive its power from the transfer of energy only from the electrons pre-heated in front of the developing channel by field concentration to the neutrals and ions behind it (detonation wave mechanism). Neutral gas in front of the channel is considered to be cold. Analytical expressions for the velocity and position of the detonation wave are derived, and experiments by the authors and by previous workers tend to confirm the mechanism for certain low-density gases. The `radiation overshoot' characteristic of shock transmission through some gases has been observed, suggesting that a shock indeed exists prior to completion of the spark channel.

Nanosecond High Radiance Standard Source

Spectral radiances R(lambda) and luminances B in the center axis of high density 8-20-nsec spark channels as produced by Nanolite sources were systematically studied by comparison with the emission from a calibrated dc-carbon arc. The maximum value of R(lambda) was 53 W cm(-2) A(-1) sr(-1) in 1 atm of air at 4850 A with a breakdown voltage of 2.5 kV when the Nanolite (C = 2.9 nF, L = 2 nH) was charged slowly. This value increased to over 70 W cm(-2) A(-1) sr(-1) when the source was pulse charged. Saturation values of R(lambda) were 38 W cm(-2) A(-1) sr(-1) in 3 atm of argon and 60 W cm(-2) A(-1) sr(-1) in 32 atm of helium (slow charged). All values were reproducible to within 10%, the absolute error being estimated to be smaller than +/-20%. Equivalent maximum blackbody temperatures of the channel surface in air were 62,000 K (pulse charged) and 38,000 K in argon and 54,000 K in helium (slowly charged). Maximum spectral radiant intensity in 1 atm of air was 0.18 W A(-1) sr(-1) at 4850 A (pulse charged).

Determination of temperatures of high current spark channels †

Abstract This paper reviews the experimental methods of determination of temperatures of recovering high current, spark channels in different, stages of the afterglow. Host of the experiments described were performed at atmospheric pressure or pressures sufficiently high that thermal equilibrium among all species is achieved so that the electron, ion and gas temperatures are considered equal. In the early afterglow (within approximately 50 μs after spark initiation) when luminosity of the spark is sufficient, spectroscopic methods are employed to determine channel temperatures. In the intermediate stage of the afterglow (50 μs-500 μs) when luminosity is not sufficient for optical techniques to be employed, measurement, of electrical conductivity or sound or shock-wave velocity can be used to derive the gas temperature. In the late afterglow (1 ms after spark initiation) when the gas has recovered to a neutral state, measurements of reignition voltage together with Poschen's law arc used to derive gas temp...

Temperature determination of a recovering spark channel from conductivity probe measurements†

The temperature of a recovering high current (25 kA) spark channel in the afterglow period from 0·2 ins to 5 ms after spark initiation has been derived from conductivity probe measurements. The model of a weakly ionized gas is used for calculating the conductivity of the recovering plasma. Temperature decay was obtained for an air spark formed between 6·4 mm tungsten electrodes spaced at 5 mm apart at 1 torr pressure at radial distances of 2 cm, 5 cm and 10 cm from the centre of the spark channel. The results are compared with temperature recovery characteristics obtained in similar air spark channels at atmospheric pressure.

Radial recovery of high current spark channels†

Abstract Radial recovery characteristics of high current (40 kA) spark channels in air at atmospheric pressure have been measured using re-ignition voltage technique and pressure probe technique. In the case of re-ignition voltage measurement the temperature was derived by assuming the validity of Paschen's law at the Into recovery period, whereas in the case of pressure probe, measurements of local sound velocity were used to derive the gas temperature. The derived temperature profiles of the spark channel under similar experimental conditions show a uniform temperature profile near the centre of the channel. The boundaries of the spark channels measured by these techniques are not well defined as compared with previous results using the shock-wave Schlieren technique. Discrepancies of results obtained by the present two different sensing techniques are found and are explained as due to the large cooling effects of the re-ignition voltage probes.