Spark Breakdowns Research Articles

Characteristics of cold atmospheric plasma source based on low-current pulsed discharge with coaxial electrodes

This work investigates the characteristics of the gas discharge system used to create an atmospheric pressure plasma flow. The plasma jet design with a cylindrical graphite cathode and an anode rod located on the axis of the system allows to realize regularly reproducible spark breakdowns mode with a frequency ∼ 5 kHz and a duration ∼ 40 μs. The device generates a cold atmospheric plasma flame with 1 cm in diameter in the flow of various plasma forming gases including nitrogen and air at about 100 mA average discharge current. In the described construction the cathode spots of individual spark channels randomly move along the inner surface of the graphite electrode creating the secondary plasma stream time-average distributed throughout the whole exit aperture area after the decay of numerous filamentary discharge channels. The results of the spectral diagnostics of plasma in the discharge gap and in the stream coming out of the source are presented. Despite the low temperature of atoms and molecules in plasma stream the cathode spots operation with temperature of ∼ 4000 °C at a graphite electrode inside a discharge system enables to saturate the plasma by CN-radicals and atomic carbon in the case of using nitrogen as the working gas.

Electron beam formation in an anomalous glow discharge

Scaling parameters that account for the efficient formation of electron beams in sources based on an anomalous glow discharge are considered, and the processes leading to the violation of scaling laws are analyzed. Estimates show that the working pressure range in such sources can be increased significantly. The results obtained can be used to optimize discharge regimes with required electron beam parameters. It is demonstrated that sources with plane-parallel continuous electrodes can operate at pressures as high as 100 Torr (which is one to two orders of magnitude higher than that in similar existing electron beam sources), provided that parasitic spark breakdowns at the insulator near the discharge cathode are suppressed. The sources designed can be used for direct excitation of laser working media or their preionization by soft X-rays.

Optical Properties of ZrO2–Containing Anodic Coatings on Aluminum

Anodic oxide coatings (AOC) formed at the potentials of spark breakdowns in aqueous solutions of potassium hexafluorozirconate (K2ZrF6) contain up to 36 wt.% of Zr. The modification of ZrO2 in AOC is tetragonal. The reflection coefficients of AOC have been measured in the range 430–670 nm. After UV irradiation for a few hours the reflection coefficient decreases by several percent. The treatment of AOC in a solution containing F− and [Fe(CN)]3− ions leads to a change in the aluminum and zirconium concentrations in the surface layer and to an increase of the AOC reflectivity.

An incremental analysis of spark paths in air using 3-dimensional image processing

A video camera has been used to record the path of spark breakdowns across a positive point/plane air-gap. By using a prism as a mirror, two images were obtained from which the path could be determined in three dimensions by subsequent computer processing and analysis. Images were analyzed for each of five angular positions of the point electrode's axis, the gap remaining constant. The fractal dimension of each path was determined and found to increase slightly with the inclination of the pointed electrode. Each spark appeared to have propagated semi-randomly in a direction which included a 'memory' factor, that is, it tended to follow the previous direction while moving monotonically towards the opposite electrode. This was confirmed by an analysis of 400 spark paths which showed that each section has a specific relationship to the previous section, but that there is a wide variation around this mean which leads to the characteristic shape of a spark path. Simulations based on these results yield spark tracks which look similar to those measured and have similar fractal dimensions. These analyses strongly suggest that the development of spark paths in air is by successive steps and in a direction suggested by an angular probability distribution which is related to the field at the tip of the propagating leader.

An analysis of spark paths in air using 3-dimensional image processing

A video camera has been used to record the path of spark breakdowns across a positive point/plane air gap. By using a prism as a mirror, two images were obtained from which the path could be determined in three dimensions by subsequent computer processing and analysis. Some 80 images were analyzed for each of five angular positions of the point electrode's axis, the gap remaining constant. The fractal dimension of each path was determined and found to increase slightly with the inclination of the pointed electrode. Each spark appeared to have propagated semi-randomly in a direction which included a 'memory' factor, i.e. it tended to follow the previous direction while moving monotonically towards the opposite electrode. This was confirmed by considering a projection of the spark path onto the plane electrode which was found to form a semi-random walk in which each step was influenced by the previous step. These, and other analysis, suggest that the development of spark paths in air by successive steps is in a direction suggested by an angular probability distribution related to the field at the tip of the propagating leader.

Separation of Surface and Bulk Gases in Contact Materials of Vacuum Switches-Determination of Gases and Effect on Vacuum Breakdown

Meaningful analysis for very small quantities of gases in contacts can be obtained only by the method which permits differentiating between surface and bulk gases. The graphite capsule method and copper bath method in vacuum fusion analysis were compared, and the surface area variation method and two-stage gas extraction method were investigated for the separation of surface and bulk gases. As a result of these investigations, the surface and bulk gas content of the copper-tellurium alloy can be determined by the surface area variation method. The bulk oxygen content of the ingot, forged- and vacuum-refined copper-tellurium alloy is 3.8 parts per million (ppm), 1.7 ppm, and 0.1 ppm, respectively. The surface oxygen content, however, is conversely 1.5 µg/cm2, 2.4µg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 3.7µg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. In high vacuum, gases are released from the electrodes as the voltage is applied between the electrodes. The released gases increase exponentially whether the spark breakdowns occur or not. It is suggested that the released gas at the spark breakdown corresponds to surface gas, which is considered as water vapor mostly absorbed on the electrode. After the spark breakdowns occur repeatedly, water vapor from the surface layers is depleted, namely, the surface conditioning takes place. Therefore, the released gas without spark breakdown corresponds to the bulk gas, which is mostly hydrogen.