The characteristics of microplasma discharge propagation over the titanium surface covered with a thin oxide film

The propagation and structure of a microplasma discharge initiated in vacuum by a pulsed plasma flow with a density of 1013 cm–3 on the surface of a titanium sample covered with a thin continuous dielectric titanium oxide film with a shickness of 2–6 nm were studied experimentally when the electric current of the discharge changes from 50 A to 400 A. It was found that the microplasma discharge glow visually at the macroscale has a branched structure of the dendrite type, which at the microscale consists of a large number of brightly glowing “point” formations – cathode spots localized on the metal surface. The resulting erosion structure on the titanium surface is visually “identical” to the structure of the discharge glow and consists of a large number of separate non-overlapping microcraters with characteristic sizes from 0.1–3 μm, which are formed at the sites of localization of cathode spots at distances of up to 20 μm from each other. It was found that the propagation of a single microplasma discharge over the titanium surface covered with a thin oxide film a thickness of 2–6 nm occurs at an average velocity of 15–70 m/s when the amplitude of the discharge electric current changes in the range of 50–400 A. In this case, the microplasma discharge propagation on the microscale has a “jumping” character: the plasma of “motionless” burning cathode spots, during their lifetime 1 μs, initiates the excitation of new microdischarges, which create new cathode spots at localization distances of 1–20 μm from the primary cathode spots. This process repeated many times during a microplasma dis- charge pulse with a duration from 0.1 ms to 20 ms.

Gas discharge sustained by powerful THz and sub-THz gyrotrons in the mixtures of noble gases with nitrogen

The discharge propagation velocity towards electromagnetic radiation of sub-THz and THz bands was measured in various noble gases (argon, krypton) mixtures with nitrogen in the wide pressure range (0.1 – 2 atm) for various field intensities into the focal spot (from dozen of kW/cm2 to several MW/cm2). In the experimental setups two different gyrotrons were used. In case of 263 GHz it was CW gyrotron with power up 1 kW, in case of 670 GHz – pulsed gyrotron (20 μs) with power up to 40 kW. In both cases the focusing system provided the size of the focal spot of (2–3)·λ, which ensured the investigation of discharge phenomena in a wide pressure range (0.1 – 2 atm). In both cases discharge appeared in the focal spot spread towards heating radiation into the area with the field intensity much less than one in the focal spot. Velocity of the discharge propagation was measured by using photos from speed camera with small exposure (down to 20 ns) and streak camera. It was demonstrated that discharge velocity increase along with pressure decrease and drops with electric field decrease as it moves away from the focal spot.

Extrusion of a nanosecond surface discharge plasma near a dielectric ledge

The presence of a dielectric ledge along the pulse discharge propagation led to a redistribution of the pulsed surface (plasma sheets) discharge glow. Discharge glow on the surface without the ledge, was uniform and lasted no more than 200 ns. Two plasma channels with increased glow intensity were observed near the rectangular ledge placed in the discharge area. The duration of these longitudinal plasma channels increased and lasted for about 0.9 μs (at a voltage of 25 kV and a density of 0.03 – 0.18 kg/m3 ). A nine-frame nanosecond camera recorded the evolution of the plasma glow. The dynamics of the flow induced by the pulse surface discharge was recorded using a high-speed shadow imaging during 40-50 μs after the ignition of the discharge.

Modelling Surface Electric Discharge Propagation on Polluted Insulators under AC Voltage

In this contribution, a mathematical model allowing for the prediction of the AC surface arc propagation on polluted insulators under non-uniform electric field is proposed. The approach is based on the experimental concept of Claverie and Porcheron. The proposed model, which makes it possible to reproduce the surface electric discharge, includes a condition for arrest of the propagating discharge. The electric field at the tip of the discharge is the key parameter governing its random propagation. A finite element approach allows for mapping of the electric field distribution while the discharge propagation process is simulated in two dimensions. The voltage drop along the arc discharge path at each propagation step is also taken into account. The simulation results are validated against experimental data, taking into account several electro-geometric parameters (distance between electrodes, pollution conductivity, radius of high-voltage electrode, length of the plane electrode). Good agreement between computed and experimental results were obtained for various test configurations.

Electromagnetic and Thermal Phenomena Modeling of Electrical Discharges in Liquids

Electrical discharges in liquids have received lots of attention with respect to their potential applications in various techniques and technical processes. Exemplary, they are useful for water treatment, chemical and thermal processes acceleration, or nanoparticles production. In this paper the special utility of discharges for cold pasteurization of fruit juices is presented. Development of devices for its implementation is a significant engineering problem and should be performed using modeling and simulation techniques to determine the real parameters of discharges. Unfortunately, there is a lack of clear and uniform description of breakdown phenomena in liquids. To overcome this limitation, new methods and algorithms for streamers propagation and breakdown phase analysis are presented in the paper. All solutions were tested in “active area” in the form of liquid material model, placed between two flat electrodes. Electromagnetic and thermal-coupled field analysis were performed to determine all the factors that affect the discharge propagation. Additionally, some circuit models were used to include the power source cooperation with discharge region. In general, presented solutions can be defined as universal and one can use them for numerical simulation of other types of discharges.