Underwater pulsed spark discharge influenced by the relative position between the top of a pin electrode and an insulating tube

Needle-to-plane geometry has been widely investigated and used in underwater pulsed discharges.The position relationship between the needle tip and insulation layer significantly affects the discharge patterns.We carried out experiments on underwater pulsed discharge with the needle tip protruding from,recessing into,and flushing with the insulating tube.The results are as follows.First,underwater pulsed discharge has a strong randomness under the experimental conditions.Different discharge patterns appeared under the same experimental environment.Second,recession into the insulator surface led to a higher probability of occurrence but a lower strength of spark discharge than protrusion.Third,between the needle tip protruding from and recessing into the insulation material,the average speed of propagation of underwater pulsed spark discharge decreased by an order of magnitude.The study shows that the optimum length of needle tip protruding from the insulation layer is 1 mm to obtain a strong underwater pulsed spark discharge.

The Optical Diagnosis of Underwater Positive Sparks and Corona Discharges

In this paper, two types of underwater discharges, spark discharge and corona discharge, are investigated by optical diagnosis using a high speed framing camera （HSFC） with the framing time within nanoseconds under the same experimental conditions. In order to capture the photographs of streamer propagation, the influence of the randomicity of the prebreakdown duration is taken into consideration. By increasing the conductivity of water, the randomicity reduces effectively. Experimental results show that, for a spark discharge, the process can be separated into three stages： the generation and propagation of a streamer, the generation and expansion of the discharge channel, and the development and annihilation of the plasma. The streamers do not directly move to the opposite electrode, but form a bush-like figure. With the increase of the number of branches, the velocity of streamer propagation slows down. The trajectory of the initial channel between electrodes is not straight. However, with the channel expanding, its shape transforms into a straight column. For a corona discharge, there are two stages： the generation and propagation of a streamer, and the stagnation and annihilation of the streamer. The initial streamer in a corona discharge is generated later than in a spark discharge. The forms of streamers for both kinds of discharge are similar; however, streamers generated by a corona discharge propagate with a slower velocity and the number of branches is less compared with a spark discharge. When the energy injection stops, the luminescence of plasma inside the discharge channel （spark discharge） or streamers （corona discharge） becomes weaker and weaker, and finally disappears.

Combination of spark discharge and nanoparticle-enhanced laser-induced plasma spectroscopy

A combination of spark discharge and nanoparticle-enhanced laser-induced plasma spectroscopy is investigated.Depositing Au nanoparticles at the surface of a brass target can enhance the coupling of the target and the laser.More atoms in the brass sample are excited.As a secondary excitation source,spark discharge reheats the generated plasma,which further amplifies the enhancement results of nanoparticles.The spectral intensity with the spark discharge increases more obviously with nanoparticle concentration increasing than without the spark discharge.Also,plasma temperature and electron density are calculated by the Boltzmann plot and Stark broadening.The changes in the plasma temperature and electron density are consistent with the spectral emission changes.

Photographic Study on Spark Discharge Generated by a Nanosecond High-Voltage Pulse over a Water Surface

Spark discharge generated by a nanosecond positive high-voltage pulse over a water surface at atmospheric pressure in air was studied using a high speed camera system. Faint streamers form near the pin electrode and propagate towards the water surface. The time for the streamer propagating across the air gap was estimated to be about 50 ns to 60 ns with a propagation velocity of ～1.3 × 10^5 m/s. It was found that the water conductivity and the gap distance have no significant effect on the propagation velocity of the streamer. After the streamers touch the water surface a brilliant spark channel forms across the air gap. The maximum diameter at the middle of the spark channel is about 1 mm, and approximately contracts with a radical velocity of about 2.0× 10^3 m/s. No significant dependence of the maximum diameter and decay velocity of the spark channel on the water conductivity and the gap distance were recognized in the present work. The maximum conduction current for a gap distance of 5 mm is significantly larger than that for a gap distance of 10 mm at the same water conductivity, and shows an increasing tendency with increasing water conductivity for a fixed gap distance. Based on the maximum conduction current, the effect of water conductivity and gap distance on the electron density of the spark discharge plasma at the peak current was investigated. Within the range studied, the electron density in the spark channel is about 10^15 cm^-3 and increases with water conductivity at a fixed gap distance.

Measurement of Temporally and Spatially Resolved Electron Density in the Filament of a Pulsed Spark Discharge in Water

The temporally and spatially resolved optical emission spectrum of Hα of a pulsed spark discharge in water was experimentally measured. The temporally and spatially resolved electron densities, along the radial direction of the spark filament, for a pulsed spark discharge in water with a conductivity of 100 μS/cm were investigated. The electron density in the spark filament was found to be in the 10^（18）/cm^3 order of magnitude. The highest electron density was measured at the primary stage of the spark filament, and it decreased with time. The radial distribution of electron density increased from the center to the edge of the spark filament.

Circle Points Discharge Tube Current Controller

Circle points discharge tube current controller is a new type device to limit the output of high voltage discharge current. Circle points uniform corona discharge to form air ionization current in the discharge tube. On the outside, even if the discharge electrode is spark discharging or the two discharge electrodes are short circuited, the air ionization current in the tube remains within a stable range, and there is no spark discharge. In this case, when the discharge current only increases slightly, the requirement to limited current is obtained. By installing the controller at a discharge pole with a small power but high voltage supply, we can realize the shift between the continuous spark line discharge and corona discharge. This provides a new simple device for spark discharge research and is a supplement to the Townsend discharge experiment.

A Green Process for High-Concentration Ethylene and Hydrogen Production from Methane in a Plasma-Followed-by-Catalyst Reactor

A green process for the oxygen-free conversion of methane to high-concentration ethylene and hydrogen in a plasma-followed-by-catalyst （PFC） reactor is presented. Without any catalysts and with pure methane used as the feed gas, a stable kilohertz spark discharge leads to an acetylene yield of 64.1%, ethylene yield of 2.5% and hydrogen yield of 59.0% with 80.0% of methane conversion at a methane flow rate of 50 cm^3/min and a specific input energy of 38.4 kJ/L. In the effluent gas from a stable kilohertz spark discharge reactor, the concentrations of acetylene, ethylene and hydrogen were 18.1%, 0.7% and 66.9%, respectively. When catalysts Pd-Ag/SiO2 were employed in the second stage with discharge conditions same as in the case of plasma alone, the PFC reactor provides an ethylene yield of 52.1% and hydrogen yield of 43.4%. The concentrations of ethylene and hydrogen in the effluent gas from the PFC reactor were found to be as high as 17.1% and 62.6%, respectively. Moreover, no acetylene was detected in the effluent gas. This means that a high concentration of ethylene and oxygen-free hydrogen can be co-produced directly from methane in the PFC reactor.

Oxygen-Free Conversion of Methane to Ethylene in a Plasma-Followed-by-Catalyst (PFC) Reactor

Oxygen-free conversion of methane to ethylene was investigated in a two-stage plasma-followed-by-catalyst （PFC） reactor. In the absence of catalyst, pulsed spark discharges and pulsed corona discharges were compared for methane conversion. The results showed that methane was mainly converted to acetylene, but pulsed spark discharges exhibited distinct advantages over the pulsed corona discharges in methane conversion. Thereby, pulsed spark discharges were employed and followed by Ag-Pd/SiO2 catalyst for achieving ethylene as a target product in the PFC reactor. Using the PFC reactor, a steady single-pass ethylene yield of 57% was obtained at a rate of methane conversion of 74%.

Role of β Phase during Microarc Oxidation of Mg Alloy AZ91D and Corrosion Resistance of the Oxidation Coating

Selective growth of oxidation coating was observed on Mg alloy AZ91 D when this alloy was treated by microarc oxidation （MAO） technique, and then the role of intermetallic phase Mg17AI12 （β phase） during MAO was investigated. Corrosion resistance and anti-corrosion mechanism of the MAO coating were also studied. Optical microscopy and scanning electron microscopy （SEM） were used to characterize β phase and coating microstructure. Corrosion properties of the coated alloy were studied by potentiodynamic polarization test and electrochemical impedance spectroscopy （EIS） in 3.5% NaCI solution. Results showed that sparking discharge preferentially occurred on ^-Mg phase rather than on β phase at the early stage of MAO; however selective growth of the coating disappeared gradually with the increasing oxidation time and β phase would not further inhibit coating growth at the prolonged stage of MAO. MglTAI12 phase ultimately was unable to destroy the integrity and continuity of MAO coating. The MAO coating could restrain charge transfer process and then greatly enhance corrosion resistance of AZ91D alloy. Sealing treatment of MAO coating by stearic acid could further improve the corrosion resistance of AZ91 D alloy.

Effect of Initial Oxide Film on the Formation and Performance of Plasma Electrolytic Oxidation Coating on 7075 Aluminum Alloy

In the early stage of plasma electrolytic oxidation(PEO),the quick formation of the initial oxide film on the surface of the aluminum substrate is necessary for the subsequent discharge spark process.Furthermore,the compactness of the initial oxide film greatly affects the quality of the PEO coating,but the related mechanisms are not investigated in detail.In this paper,the status of the initial oxide film was adjusted by adding the(NaPO)into the based electrolyte,and then the effect of initial oxide film on the formation of the PEO coating was compared.Microstructure and chemical composition were analyzed by scanning electron microscope,transmission electron microscopy and X-ray photoelectron spectrometer.The compactness of the initial oxide film was characterized by Mott-Schottky plots.The corrosion resistance was measured by electrochemical impedance spectroscopy.The results show that the addition of(NaPO)can promote the co-deposition of phosphide compound into the initial oxide film and improve the film compactness,which is beneficial for the more uniform spark discharge in the later stage.As a result,the addition of(NaPO)is helpful to obtain PEO coating with better performance.