Anode Spot Research Articles

Determination of Cr Density After Current Zero in a High-Current Vacuum Arc Considering Anode Plume

Results of broadband absorption spectroscopy in high-current vacuum arcs after current zero (CZ) are presented. Vapor density of atomic Cr is determined within 3 ms after CZ in cases with an anode plume which appears at about 400 $\mu \text{s}$ before CZ and after extinction of anode spot type 2. The anode plume is a cloud of metal vapor which develops on the anode after the extinction of one specific high-current anode mode, namely, anode spot type 2. An ac current pulse of about 7 kA from a high-current generator with operation frequency of 100 Hz was applied. Cr I resonance lines at 425.43, 427.78, and 428.97 nm were chosen for the analysis. The temporal evolution of the ground state density of Cr atoms within first 3 ms after CZ starts with about $2.5 \times 10^{18}~\text{m}^{-3}$ and decreases almost linearly with $0.65 \times 10^{18}~\text{m}^{-3}$ /ms. The Cr I densities in cases with and without anode plume corresponding to anode spot type 2 and 1, respectively, are compared at 100 $\mu \text{s}$ after CZ. In cases with anode plume, the metal vapor density is about 60% higher than in cases without plume, although the interrupting current is the same. The general behavior of the copper component is expected to be very similar to that of chromium.

Time and space resolved spectroscopic investigation during anode plume formation in a high-current vacuum arc

This paper presents time and space resolved results of spectroscopic measurements during the formation of an anode plume in the late current pulse phase of a high-current vacuum arc. The formation of the anode plume is investigated systematically based on the occurrence of high-current anode spots, depending on gap distance and current for AC 100 Hz and CuCr7525 butt contacts with a diameter of 10 mm. The anode plume is observed after the extinction of anode spot type 2 in which both the anode and cathode are active. It is concluded from the spatial profiles of the atomic and ionic radiation, parallel and perpendicular to anode surface, that the inner part of the plume is dominated by Cu I radiation, whereas a halo of light emitted by Cu II covers the plume. The radiation intensity of Cu III lines is quite low across the whole anode plume. Upper level excited state densities corresponding to Cu I lines at 510.55, 515.32, 521.82, 578.21 nm are determined. The temporal evolution of the resulting excitation temperature in the centre of the plume varies from 8500 K to 6000 K at 500 µs to 100 µs before current zero, respectively. The density calculated for Cu I at position in the plume is in the range of 1–5 × 1019 m−3.

Diagnostics of Various Phenomena in LV Devices Under Real Switching Conditions

The article deals with issues to be tackled when performing experiments with low voltage devices under real switching conditions and subsequently discusses various phenomena in an experimental device. The first part describes optimum setting of diagnostic equipment - mainly for optical diagnostic methods. The second part describes some phenomena encountered during switching process under real switching conditions - arc roots movement (cathode and anode spots). These phenomena are not only important for experimental study itself but also form necessary input data for numerical models and their validation.

Gas flow influence on streamer-to-leader transition in surface barrier discharge in air at atmospheric pressure

The experimental results on study of a sinusoidal surface barrier discharge (SBD) in airflow at different velocities are presented. Influence of gas flow velocity and its orientation on plasma structure of SBD is established and the conditions providing transition of SBD from the streamer mode to the mode with surface leaders (SL) are found out as well. It is shown the formation of the SL in SBD happens owing to thermal effects associated with a local gas heating in both the bright current spots disposed at the sharp edge of high-voltage electrode and thin current channels (streamers) originated from these spots. It was revealed that gas flow with a high velocity directed against the propagation of surface streamers leads to destruction of the bright anode spots and elimination of the SL (but not the streamers) on a barrier surface. However the gas flow with the same velocity but opposite direction leads to regular disposition of the bright current spots at the edge of high-voltage electrode and regular spatial structure of the thin current channels (streamers and leaders) originated from the current spots. It was found out that gas flow of any orientation leads to diminishing the gas temperature in the plasma channels compared to that in SBD in gas at rest.

Emission Spectroscopy During High-Current Anode Modes in Vacuum Arc

A vacuum interrupter reaches its interruption limit once high-current anode phenomena occur. High-current anode modes lead to an increase of the anode surface temperature and an increased generation of metal vapor, which may result in a weakening of the dielectric recovery strength after current zero. In this work, different discharge modes in a vacuum arc for AC 50 Hz including diffuse, footpoint, anode spot type 1 and type 2, and anode plume are investigated. Electrodes made of CuCr7525 with diameter of 10 mm are used. The final gap length is about 20 mm. Time and space resolved optical emission spectroscopy is used to examine the temporal and spatial distribution of atomic and ionic copper lines. The distribution of atomic and ionic lines parallel and perpendicular to the anode surface is investigated. Radiator density is also determined for CuI, CuII, and CuIII near the anode surface.

Determination of Cr Density in the Active Phase of a High-current Vacuum Arcs

Melting and evaporation of the anode surface strongly influence the interruption capability of vacuum circuit breakers, because they lead to injection of atomic vapour into the inter-electrode gap. Determination of the vapour density and its dynamics with respect to different anode phenomena is therefore of great importance. Results of Cr density measurements in a high-current vacuum arc by using broadband absorption spectroscopy are presented. The vapour density of atomic Cr is determined after the formation of anode spots as well as close to the current zero. Cr I resonance lines at 425.43 nm have been used for the analysis. An AC current pulse with maximum value of 7 kA and a frequency of 100 Hz is applied to a vacuum arc between two cylindrical butt electrodes made of CuCr7525 with a diameter of 10 mm. The high-current anode modes are observed by means of high-speed camera imaging. The temporal evolution of the Cr ground state density is presented and discussed.

Impact of Different Vacuum Interrupter Properties on High-Current Anode Phenomena

This paper presents the impact of current waveform and frequency on the formation of high-current anode phenomena in a vacuum interrupter experiment. Different waveforms including the alternative current pulses of 50, 180, and 260 Hz and direct current pulses of 5 and 10 ms are compared. The impact of different opening times and contact speeds on the high-current anode mode formation is investigated. The results show that both instantaneous current and total transferred charge are important in the formation of high-current anode modes. Therefore, the arcing time has a strong influence. Two types of anode spot modes with different electrical and optical characteristics are also observed. The transitions between different high-current modes are examined systematically, resulting in existence areas dependent on threshold current and gap length. The latter are determined for different contact materials including Cu, CuCr7525, and CuCr50 and different contact diameters.

Fundamental investigation of dry electrical discharge machining (DEDM) by optical emission spectroscopy and its numerical interpretation

Dry electrical discharge machining (DEDM) has been developed as an alternative manufacturing process to the traditional EDM in liquid dielectric media. The absence of the liquid dielectric allows DEDM to be performed by simpler and environmentally friendlier machines. The erosion in DEDM mainly occurs due to the bombardment of the workpiece electrode surface by charged particles produced by micro electric discharges. Thus, the understanding of the fundamental properties of the micro plasma is necessary to explain the erosion mechanisms in this process. Optical emission spectroscopy of DEDM single discharges and its numerical interpretation by emission spectra simulation are developed in the present work. The hypothesis of plasmas in local thermal equilibrium (LTE) is developed, whereas the formation of an electron beam in non-LTE plasmas is also considered and briefly introduced. The simulations show that large amount of different ionic species is produced from the anode workpiece material, and the estimated electron temperature profile is peaking at the plasma centre. Moreover, hot anode spots formed on the workpiece surface due to the plasma-material interactions seem to be considerably smaller than the total plasma diameter and the respective eroded crater. These characteristics indicate that DEDM produces discharges similar to anode dominated vacuum arcs, which present properties very different from EDM discharges in liquid dielectric.

Video Spectroscopy of Vacuum Arcs During Transition Between Different High-Current Anode Modes

This paper presents spectroscopic results of high-current anode phenomena in a vacuum arc discharge between CuCr electrodes. AC (alternative current) 50-Hz and 10-ms pulsed dc (direct current) are applied as interrupting current. Time and space resolved optical emission spectroscopy (video spectroscopy) is used to examine the temporal and spatial distribution of different atomic and ionic copper lines. During the transition from low-current mode to different high-current modes, including footpoint, anode spot, and intense arc mode, the intensity of Cu I, Cu II, and Cu III line radiation is examined near the anode, the cathode, and in the interelectrode gap. The results show that during the formation of anode spot and intense mode the intensity and the distribution of all lines change noticeably in the different spectral regions. In fact, higher ionization states represent the arc dynamics behavior during transition to high-current anode modes. Significant differences have been found, for example, in the spatial structure of Cu II and Cu III lines in the anode spot mode. The results for Cu I lines indicate an active role of atoms together with the ions in different charge states in high-current anode modes. The impact of threshold current and transferred charge of the formation of high-current anode modes in case of ac and pulsed dc is also investigated regarding the intensity of copper lines near the anode.

Dynamics of the spatial structure of pulsed discharges in dense gases in point cathode−plane anode gaps and their erosion effect on the plane electrode surface

The dynamics of the spatial structure of the plasma of pulsed discharges in air and nitrogen in a nonuniform electric field and their erosion effect on the plane anode surface were studied experimentally. It is established that, at a nanosecond front of the voltage pulse, a diffuse discharge forms in the point cathode–plane anode gap due to the ionization wave propagating from the cathode. As the gap length decreases, the diffuse discharge transforms into a spark. A bright spot on the anode appears during the diffuse discharge, while the spark channel forms in the later discharge stage. The microstructure of autographs of anode spots and spark channels in discharges with durations of several nanoseconds is revealed. The autographs consist of up to 100 and more microcraters 5–100 μm in diameter. It is shown that, due to the short duration of the voltage pulse, a diffuse discharge can be implemented, several pulses of which do not produce appreciable erosion on the plane anode or the soot coating deposited on it.

The Influence of the Current Density in the Open Welding Arc Active Spots on Cathode and Anode Voltage Breakdowns

Surfacing in gas-shielded as well as by the open arc burning between consumable electrodes, is usually made in reverse polarity. It is connected with the stable burning of arc and the minimum spatter of electrode metal. In this case, the cathode spot of the welding arc is on the plate, and the anode spot is on the wire end. These conditions of the arc spots existence affect the processes proceeding in the cathode and anode areas of the arc. The article considers the influence of the arc active spots formation conditions and its modes on the cathode and anode voltage breakdowns.

<i>In-Situ</i> Corrosion Monitoring of Scratched Epoxy Coated Carbon Steel in Saturated Ca(OH)<sub>2</sub> with or without 3% NaCl by Scanning Electrochemical Microscopy and Electrochemical Impedance Spectroscopy

The present work is investigated the in-situ monitoring of local corrosion process of scratched epoxy coated carbon steel in saturated Ca(OH)2 with and without 3% NaCl using SECM and correlated with EIS. The results obtained from EIS analysis showed that the corrosion resistance of scratched epoxy coated carbon steel decreases in Cl- containing solution as the increase in wet/ dry corrosion cycles. This was indicated by decrease in film resistance (Rf) and charge transfer resistance (Rct), while the coated steel maintain the resistance values in saturated Ca(OH)2, most of which recovered after drying. The corrosion process was monitored using SECM by setting the tip potential at -0.70 V vs Ag/AgCl, where the consumption of dissolved oxygen occurred at the surface of test sample. The consumption of dissolved oxygen current (I’oxy-c) values was increased during the immersion in a solution with 3% NaCl. However, in wet/dry corrosion cycles, I’oxy-c was decreased due to the coverage of hydroxides/oxides at scratch area which suppressed the consumption of dissolved O2. It was found that the continuous decrease in corrosion was mainly attributed to continuous formation of corrosion products at anodic spots.

Feasibility study on high current ion beam extraction from anode spot plasma for large area ion implantation

In this paper, we have investigated the feasibility of the high current beam extraction from anode spot plasma as an ion source for large area ion implantation. Experiments have been carried out with the ambient plasma produced by inductive coupling with radio-frequency (RF) power of 200 W at the frequency of 13.56 MHz. Anode spot plasmas are generated near the extraction hole of 2 mm in diameter at the center of a bias electrode whose area exposed to the ambient plasma can be changed. It is found that the maximum ion beam current is extracted at the optimum operating pressure at which the area of bias electrode exposed to ambient plasma is fully covered with the anode spot plasma whose size is dominantly determined by the operating pressure for given gas species. It is also observed that the extracted ion beam current increases nonlinearly with the bias power due to the changes in size and shape of the anode spot plasma. With the well-established anode spot plasma operating at the optimum gas pressure, we have successfully extracted high current ion beam of 6.4 mA (204 mA/cm2) at the bias power of 22 W (∼10% of RF power), which is 43 times larger than that extracted from the plasma without anode spot. Based on the experimental results, criteria for electrode design and operating pressure for ion beam extraction from larger extraction aperture are suggested. In addition, the stability of anode spot plasma in the presence of ion beam extraction through an extraction hole is discussed in terms of the particle balance model.

Electrical and plasma flow characteristics of a segmented plasmatron operating with mixture of gases

The electric characteristics of a segmented plasmatron and the results of optical emission spectroscopy of Ar-air, N2, and N2–CO2 are presented. The main working gas forming the plasma stream was fed near the cathode into the arc region and another additional gas was injected into the plasma stream beyond the arc. It is shown that the gas injected into the plasma stream is drawn to the arc area due to arc spot movement and cyclic arc shrinking and expanding due to the power supply pulsation. It was found that when the anode spot moves upstream, the additional gas is retracted into the arc region, changing the operating conditions of the plasmatron. The retraction mechanism depends on the gas type and is different in argon and molecular plasmas. The results of the plasma emission spectroscopy show differences in the electron excitation and rotational temperatures for the plasmas studied and are used to explain the mechanism of functioning of a segmented plasmatron.

Anode Current Density Distribution Measurements for Different Vacuum Arc Modes Subjected to Axial Magnetic Field

We report our measurements of anode current density distributions for different modes in vacuum arc discharge using a split-anode and a cup-shaped axial magnetic field cathode configuration system. The modes investigated included the diffuse arc and anode spot. The anode surface was divided into four regions: one central region and three symmetrical-disposed peripheral annular-arc regions. Three different-sized central regions were selected with diameters of 12, 18, and 20 mm. The contact material was CuCr25 (25% Cr). The arc current in the tests ranged from 6 to 14 kA (rms) at 50 Hz. The opening velocities were 1.8 and 2.4 m/s, respectively. The currents of the four areas on the anode contact were measured by four Rogowski coils situated outside the vacuum chamber. The experimental results show the current density distribution in different arc modes. For the diffuse arc mode with arc currents of 6 and 8 kA (rms), the average current density in the central region at current peak ranged from 8.5 to 15.9 A/mm $^{2}$ . The anode spot current density ranged from 31.9 to 37.7 A/mm $^{2}$ , estimated from the current density in the eroded region caused by the anode spots covering the entire central region of diameters 12 and 18 mm, at currents of 12 and 14 kA, respectively, regarded as the lower limit current density of the anode spot. Moreover, the dependence of the current density distribution on opening velocity in diffuse arcs was slight. For the anode spots, the current density distribution with higher opening velocity was more concentrated.

On Conductivity of Cold Gas Layer Separating Arc Column and Nozzle in Nontransferred Plasma Arc (Anode Reattachment Process in Plasma Spray Systems)

The nozzle serves as an anode in systems for plasma arc spraying. In these systems, the anode attachment moves downstream along the nozzle under the influence of the gas flow. The arc length increases and the arc voltage rises. At a specific arc voltage, the anode attachment jumps upstream and the arc voltage drops: reattachment occurs. The frequency of reattachment and the location of the new anode spot are determined by the conductivity of the cold gas layer that separates the extended arc column and the metallic anode. The conductivity of this layer in calculated in this paper. Two important effects are taken into account: 1) the difference between the electron and the heavy particle temperatures and 2) deviation from ionization equilibrium. Disregarding these effects leads to, practically, a completely insulating characterization of the cold gas layer. Calculations for an argon plasma showed that as opposed to the frequently accepted opinion, the electrical conductivity of the layer in the arc trail remains at a very substantial level. They also showed, in accordance with experiments, that admixing even small amounts of helium to argon decreases the layer conductivity that leads to reattachment delay and high voltage fluctuations. Analysis of the spatial distribution of the layer conductivity allows one to predict the new location of the anode spot after reattachment.

Influence of Technological Parameters of High-Precision Plasma Cutting on the Position of the Anode Spot on the Cut Edge

The paper is devoted to investigating the position of the anode spot on the cut edge during high-precision plasma processing and studying the trajectory of the melting products flowing from the cut edge. The material of samples is structural steel of St3 (<strong>GOST 14637-89)</strong> having the thickness of 3, 4, 5, 6 and 8 mm. The formation of the cut edge was carried out according to the HiFocus technological scheme of cutting by using the equipment manufactured by the Kjellberg firm (Germany). It was found that among technological parameters such as current, cutting speed, pressure and flow rate of cutting and swirl gases considered in the paper the greatest influence on the position of the anode spot on the cut edge is made by the cutting speed. For the investigated technological scheme of cutting the ratio between the anode spot and the thickness of the cut material lies in the range from 0.42 to 0.47. The optimization of the processing speed is based on studying the trajectory of the melting products outflow from the cut edge.

RESEARCH ON THE PLASMA SPRAY PROCESS APPLYING THE FINITE ELEMENT METHOD / PLAZMINIO PURŠKIMO PROCESŲ TYRIMAS BAIGTINIŲ ELEMENTŲ METODU

The article investigates the physical processes of plasma spraying. The application of the finite element method has assisted in establishing the distribution of the voltage of the plasma arc and current density in the plasma stream during numerical simulation. With reference to the results of experimental data, the real location of an anode spot of the electric arc in the plasma spray process has been evaluated. The paper has calculated the values of electromagnetic Lorentz forces and established their influence on plasma flow. With the help of the two-layer model for the semi-molten nickel particle, contact between the particle and substrate during plasma spraying has been simulated. Straipsnyje nagrinėjami fizikiniai procesai, vykstantys plazminio purškimo metu. Taikant baigtinių elementų metodą, skaitinio modeliavimo metu buvo nustatytas plazmos lanko įtampos ir srovės tankio pasiskirstymas plazmos sraute. Panaudojant eksperimentinių rezultatų duomenis, buvo įvertinta reali elektros lanko anodo dėmės padėtis plazminio purškimo procese. Buvo apskaičiuotos elektromagnetinių Lorenco jėgų reikšmės ir nustatyta jų įtaka plazmos srautui. Taikant dvisluoksnės aplydytos nikelio dalelės modelį, buvo imituojamas dalelės kontaktas su substratu purškiant.

Realization of DC atmospheric pressure glow discharge without external airflow

A new needle-to-cylinder electrode structure with small gap (1.5 mm) was designed to realize the stable gas discharge in ambient air. The stable corona discharge and the stable glow discharge were realized without external airflow, which was proved by the discharge waveform stored in the oscilloscope, the discharge pictures recorded by the digital camera and the voltage–current plot. The image of the glow discharge showed that there was a negative glow region, Faraday dark space, a positive column region and an anode spot. Also the experiment and the COMSOL simulation showed that the needle with a diameter of 56.4 μm can realize the glow discharge while the needle with a diameter of 626 μm can not. The reasons for the realization of the glow discharge by the small gap between the needle and the cylinder were analyzed. It also shows that the glow discharge is easily realized with the black spot on the cylinder surface and a proper ballast resistor value (larger than 2 MΩ and less than 30 MΩ). The needle-to-cylinder electrode structure is easy to fabricate by MEMS technology, which can be used in portable analytical instruments.

Arc Discharge Anode Reattachment: Simple Model

Anode attachment of a nontransferred dc arc moves along the anode nozzle. Under the influence of the gas flow, it moves downstream thus elongating the arc. This leads to an increase of the arc voltage. The voltage across the cold layer that separates the arc column and the nozzle increases accordingly. This voltage increase leads to the breakdown of the cold anode layer. After this breakdown, the anode attachment jumps to the new upstream location (reattachment). The phenomenon of reattachment of the anode spot has not been completely understood yet. A simple model of the reattachment is suggested. Based on a suggestion that electron temperature inside the cold layer does not drop as low as the heavy particles' temperature does, it is assumed that some residue electrical conductivity (on the level of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> Ω <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) exists in this layer. It is shown that the voltage drop across the layer rises in time, leading to a fast developing overheating instability. This creates a electrically conductive column at an upstream location and results in reconnection of the arc to this new anode location. Simple estimations of the amplitude of the voltage oscillations and their frequency correspond to the experimental data.