Arc Spot Research Articles

Investigation of the interaction of dense noble gas plasmas with cold cathodes: II Arc spot ignition on Au, Pd and Pt cathodes

AbstractInvestigations of the ignition of cathode spots by arcs operated in atmospheric pressure noble gas atmosphere on electrodes made of Al, Cu, Ti and graphite are extended to electrodes made of Au, Pd and Pt. Rods with a diameter of 2 mm are inserted in a UHV tight stainless steel vessel filled with Ar 5.6 or Kr 4.8. The negatively biased electrodes are brought into interaction with arc plasma by a magnetic blast field. Their end faces are mostly polished with diamond grinding powder, some Pd electrodes are additionally glowed, and other Pd electrodes are pasted with Pd powder. The arc spot ignition is observed by short time photography, streak camera records and temporally highly resolved optical spectroscopy for a lower and higher voltage applied between the arc plasma and the commutation electrode. Measurements of the commutation time tc elapsing between a signal generated by the arc plasma in front of the commutation electrode and the start of the commutation current show that at low voltages tc is in parts drastically increased by oxide layers. The measurements of tc for different electrode materials demonstrate that tc decreases with decreasing boiling temperature of the material. It confirms that not only ions of the filling gas but also of vaporized electrode material in front of the commutation electrode initiate and promote the arc spot ignition. SEM records show that different electrode treatments cause primarily different arc traces on the electrode surface.

Investigation of the interaction of dense noble gas plasmas with cold cathodes:III—Arc spot ignition on pure and doped W cathodes

AbstractThe study of the commutation of atmospheric pressure noble gas arcs on cold cathodes made of Al, Cu, Ti, graphite, Au, Pd, and Pt is extended to W cathodes. Rods with a diameter of 2 mm are inserted in a UHV tight stainless steel vessel filled with Ar 5.6 or Kr 4.8. The negatively biased electrodes are brought into interaction with arc plasma by a magnetic blast field. Their end faces are mostly polished with diamond grinding powder; some are electrolytic polished, others additionally covered with a thick oxide layer or pasted with W powder. Moreover, electrodes are investigated being doped with Al, K, and Si or doped with ThO2. The arc commutation is observed by short time photography, streak camera records, and temporally highly resolved optical spectroscopy for a lower and higher voltage applied between the arc plasma and the commutation electrode (CE). Varying the electrode properties revealed that basically ionized vapour of electrode material and less distinctly lowering of the work function by doping accelerate the arc commutation. It is initiated by plasma ions, which initially generate secondary electrons across the whole end face of the electrode. Since the electron emission varies locally, the multiplication of ions within the plasma layer by emitted electrons varies too. The positive feedback between both provokes a constriction of the commutation current and the power input into an arc spot. The electrode vapour promotes at first the development of an arc spot but finally quenches it by cooling the plasma layer in front of it.

Investigation of the interaction of dense noble gas plasmas with cold cathodes: I—Experimental setup and application to Al, Cu, Ti, and graphite cathodes

AbstractThe formation of arc spots on cold cathodes, called arcing, is an interference factor in all kinds of plasma technical devices. It is investigated by igniting an arc in a clean atmospheric pressure Ar or Kr filling of a stainless steel vessel. It is brought into interaction by a magnetic blast field with a negatively biased commutation electrode (CE), formed by a rod with a diameter of 2 mm consisting of pure Al, Cu, Ti, or graphite; its end face is polished with diamond grinding powder. The arc commutation is observed by short‐time photography, streak camera records, and temporally highly resolved optical spectroscopy for different applied voltages. The emission of ion lines of the filling gas and of vaporized electrode material by the plasma in front of the cathode is indicating the formation of a positive space charge layer in front of its surface. It provides the ions, which induce secondary electron emission of the cathode surface and with it, the arc commutation. The commutation time tc elapsing between a signal generated by the arc plasma in front of the CE and the beginning of a current flow through the electrode increases for low voltages with increasing permittivity of a surface layer formed by electrically insulating metal oxide and decreases with increasing electrical conductivity of the layer. It is lowest for graphite without an oxide layer. On scanning electron microscope pictures, taken of the end face of the electrode after arc commutation, craters with diameters of less than 1 μm are visible.

Amorphous Carbon Coatings with Different Metal and Nonmetal Dopants: Influence of Cathode Modification on Laser-Arc Evaporation and Film Deposition

In this study, the arc evaporation of pure graphite and composite cathodes with small amounts of metals (Mo, Fe) or nonmetals (B, Si) was investigated by means of a laser-arc process. Both specific aspects of the arc evaporation and the effects on the deposition of the doped and undoped carbon coatings were studied. The deposition rate, the chemical composition and the mechanical properties of the generated films were evaluated. In addition, the dependence of the deposition rate and the composition on the height position of the substrates in relation to the cathode were also the subject of the investigations. Finally, the erosion rate and the arc spot behavior on the cathode were analyzed. It is shown that homogeneously doped (t)a-C:X coatings can be reliably synthesized with the laser-arc technique. There are differences in the various properties of the coatings and the deposition rate. The latter is attributed in particular to the erosion behavior of the cathodes.

Effect of Arc Currents on the Mechanical, High Temperature Oxidation and Corrosion Properties of CrSiN Nanocomposite Coatings

In methods for multi-arc ion plating technology, the behavior and characteristics of the arc spot determine the physical characteristics of arc plasma and the properties of the subsequent deposited coatings. In this paper, the effect of arc currents on the hardness, friction coefficient, high temperature oxidation, and corrosion properties of the CrSiN coatings was studied. According to the XRD and SEM results, with the increase of arc currents, the coatings grew preferentially to the CrN (111) crystal direction, and the CrN (220) crystal phase appeared at high currents of 90 A. In addition, the number of large particles increased when the current exceeded 70 A. The HR-TEM results confirmed the formation of nanocomposite structure of nanocrystalline of CrN embedded into the amorphous phase of Si3N4 as explored by XRD. The maximum hardness was achieved at 3120 Hv when the coatings were deposited under currents around 70 A. However, the hardness values decreased with further increase of arc currents. From the contact of ceramic balls with the wear of coatings, the surface of coatings gradually produced friction marks, and the friction force increased from a steady friction force to a dynamic friction force. The high temperature oxidation results showed that fewer oxides were formed on the surface of the coatings when oxidized at 800 °C. It was also found that CrSiN nanocomposite coatings prepared at an arc current of 70 A had a larger corrosion potential and polarization impedance, which could effectively protect the tool matrix.

Discussion on the Application and Common Problems of Arc Welding and Spot Welding Robot

Discussion on the Application and Common Problems of Arc Welding and Spot Welding Robot

High-resolution observation of cathodic arc spots in a magnetically steered arc plasma source in low pressure argon, nitrogen, and oxygen atmospheres

Cathode spots in a magnetically steered arc source were studied under low-pressure noble gas (Ar) and reactive gas (N2, O2) atmospheres. The plasma was observed using a streak camera coupled with a long-distance microscope to study the evolution of cathode spots with high temporal and spatial resolution. We find two well-known types of cathode spots: “type 1” for less bright spots eroding the compound layer on the cathode surface and bright “type 2” spots on (clean) metallic surfaces. Cathode spots are characterized by a sequence of microexplosions that give the impression of a moving spot, which, in the presence of a magnetic field, is generally in the retrograde direction. However, the apparent displacement can also go in the opposite, the Amperian direction, especially when nitrogen is present. In oxygen, spot ignition often happens in approximately the same location repeatedly. For type 2 spots, we detected an apparent motion mainly in the retrograde direction with distinct jumps to new locations. Via the effects of spot appearance, we note the competing effects of cathode cleaning by spot-induced material removal (erosion) and compound formation in the presence of reactive gas. The streak images were analyzed by fast Fourier transformation, and we found that the arc fluctuations are stochastic without specific frequencies. The colored random noise (CRN) index tends to be reduced in the presence of a compound layer, indicating an enhanced spot ignition probability. A reduced CRN index implies reduced feedback (influence) of previously active spots, which is most apparent in the presence of elevated oxygen pressure.

Influence of lithium and silicon coatings on tungsten erosion in arc plasma

• Arc erosion of W substrate can be effectively mitigated by Li and Si coatings. • Selective formation of spots on coatings is responsible for erosion mitigation. • Vapor shielding of Li coating provides effective mitigation of tungsten erosion. Arc spots formed on a plasma-facing component (PFC) have a stunning resemblance with the arc spots formed on a cathode of a vacuum arc. Hence, in this paper, a laboratory-scale vacuum arc process is used to get a better understanding of the arc erosion process of the tungsten (W) material. The performance of lithium (Li) and silicon (Si) coatings in erosion mitigation of W material is analyzed. The virgin W substrate was eroded significantly by arc spots and numerous W droplets were ejected into the interelectrode plasma from cathode with velocity of tens m/s. However, the W substrates can be effectively protected by Li and Si coatings from arc erosion. Preferential formation of arc spots on the coatings is responsible for the arc erosion mitigation. Compared with the virgin W substrate, no ejection of W droplets from the cathode was observed in the cases of coatings. Li and Si droplets were accidently ejected from the cathode and the ejected droplets were rapidly ablated in the arc plasma. For the case of Si coating, the W substrate was slightly melted and eroded due to the arc spots formed on the coating. By contrast, no obvious melting and erosion were observed on the W substrate for the case of Li coating, which is possibly due to the vapor shielding effect of Li between the W substrate and arc spots. These results confirm coatings served as a sacrificial protective layer and can reduce tungsten surface erosion caused by arcs, which provides a new reference for protecting first wall materials in future fusion devices.

Arc spot formation conditions and influencing factors of a micro multi-electrode technology

Arc heaters are important items of equipment for ground thermal appraisal of space probes returning to Earth. However, cathodes of arc heaters are seriously ablated, which limits the appraisal. Micro multi-electrode technology is characterized by multiple spots, which has the potential to reduce cathode ablation. In order to develop this technology, Cu–LaB6 alloys were prepared by using a hot press sintering furnace with a LaB6 content of mol. 0.03%–1% and average particle size of 18-60 μm. The influence of current, discharge distance, LaB6 content, LaB6 size and magnetic field on the arc spot mode was studied. The similarity parameter of the arc spot mode was analyzed. The arc spot has three modes: single-spot mode (SSM), multi-spot mode (MSM) and branched-spot mode (BSM). With the increase in the discharge current, the arc spot changes in the order of the BSM, the MSM and the SSM. With the increase in the discharge distance, the arc spot changes in the opposite order. Increasing the content or decreasing the size of LaB6 can make the MSM region move in the direction of decreasing current and increasing discharge distance. A model for arc spot transition is proposed. The change of the arc spot from the MSM to the BSM is related to the arc constriction.

Observed Oscillating Anodic Plasma Plume Phenomena in High Current Vacuum Arcs

The impact of current density on the anode attachment characteristics of high current vacuum arcs was investigated using high-speed imaging techniques. The current density determines the occurrence of attachment transition into an anode spot type 2. Electrodes were made of Cu/Cr 50%/50%. In combination with the adequate amplitude of current, the anode spot type 2 was accompanied by first observed oscillating anodic plasma plumes by the higher current densities. Resulting electrode overheating processes, which depend on the instantaneous power delivered, determine the formation of a plasma region in front of the anode. Image processing indicates that this region interacts actively with the cathode, displacing the radiation intensity region, and influencing the behavior of its arc spots. In addition, increased high-frequency contents on the measured voltage signals accompanied the observed oscillations of the anodic plume. An explanation through a double-layer effect for the anode spot type 2 is proposed.

High-resolution observation of cathode spots in a magnetically steered vacuum arc plasma source

The properties of cathode spots in a magnetically steered vacuum arc plasma source were investigated. The cathode spots were observed with a long-distance microscope coupled to a streak camera, where the entrance slit of the camera was aligned with one of the linear sections of the steered arc source. This is a key feature that allows the observation of spots as they appear along the erosion racetrack. The trends of spot motion and patterns in the emitted light were observed with different sweep times. The streak images were analysed by fast Fourier transformation to check for the presence or absence of characteristic features like characteristic spot lifetimes, since their existence remains a disputed issue. We found that the power spectrum of the arc spot fluctuations does not show any specific frequencies or related times, rather, the spot fluctuations have a broad spectrum following a power law (spectral power density versus frequency), which is indicative for the fractal (self-similar) nature of spot processes. Moreover, we show that—at about 300 MHz under our conditions—we reach the ultimate temporal resolution limit of the method, which is associated with the finite lifetime of excited atomic and ion states. This is a fundamental limit given by the physics of atoms and ions, not a measurement flaw. The temporal resolution limit of about 3 ns also implies a spatial resolution limit of order of a few μm given that radiating species move laterally to the observation direction with at least 103 m s−1.

Enhancement of discharge and deposition rate by imposed pulse current during the cathodic arc deposition of ta-C films

The deposition rate of ta-C films was increased by imposed pulse cathodic arc deposition (IP-CAD). The arc discharge behavior, deposition rate and micro-hardness of the ta-C films were investigated at average arc currents (Iaverage) ranging from 60 A to 100 A. The average substrate current density of IP-CAD was higher than that of DC-CAD at the same Iaverage. The optical emission spectra demonstrated that a dramatic increase in the evaporation rate of both carbon atoms and ions were achieved by imposing high pulsed current with Iaverage of 80 A. As the IAverage increased from 60 A to 100 A, the deposition rate increased by about 110% for the IP-CAD and by 33% for DC-CAD. This might be attributed to the cooperative actions of high evaporation rate of the carbon, large arc pressure near the arc spots and the focusing effect induce by high current. As compared to DC-CAD, the IP-CAD mode led to smaller concentration of sp3 (44%) than DC-CAD (59%) at the IAverage of 80 A. However, the IP-CAD ta-C films exhibited higher micro-hardness (~44.7 GPa) than that of DC-CAD (24 GPa). A preferable toughness was simultaneously obtained for IP-CAD.

Exploration of a micro multi-electrode technology applied in an air arc heater

Arc heaters are vital equipment for the ground thermal appraisal of space probes reentering Earth’s atmosphere. However, the cathodes of arc heaters are currently made of pure copper, and cathode erosion limits the appraisal. A micro multi-electrode technology was proposed to reduce cathode erosion. Cu–Y, Cu–Ba, Cu–Hf, Cu–La, Cu–Cr2Nb, Cu–LaB6, Cu–HfO2, and Cu–Y2O3 alloys were prepared with a content of mol. 0.3%. The microstructure, electron-emission abilities, and arc-attraction abilities of the prepared alloys were analyzed. These alloys consist of low work function (LWF) phases and copper. The electron-emission abilities of these alloys are 10–30 times higher than that of copper. The LWF phase can attract the arc, and its arc-attraction ability can be characterized by a parameter, . The arc-attraction ability of these LWF phases from low to high is Cu, Cu6Y, Cu6La, Cu13Ba, Cu5Hf, Cr2Nb, HfO2, Y2O3, and LaB6. The arc-attraction ability of the LWF phase is related to its work function and its melting point. Cr2Nb, HfO2, Y2O3, and LaB6 successfully attracted and separated the single arc spot into multiple micro spots, which verified the feasibilty of the micro multi-electrode technology.

Effect of Energy and Temperature on Tetrahedral Amorphous Carbon Coatings Deposited by Filtered Laser-Arc.

In this study, both the plasma process of filtered laser-arc evaporation and the resulting properties of tetrahedral amorphous carbon coatings are investigated. The energy distribution of the plasma species and the arc spot dynamics during the arc evaporation are described. Different ta-C coatings are synthesized by varying the bias pulse time and temperature during deposition. An increase in hardness was observed with the increased overlapping of the bias and arc pulse times. External heating resulted in a significant loss of hardness. A strong discrepancy between the in-plane properties and the properties in the film normal direction was detected specifically for a medium temperature of 120 °C during deposition. Investigations using electron microscopy revealed that this strong anisotropy can be explained by the formation of nanocrystalline graphite areas and their orientation toward the film’s normal direction. This novel coating type differs from standard amorphous a-C and ta-C coatings and offers new possibilities for superior mechanical behavior due to its combination of a high hardness and low in-plane Young’s Modulus.

Using of mathematical methods in the study of temperature-time conditions of the arc surfacing upon manufacturing of steel-aluminum compositions

A mathematical model for studying temperature-time conditions of arc surfacing upon manufacturing of steel-aluminum compositions has been developed and verified. During simulation, the dependences of the thermophysical properties of the materials under study (thermal conductivity and heat capacity of a unit mass of the substance at constant pressure and volume) on the heating temperature were added to the database of the «SVARKA» software to solve the thermal problem. When simulating the arc surfacing, the geometric model of the object was set in the form of a single body, which, e.g., in case of formation of functional coatings based on non-ferrous alloys on steel substrates, can consist of various materials. The parameters of the heat loads of the heating source are: the advance speed, power, distribution along and across the weld, as well as the presence and grade of the filler material. The calculation of heat propagation upon argon-arc surfacing with a non-consumable electrode was carried out according to the scheme with a normal-circular heat source located on the surface of a flat layer with the limiting effect of the bottom plane of the sheet. The considered calculation scheme includes all the main features of the argon-arc surfacing process, i.e., introduction of the heat of the welding arc into a massive body from the surface; small pressure of the welding arc; slight immersion of the active arc spot in the liquid metal. It is shown that allowance for the thermophysical properties of the intermetallic layer of the Fe – Al system located in the diffusion zone, provided determination of the heating temperature with the uncertainty of no more than 8 % not only at the steel-aluminum interface, but also at any point of the samples both in the transitional bimetallic steel-aluminum elements joined with aluminum or steel structures, and in functional aluminum coatings formed by surfacing, including those made of composite materials.

Cyclic Experiments on Sidelap and Structural Connectors in Steel Deck Diaphragms

This work studied the cyclic shear performance of connectors used for sidelap and structural connectors in bare steel deck roof diaphragms. Typical bare steel deck roof diaphragms include thousands of such connections, and knowledge of the connection shear performance is important for understanding and predicting diaphragm performance under seismic loads. The experimental program included four types of sidelap connectors: screw nestable, top arc seam weld, button punch, and the proprietary PunchLok II crimp. The testing included three types of structural connectors: power-actuated fasteners, arc spot welds, and arc seam welds. A total of 64 sidelap and 60 structural connector tests were performed under a cyclic loading protocol, to evaluate behavior at small deformations as well as under large excursions, providing the shear response up to and through the peak response, including postpeak response until material separation and failure. The connection test results were used to establish experimental multilinear backbone curves. From these backbone curves, average connection shear stiffness, strength, and ductility can be estimated and compared for all studied configurations. The connection response for the mechanical connections and welded connections exhibited significantly different behavior, particularly in terms of postpeak ductility and residual force. The connector backbone response is appropriate for use as nonlinear shear springs to model the response of the connections in high-fidelity models of bare steel deck roof diaphragms.

Numerical Simulation of Heat Source Characteristics in Arc Spot Welding with Constricted Nozzle

Numerical Simulation of Heat Source Characteristics in Arc Spot Welding with Constricted Nozzle

Study of variable range hopping conduction mechanism in nanocrystalline carbon thin films deposited by modified anodic jet carbon arc technique: application to light-dependent resistors

In this study, we report the deposition of nanocrystalline carbon thin films by modified anodic jet carbon arc technique assisted with inert-helium and reactive-nitrogen gaseous environments. The modified anodic jet technique facilitates to generate a very high localized pressure near the arc spot for the growth of nanocrystallites of carbon at high arc temperature and high pressure. The deposited films were analyzed for the growth of nanocrystallites in amorphous carbon structure under high-resolution transmission electron microscopy (HRTEM). The HRTEM studies reveal the distribution of nanocrystallites in the amorphous carbon matrix. The temperature-dependent conduction behavior of the deposited films has also been analyzed under Mott’s variable range hopping conduction mechanism. Both sets of the film are found to follow three-dimensional variable range hopping conduction mechanisms for the transport of charge carriers. The deposited films are also analyzed for their applications to energy-saving light-dependent resistors under an illumination intensity of ~ 100 mW/cm2 of white light. The reasonably high value of detectivity values ~ 1.26 × 106 and 3.12 × 106 Jones and relatively lower values of trap depth 0.6826 and 0.6834 eV for the nanocrystalline films deposited in helium and nitrogen environments suggest trapped state-assisted significantly high power conversion efficiency. This supports the suitability of the deposited films for light-dependent resistor applications.

3D computational study of arc splitting during power interruption: the influence of metal vapor enhanced radiation on arc dynamics

Arc splitting is one of the most important processes in accomplishing a power interruption by multiplying the number of voltage drops. During arc-plate interaction, the arc roots erode and vaporize the metals which significantly alters the gas composition and plasma properties, such as the radiation absorption coefficient. In this work, we perform a 3D computational study of arc splitting in a circuit breaker. In order for the study to be systematic, the metal vaporization, species transport, and radiative heat transfer are integrated into the magnetohydrodynamics modeling with some special considerations. Firstly, the simulation considers the ferromagnetic effect of steel plates. Secondly, the metal-vapor-enhanced radiation is numerically implemented by the discrete ordinate method with consideration given to the banded radiation spectrum. Thirdly, the simulation model incorporates a near-electrode layer to implement the voltage drop and imposes additional heat flux on the arc spots. The simulation results show that the metal vaporization not only influences the arc dynamics (via Stefan flow) but also enhances the local radiation intensity. Besides, due to the ferromagnetic effect, the magnetic field increases dramatically during arc splitting. However, the self-induced magnetic force has quite a different influence on the motion of sub-arcs, which prevents even and concurrent arc splitting. This simulation reveals that the magnetic-field-induced uneven splitting can be compensated by the enhanced pressure wave or externally applied transversal magnetic field. This study is expected to explore more applications in simulating arc interruption and improve the design of highly-efficient circuit breakers.

Unipolar arc plasmas on nanostructured tungsten surfaces under perpendicular magnetic field

Unipolar arcing is an important phenomenon in terms of wall erosion and the subsequent plasma contamination in nuclear fusion devices. In this study, unipolar arc plasmas are experimentally explored under the presence of the external magnetic field perpendicular to a tungsten (W) sample on which a ‘fuzz’ nanostructure surface is formed. It is found that the fuzz layer thickness, d fuzz, plays a dominant role in determining the motion of arc spots, the arc ignition probability, and arc plasma parameters. High-speed imaging reveals that the motion of arc spots becomes more collective, as d fuzz increases. Interestingly, arc spots make a circle while moving outward. With increasing d fuzz, the amount of eroded W atoms increases, as indicated from an observed increase in the total visible emission intensity, while the arc ignition probability is found to drop. The physical mechanisms are discussed, in detail, based on spectroscopic measurements of the electron temperature, obtained from Boltzmann plots of both W I and W II, and the electron density, determined from Stark broadening of W I lines.