Behavior Of Cathode Spots Research Articles

An Investigation into the Behavior of Cathode and Anode Spots in a Welding Discharge

The effective development of modern welding technologies and the improvement of equipment and materials inevitably require deep theoretical knowledge about the physical phenomena occurring in the electric arc column and in the near-electrode region. However, there is still no convincing theoretical description of an arc discharge. This article demonstrates, through the generalization of known experimental facts and studies using a high-speed camera, that the conductive channel of an electric arc has a discrete structure, consisting of a set of thin channels through which the main discharge current passes. The cathode spot of an arc discharge is a highly heated and brightly glowing area on the cathode’s surface. Electron emission occurs from this region, which supports the discharge as well as the removal of the cathode material. We propose a new technique to study the reverse side of the cathode spot, revealing a structure consisting of individual cells or fragments of the cathode spot. For the first time, we present the anode spots captured by a high-speed camera. We carry out an analysis of the spots’ structure. We determine the parameters affecting the mobility of cathode and anode spots. We propose a hypothesis based on the obtained experimental facts about the heterogeneous structure of cathode and anode spots in an arc discharge and the existence of current filaments that affect the mobility of spots during arc combustion.

Study on cathode spot expansion process in vacuum arc with high di/dt

The study on cathode spot under high di/dt condition promotes the application of vacuum circuit breaker in DC transmission and aerospace technology. Cathode spot, as a source of microscopic particles, directly affects the breaking capacity of vacuum circuit breaker. In this paper, vacuum arc with high di/dt was generated in L-C oscillating circuit. The cathode spot behavior was obtained by a high-speed camera. There are two types of cathode spots with different size, different expansion velocity and brightness. The difference in the size of different types of cathode spots leads to the difference in the thermal diffusion velocity of the spots, which is also the main reason for the difference in the expansion velocity of the two types of spots. The influence of spot current density on temperature and electric field is discussed. With the increase of di/dt, the phenomenon of unstable expansion of cathode spots is observed. A new cathode spot is generated in the cold electrode area far away from the previous cathode spot area, resulting in a significant increase in the arc voltage. The expansion characteristics of cathode spot under different current amplitude, frequency and di/dt conditions are obtained.

Three-dimensional hybrid simulation of vacuum arc extinction considering dynamic behavior of cathode spots

Vacuum arc extinction directly determines the plasma distribution at the current zero point, which affects the post-arc insulation recovery. In that regard, studying the extinction process to improve the post-arc insulation recovery ability is of great significance. In the extinction process, as the current decreases, cathode spots are extinguished and their distribution exhibits apparent discreteness and asymmetry, while the plasma density decays drastically. On the one hand, the discreteness of cathode spots makes the simulation setup regarding the whole cathode surface as a continuous emission source no longer applicable. On the other hand, the drastic decay of plasma density requires a simulation tool that can balance the computational efficiency and accuracy of the extinction process. To deal with these problems, the dynamic behavior of cathode spots is considered in this paper, and a three-dimensional hybrid plasma model is established. In order to simulate a more realistic situation, two applied axial magnetic field configurations for practical applications—bell-shaped and saddle-shaped configurations—are introduced. Both configurations are used to compare the position distribution and extinguishing sequence of cathode spots, as well as the evolution of plasma. More importantly, the method applied in this study can be employed to evaluate the effect of different magnetic field conditions on vacuum interruption, which would enable one to simulate the extinction process in a realistic manner.

Cathode spot behavior in nitrogen and oxygen gaseous atmospheres and concomitant cathode surface modifications

The cathode spot behavior influences the arc plasma chemistry and film growth conditions during reactive cathodic arc deposition of nitride and oxide films. Cathode spots can be studied using their characteristic craters left behind on the cathode surface. The multilayer cathode design used in this study reveals temporal and spatial progress of cathode spots by enabling three-dimensional visualization of the craters. The surface nitridation or oxidation of the cathode, also known as cathode poisoning, was found to give rise to a repeated switching between cathode spots of type 1 and 2. The surface oxide layers, however, more significantly promote the ignition of type 1 spots due to their thermodynamically privileged formation and/or their more favorable physical properties building up a stronger electric field within the insulating layer. The crater depths and their contribution to the surface modification of multilayered cathodes are discussed in detail. These results may contribute to a better understanding of macroparticle generation and arc plasma properties in cathodic arc deposition processes.

Investigating cathode spot behavior in argon alternating current tungsten inert gas welding of aluminum through experimental observation

The behavior of cathode spots in stationary argon alternating current tungsten inert gas welding of aluminum was clarified in this study. The temporal variation in cathode spot distribution during electrode positive (EP) polarity was, for the first time, revealed quantitatively based on high-speed video camera observation with a frame rate of 500 000 fps. In addition, the weld pool surface temperature at the start and end of EP polarity was also measured using the two-wavelength temperature measurement method to clarify the relationship with the cathode spot behavior. As a result, the general tendency of the life cycle of the cathode spot was clarified. The first cathode spot appeared and divided outside the weld pool. Following this, they moved rapidly toward the weld pool center within 100 µs. After that, the cathode spot group became an uneven and ring-shaped distribution, which expanded outward with time with a velocity of approximately 1 m s−1. The cathode spots have a random movement with the average velocities of each cathode spot on the liquid and solid surface being about 142 ± 59 m s−1 and 87 ± 44 m s−1, respectively. The ratio of the number of cathode spots located in the weld pool to the total number of cathode spots decreased from 100% to 37.4% with elapsed time, except right at the beginning. Furthermore, the temperature of the center surface of the weld pool was 1400 K, and this gradually increased to a slightly higher temperature than the melting point of aluminum around the weld pool boundary at the start of EP polarity. The maximum temperature at the center is considered to be linked with absence of a cathode spot. The temperature at the center decreased by 150 K at the end of EP polarity due to the temporal expansion in the ring-shaped distribution of the cathode spot group leading to a decrease in heat input around the center.

Influence of the magnesium content on cathode spot behavior in AC TIG welding of aluminum alloy

Influence of the magnesium content on cathode spot behavior in AC TIG welding of aluminum alloy

Experimental Investigation on Triggered Vacuum Arc and Erosion Behavior Under Different Contact Materials

In order to improve the breaking capacity of contacts, it is very important to find a better contact material. In this paper, the triggering experiments of plate contact with different materials are studied to investigate the characteristics of vacuum arc. The current range is from 1 to 9 kA (rms). The gap distance is selected as 3 and 5 mm. The arc appearance and the behavior of cathode spots are captured by high-speed camera with an exposure time of $2~\mu \text{s}$ . The surface topography of contacts after experiments is evaluated by scanning electron microscope. The arc current and arc voltage are measured at the same time by Rogowski coil and high-voltage probe. The diameter of the contact is 25 mm. There are seven kinds of materials (CuCr10, CuCr25, CuCr30, CuCr40, WCu10, WCu30, and CuW–WC) conducted in this paper. Compared with the experimental results, the arc of WCu alloy distributes more evenly on the contact plane with many bright cathode spots. The small gap distance makes the burning point more fixed. The arc of CuW–WC is unstable. During the arc trigger moment to the arc stable burning stage, the radial diffusion velocity of WCu10 cathode spot is the lowest with about 25% of CuCr10. CuCr10 alloy has the worst melting resistance. WCu alloy has good melting resistance and bad toughness. The toughness of CuW–WC is even worse. Under the low current, CuCr30 might be preferred for vacuum interrupters from the cathode spots’ radial diffusion speed, arc appearance, and erosion result. When the current increasing, the arc can easily spread to the entire surface of WCu contact, and WCu10 would be the prefer material with the diffusion arc and good erosion resistance.

Influence of Admixture of Oxygen into Shielding Gas on Cathode Spot Behavior

Influence of Admixture of Oxygen into Shielding Gas on Cathode Spot Behavior

Experimental investigation and numerical simulation of triggered vacuum arc behavior under TMF/RMF-AMF contact

A series of triggering experiments was carried out to investigate the characteristics of vacuum arc controlled by TMF/RMF-AMF contacts. During all the experiments, the current ranged from 5–20 kA (rms) and both the arc appearance and behavior of cathode spots were captured by high-speed camera with corresponding arc current and arc voltage. A 3D steady magnetohydrodynamics (MHD) model was built to simulate and analyze the vacuum arc behavior under TMF/RMF-AMF contacts, and arc plasma parameters were calculated based on the above model. The experimental results showed that arc deflection was visible under both low and high current. Under high current, arc core formed, which meant the arc contracted significantly. In addition, the anode became much more active under high current. The behavior of the cathode spots showed that they split themselves into other new cathode spots. Under high current, the bulk of the spots rotated along a clockwise direction on a transverse magnetic field (TMF) plate, which caused much noise and oscillation in the arc voltage. The simulation results show that ions are likely to gather on the branches of the TMF plate on the anode plane, as a result of the effects between the electromagnetic force and pressure gradient of the arc plasma. The current contracts in the center of the TMF plate on the cathode which was due to the thin connecting rod there. The anode contraction of the current is caused by the Hall effect. Ions move along a clockwise direction on the TMF plate, which is driven by Ampere force. The current contraction resulted in significant melting in the center of the cathode surface while the other region suffered from uniform melting. The melting caused by the anode contraction is more significant than that of the cathode.

Cathode Spots of Vacuum Arc Discharges

AbstractVacuum arc discharge is used for formation of pure metal and complex combined coatings, and also for the impact of the discharge on a surface to remove superficial contaminants. Physical processes observed in an arc discharge are mostly determined by the behavior of cathode spots, which are fundamentally unstable plasma formations and are characterized by a certain value of average lifetime. Reliable stabilization of the cathode spots and controlling their movements on the working surface grants the technology a grater importance in application.

Observation of the Behavior of Cathode Spots in AC Tungsten Inert Gas Welding on Aluminum Plate

The cleaning action in AC TIG welding is attributed to the oxide film removal process caused by cathode spots. Clarifying the behavior mechanism of these cathode spots will enable the control of the cleaning action. However, the behavior mechanism of cathode spots has not yet been experimentally clarified. This study reports that observations by high-speed video camera revealed the absence of cathode spots at the center of the molten pool, when shot in the vicinity of a molten pool after AC TIG welding using helium as shield gas on aluminum plates.

Controlled bridge transfer (CBT) gas metal arc process for steel sheets joining

In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by lowering the short-circuit current to a low level just before the re-arcing. The reduction in spatter requires an improvement in the accuracy of predicting the re-arcing by stabilizing the metal transfer and improving the robustness of the accuracy against disturbances. The controlled bridge transfer (CBT) process optimizes the accuracy of predicting the re-arcing in real time in response to the metal transfer, realizes spatter reduction and stable arc in non-pulsed GMAW. Traditionally, GMAW is carried out using electrode positive polarity. However, this polarity is not sufficient for welding extra-thin steel sheets, specifically those thinner than 1.0 mm. With electrode negative (EN) CBT process, although slight arc voltage fluctuation occurs caused by the behaviour of cathode spots on the tip of the wire during EN polarity GMAW, instantaneous voltage uses command computation to improve the transient response against the disturbance. Consequently, a stable arc can be obtained without increasing the number of short circuits in a unit time to obtain spatter-free welds.

TIG-MIG複合溶接法の基礎的検討

TIG and MIG welding are most popular gas shielded arc welding processes used on many industrial fields. MIG welding is high efficiency process compared to TIG welding, however it is needed to improve the quality about spatter and weld metal toughness for advanced GMA process. Although pure Ar shielding gas is desirable for weld metal toughness, MIG arc is unstable in pure Ar shielded condition due to the irregularly behavior of cathode spot on plate surface and suitable welding execution is difficult. We found that MIG arc becomes stable even though in pure Ar shielded condition by simple hybridization of TIG and MIG. Therefore, this process has high possibility as new type of high quality and high efficiency welding process. In this study, we investigated the influence of hybridization condition such as TIG-MIG current balance, distance between the both arcs and torch angles on stabilization of the process and confirmed the suitable range of the condition. The main results are as follows: (1) TIG current is needed to be larger than MIG current for stabilization of MIG arc. (2) According to increase of the distance between the both arcs, MIG arc shows high arc voltage and becomes unstable. (3) On the condition of small angle between the both torches, as thirty degree, MIG arc is deflected toward back by repulsion with TIG arc and bead shape becomes convex.

G060014 金属平板の陽極に対向していない面に対する真空アーク酸化膜除去([G06001]熱工学部門一般セッション(1):電気・化学過程を伴う熱流体,およびプラズマ・放電)

Vacuum Arc cleaning to remove oxide layer on metal plate opposite side to anode was demonstrated. It was confirmed that cathode spots could exist on the opposite side and removed the oxide layer as well as the case on the anode side. The cathode spot behavior and the removal characteristics of oxide layer were investigated, and some charges caused by different experimental conditions were revealed.

Cathode Spot Behavior Variation in Oxide Layer Removal Caused by Potassium Hydroxide Pre-treatment

Cathode Spot Behavior Variation in Oxide Layer Removal Caused by Potassium Hydroxide Pre-treatment

Measurement of Charged Particle Current Diffused from Vacuum Arc

Particles diffused from vacuum arc can be used for thin film formation. Diffusion of particles from vacuum arc influences current breaking properties of a vacuum interrupter. For these application it is essential to clarify the behavior of particles diffused from vacuum arc at around current zero point. In this paper dependence of diffused charged particles current on the crest value of arc current before current interruption point was shown with Faraday cup in case of oxygen-free copper electrodes and CuCr (Cu:Cr=50:50) electrodes. Experimental results were as follows. Ions and electrons diffused from vacuum arc entered into the Faraday cup with pulsive and intermittent states, when vacuum arc of DC 100 A∼150 A was generated in case of oxygen-free copper electrodes. This phenomenon corresponds to the behavior of cathode spots. Electron current value was about 100 times as large as ion current value. The pulse width is about 1∼10μs. When the crest value of arc current were 2,000 A∼10,000 A in case of oxygen-free copper electrodes and CuCr (Cu:Cr=50:50) electrodes, the amount of charged particle current for CuCr before the current interruption point was larger than those for Cu. The ratios of the ion current to the electron current diffused from vacuum arc were calculated by the waveforms of electron and ion currents. The current ratios of 0∼0.02 were taken when the crest value of arc current 2,000 A∼10,000 A.

Controlled Bridge Transfer（CBT法）の適用による薄鋼板の溶接

In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by lowering the short-circuit current to a low level just before the re-arcing. The reduction of spatter requires an improvement in the accuracy of predicting the re-arcing by stabilizing the metal transfer and improving the robustness of the accuracy against disturbances. The controlled bridge transfer (CBT) process optimizes the accuracy of predicting the re-arcing in real time in response to the metal transfer, realizes spatter reduction and stable arc in non-pulsed GMAW. Traditionally, GMAW is carried out using electrode positive polarity. However, this polarity is not sufficient for welding extra-thin steel sheets, specifically those thinner than 1.0 mm. With electrode negative (EN) CBT process, although slight arc voltage fluctuation occurs caused by behavior of cathode spots on the tip of the wire during EN polarity GMAW, instantaneous voltage is used command computation to improve the transient response against the disturbance. Consequently, a stable arc can be obtained without increasing the number of short circuits in a unit time to obtain spatter-free welds.

Cathode spot craters in pulse vacuum arc cleaning of a metal surface oxide layer

Cathode spot behavior during vacuum arc cleaning (VAC) of a metal surface covered with an oxide layer is investigated using a pulse discharge. During VAC, a bright spot is visible. A dot-like trace is marked on the cathode surface by the removed oxide layer. Detailed surface analyses show that the surface roughness is increased and that the oxide layer is stripped off almost uniformly in the trace. The cathode spot size adjustment is more precise than in our previous experiment. Spot characteristics change concomitantly with variation of the discharge electric power.

Effect of Pressure on the Behavior of Cathode Spots in Oxide Removal by Arc Discharge

The effects of pressure on the cleaning action of the cathode spots were investigated in a wide pressure range from 10 to 20 000 Pa of inert atmosphere.Experiments were carried out by using hot rolled steel plates, the surfaces of which were covered with thick oxide of 5–10 µm.Experimental observation revealed the following results:(1) The cleaning action by cathode spots is confirmed even under a pressure as high as 20 000 Pa.(2) The number of cathode spots on the oxide surface increased with the increase of pressure in the range of 10–20 000 Pa.(3) The removal rate of oxide decreased with the increase of pressure.(4) The energy density of the cathode spot decreases with the increase of pressure.(5) The higher the pressure increases, the smaller area the migration of the cathode spot was restricted to. The migration area decreased with the increase of pressure.

Change in cathode spot behavior in vacuum arc cleaning with application of polyethylene glycol

The authors investigated the effect of polyethylene glycol (PEG) application on the oxide removal action of cathode spots. Experimental observations revealed that the cathode spots formed on the oxide layer on the metal surface can be classified into two different sub-types, slow cathode spot (SCS) and fast cathode spot (FCS). Two different sub-types of cathode spots could exist in a same test piece at a same time. The removal rate of oxide depends on the mixture ratio between FCS and SCS. The more the cathode spots belonged to FCS, the faster the oxide layer is removed. The application of PEG enhances the probability of the presence of FCS.