Arc Cathode Spot Research Articles

Split Process of Low‐Pressure Arc Cathode Spots on SS400(Fe + C) Surface with Oxide Layer

AbstractA remarkable characteristic of a cathode spot of a low‐pressure arc is that the cathode spot moves around on the metal surface. A cathode spot can remove an oxide layer. Cathode spots of low‐pressure arcs have been used for cleaning metal oxide surfaces. However, the influence of an oxide layer on the cathode spot movement remains unclear. As described herein, the cathode spot movements on the oxide layer surface and on the metal surface were measured to determine the cathode spot movement. Experiments were performed using an SS400 cathode and a cylindrical copper anode. A high‐speed video camera recorded the cathode spot movement. Then the obtained images were analyzed using plasma image processing. The cathode spot movement was generated mainly on the boundary layer, but sometimes on the processed surface. The first spot remains at the boundary when the cathode spot splits; the second spot moves on the bulk surface and reappears at a different boundary. Therefore, the cathode spot does not appear suddenly; instead, it moves continuously during the splitting process. The oxide layer serves an important role for cathode spot existence because of oxide layer vaporization. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

INFLUENCE OF AXISYMMETRIC MAGNETIC FIELD ON CATHODE SPOTS MOVEMENT IN ARC ION PLATING

Arc ion plating(AIP) has been widely utilized in the deposition of various kinds of thin solid films due to the excellent characteristics of the arc plasma produced from an active cathode spot that emits ions of cathode material,as well as electrons.In AIP process,the cathode spot is usually steered by an external magnetic field.Cathode spot motion is the key factor because it affects the physical characteristics of the vacuum arc plasma,the utilization of the cathode material, the emission of macroparticles(MPs) and the quality of subsequent films containing these MPs. Therefore,cathode spot dynamics should be understood practically under a compound external magnetic field,such as in axisymmetric magnetic field(AMF),for industrial applications.An AMF produced by using an adjustable electromagnetic coil associated with a concentric magnetic flux guider was applied to the cathode surface to investigate the influence of the AMF on the arc cathode spot motion.The distribution of the magnetic field was simulated by the finite element method (FEM) software.The magnetic field intensity was measured by an SHT-V magnetometer and the distributions of magnetic field with different intensities were analyzed.Based on the results of FEM simulation and the physical mechanism of the arc cathode spot discharge,the effects of magnetic-field components and AMF on the cathode spot movement were discussed.The results showed that increasing the AMF intensity can strongly influence cathode spot movement.In the case of a weak AMF, the cathode spot moves randomly on the cathode surface.With increasing AMF,there is an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge.The increase in the transverse magnetic field(TMF) intensity,B_T,can accelerate the rotational velocity of the cathode spot,increase the arc voltage and decrease the arc current.With a relatively strong AMF(B_T≈30 Gs), the cathode spot rotates near the edge of the cathode surface and is confined to a circular trajectory. A new arc cathode spot is ignited,splits,and is extinguished repeatedly on the cathode surface,which can be observed at intervals of about 0.5 s,while there is an obvious erosion track left at the bottom of the cathode edge.

Generation of multicomponent ion beams by a vacuum arc ion source with compound cathode

This paper presents the results of time-of-flight mass spectrometry studies of the elemental and mass-to-charge state compositions of metal ion beams produced by a vacuum arc ion source with compound cathode (WC-Co(0.5), Cu-Cr(0.25), Ti-Cu(0.1)). We found that the ion beam composition agrees well with the stoichiometric composition of the cathode material from which the beam is derived, and the maximum ion charge state of the different plasma components is determined by the ionization capability of electrons within the cathode spot plasma, which is common to all components. The beam mass-to-charge state spectrum from a compound cathode features a greater fraction of multiply charged ions for those materials with lower electron temperature in the vacuum arc cathode spot, and a smaller fraction for those with higher electron temperature within the spot. We propose a potential diagram method for determination of attainable ion charge states for all components of the compound cathodes.

A study of vacuum arc ion velocities using a linear set of probes

The most likely velocity of ions moving away from vacuum arc cathode spots was measured using a set of probes along the path of plasma expansion. The goal was to determine how much, if any, change in the ion drift velocity occurs in the expanded plasma. The arc discharge current was perturbed to create plasma density markers whose travel is picked up by the set of probes. It was found that the perturbation with current oscillations did not result in consistent data because ion current maxima and minima are determined not only by the plasma production but also by the transients of the arc pulse and by the asymmetry of the ion velocity distribution function. Perturbation with a short current spike was more conclusive. The most likely ion velocity, which depends on the cathode material but is generally (1–3) × 104 m s−1, was measured to be reduced (about 25%) with increasing distance (0.5 m) from the cathode, which can be explained by collisions of ions with the background of neutrals. The ion velocity was slightly increased when the arc current was increased (about 15% when going from 50 to 400 A), which correlated with enhanced arc voltage and power dissipation. The ion velocity could be enhanced (by about 20%) when the plasma was produced in a non-uniform magnetic field (up to 300 mT).

Continuous phase transition in the region of the vacuum arc cathode spot

A model for the near-cathode region of electric arcs is presented to investigate the liquid-plasma phase transition in the cathode spot region. Due to the high values of pressure and temperature after spot ignition, a “continuous phase transition” occurs in the liquid-vapor interface. A set of fluid equations with suitable boundary conditions have been solved to obtain diagrams of the spot plasma in the temperature-density plane during the spot evolution for a typical spot. To evaluate the model, the magnitude of some essential quantities such as the mean ion charge state of plasma and current density have been calculated, which are in accordance with experimental results.

Relation between surface roughness and number of cathode spots of a low-pressure arc

A remarkable characteristic of the cathode spot of a low-pressure arc is that it can remove an oxide layer preferentially. Recently, cathode spots of a low-pressure arc have been used for cleaning metal oxide surfaces before thermal spraying or surface modification. Nevertheless, few reports have described the cathode spot movement or the oxide removal process. This experiment was carried out using a Fe+C cathode workpiece and a cylindrical copper anode. The cathode spot movement was recorded using a high-speed video camera. The images were later analysed using plasma image processing. The workpiece surface, which was covered with a 9.67 µm thick oxide, was analysed using laser microscopy after processing. The surface roughness and the number of cathode spots showed no direct relation because the current density per cathode spot did not change according to the number of cathode spots.

Experimental investigation on the motion of cathode spots in removing oxide film on metal surface by vacuum arc

The motion of vacuum arc cathode spots has a very important influence on the efficiency of removing the oxide film on the metal surface. In this paper, the characteristics of cathode spot motion are investigated experimentally. Experiments were conducted in a detachable vacuum chamber with ac (50 Hz) arc current of 1 kA (rms). A stainless steel plate covered by an oxide layer was used as the cathode. The motion of cathode spots during the descaling process was photographed by a high-speed digital camera with an exposure time of 2 µs. Experimental results indicate that the motion of cathode spots is influenced by the interaction among individual cathode jets and the position of the anode as well as the surface condition. The waveform of arc voltage is also influenced by the motion of cathode spots.

Bonding adhesive strength for plasma spray with cathode spots of low-pressure arc after grit blasting and removal of oxide layer on metal surface

Cathode spots of a low-pressure arc can remove oxide layers and evaporate impurities on metal surfaces. Removal of the oxide layer using cathode spots is expected to solve recent obstacles due to chemical and mechanical cleaning methods. Various phenomena of cathode spots have been investigated for pre-treatment of atmospheric pressure plasma spray (APPS). This study treated the surface shapes of oxide and non-oxide samples using a composite pre-treating method: cathode spots after grit blasting. In addition, the samples are compared with conventionally treated cathode spots and mechanical blasted surfaces. Results show that roughness on the sample surfaces becomes higher in concomitance with the initial oxide layer thickness. This fact reveals the factors those dominant bonding strength on the pre-treated surface using cathode spots of a low-pressure arc. Bonding strength becomes higher in relation to the arithmetical mean height of the surface roughness caused by the initial oxide layer thickness. Bonding strength is higher when the mean spacing of profile irregularities is narrower. The bonding adhesive strength after treatment using cathode spots after grit blasting is greater than 90 MPa. However, cathode spot treatment must be limited not to destroy the projection formed by melting after grit blasting in an arc time up to 5 s. And basically the removal characteristics of oxide layer using the cathode spots were discussed on the trace, roughness, number of cathode spots in time and pressure, moving speed on the metal surface, etc.

Distribution of Cathode Spots in Vacuum Arc Under Nonuniform Axial Magnetic Fields

Recent results on the distribution of vacuum arc cathode spots (CSs) in nonuniform axial magnetic field (AMF) are presented. Based on previous studies, we deem that two contrary influences of AMF, inward effect and outward effect, are attributed to CSs distribution. With this notion, we have analyzed the controlling effectiveness of nonuniform AMF on CSs distribution. Experiments were conducted in a detachable vacuum chamber with iron-style AMF electrodes. Images of vacuum arc column and the distribution of CSs were photographed with a high-speed charge coupled device (CCD) camera. Experimental results agreed well with the theoretical analysis.

Physical model of the cathode spot retrograde motion

A physical model is proposed that describes the retrograde motion of a dc arc cathode spot as a process of plasma propagation in the direction opposite to the Poynting vector (against the Lorenz force).

Use of kinetic simulations for the determination of particle and energy fluxes at the cathode surface of a vacuum arc

A kinetic model and the associated PIC-MCC numerical technique have been used to simulate the plasma generated by a vacuum arc cathode spot fragment and calculate the current densities and net energy fluxes at the cathode surface. The model is based on a comprehensive description of the plasma generation processes, including a detailed representation of metal vapor particle ionization and plasma-cathode interaction. An important feature of this model is its ability to provide details of the spatial variations of plasma parameters within the near cathode region. Calculations are carried out for nickel, titanium and zirconium cathodes as a function of the surface temperature. It is found that below a threshold surface temperature the energy input to the cathode is negative and the spot fragment can not operate in a self-sustained way. The relative importance of the different mechanisms controlling current and energy transfers at the cathode surface are analyzed in detail.

Measurements of the total ion flux from vacuum arc cathode spots

The ion current from different cathode materials was measured for 50-500 A of arc current. The ion current normalized by the arc current was found to depend on the cathode material, with values in the range from 5% to 19%. The normalized ion current was generally greater for elements of low cohesive energy. The ion erosion rates were determined from values of ion current and ion charge states, which were previously measured in the same ion source. The absolute ion erosion rates ranged from 16-173 /spl mu/g/C.

Light emission of droplet spots at vacuum arc and after arc extinction

Some droplets emitted by vacuum arc cathode spot and surrounded by dense plasma are referred to as droplet spots. The goal of this work is the study of spectral characteristics of the droplet spot light emission. The spectrum of droplets was compared with the spectrum of the cathode spot. The light emission of droplet spots after arc extinction was also studied. Non-monotonous (twinkling) glow of droplet spots was found to last for a long time (up to 1 ms) after arc extinction.

The fractal nature of vacuum arc cathode spots

Cathode spot phenomena show many features of fractals, for example self-similar patterns in the emitted light and arc erosion traces. Although there have been hints on the fractal nature of cathode spots in the literature, the fractal approach to spot interpretation is underutilized. In this work, a brief review of spot properties is given, touching the differences between spot type 1 (on cathodes surfaces with dielectric layers) and spot type 2 (on metallic, clean surfaces), as well as the known spot fragment or cell structure. The basic properties of self-similarity, power laws, random colored noise, and fractals are introduced. Several points of evidence for the fractal nature of spots are provided. Specifically, power laws are identified as signature of fractal properties, such as spectral power of noisy arc parameters (ion current, arc voltage, etc.) obtained by fast Fourier transform. It is shown that fractal properties can be observed down to the cutoff by measurement resolution or occurrence of elementary steps in physical processes. Random walk models of cathode spot motion are well established: they go asymptotically to Brownian motion for infinitesimal step width. The power spectrum of the arc voltage noise falls as 1/f/sup 2/, where f is frequency, supporting a fractal spot model associated with Brownian motion.

Refraction and absorption shadow imaging of the vacuum arc cathode spot at an atomic resonance line of cathode vapors

The resonance laser interference and absorption shadow imaging combined with the high-speed IMACON 468 camera was used for studying the dynamics of metal vapors supply into the cathode spot plasma at operation of low-current vacuum arc discharge with liquid-metal cathode. The experiments were performed with a capillary-type liquid-metal cathode made of gallium-indium alloy. The data are analyzed in terms of cyclicity of cathode spot operation. Two distinct modes of cathode spot operation were discerned in images, which are the explosive mode and the steady mode. The explosive mode takes the 2%-part of the spot operation cycle. It follows by the steady mode (/spl sim/20% of the spot cycle time). The rest time the spot is transparent at the resonant line of cathode vapors. At a discharge current above 50A, the spot is always in the transparent mode.

Laser stabilisation of arc cathode spots in titanium welding

Cathode spot formation is very pronounced during arc welding of titanium and titanium alloys. The dynamic behaviour of these spots was observed to interfere with metal transfer during welding, this interference being a fundamental cause of poor weld quality in these alloys. In the present work, stabilisation of the arc cathode spot with a focused Nd–YAG laser beam during pulsed gas metal arc welding of titanium was investigated. The laser beam was focused near the leading edge of the weld pool and the laser power and focus spot size were varied to determine the values required to confine the cathode spot to the laser focus position. The results showed that, for fixed welding conditions, the laser power required to prevent cathode spot motion varied as a function of focus spot size. The required laser power was minimised at 200 W for a spot size of 0.6 mm. The laser stabilised arcs had lower voltage but approximately the same current density as stabilised arcs. Increased welding speeds required marginally higher laser powers to stabilise the spot, but the minimum power was still attained with a 0.6 mm focus spot diameter. The laser power density required for stabilisation decreased as spot size was increased, varying from almost 106 W cm−2 at the smallest spot size to approximately 104 W cm−2 at the largest. Cathode spot stabilisation improved weld quality by reducing spatter generation and weld bead irregularity.

Pre-treatment on metal surface for plasma spray with cathode spots of low pressure arc

The cathode spots of the low pressure arc can remove the oxide layer and evaporate impurities on the metal surface. The removal of the oxide layer by using the cathode spots is expected to solve the recent problems of the chemical and mechanical cleaning methods. The phenomena of the cathode spots for pre-treatment of Atmospheric Pressure Plasma Spray (APPS) have been investigated. In this paper, the surface shapes of oxide and non-oxide work pieces were treated by using a composite pre-treating method: the cathode spots after grid blasting. In addition, they are compared with conventional cathode spots treated and mechanical blasted surfaces. As the results, the roughness on the surface of work pieces is arisen with the initial oxide layer thickness. It reveals the factors dominating bonding strength on the surface pre-treated by using the cathode spots of the low pressure arc. The bonding strength increases with the arithmetical mean height of the surface roughness due to the initial oxide layer thickness. The bonding strength is raised when the mean spacing of profile irregularities becomes narrow. The bonding adhesive strength treated by the cathode spots after the grid blasting is over 90 MPa.

Erosion of a Copper Cathode in a Nonstationary Arc Spot. II. Determination of the Energy Parameters of the Arc Spot by the Thermophysical Method

The results of investigation of the energy parameters of a nonstationary electric‐arc spot on a copper cathode in an air atmosphere have been presented. The data published earlier on the thermal volt‐equivalent of the cathode arc spot have been generalized. A nonlinear dependence of the current density on the magnetic field has been obtained. A comparison of the new results and those obtained earlier on other setups and their generalization have been made.

Mechanism for small electron current fraction in a vacuum arc cathode spot on a refractory cathode

A mechanism for refractory cathode spot operation in a vacuum arc is proposed, based on the presence of a finite electric field Ep at the cathode sheath–plasma interface. The proposed model determines the relationship between the electron current and the total spot current density j and includes a system of equations solved for cathode plasma containing a mixture of ion states. The calculation shows that Ep reaches a value, ∼106 V/cm, and j is 104–108 A/cm2, when the electron current fraction s increases from 0.5 to 0.9. It is discovered that for a refractory cathode, ions with high ionicity (from 2 to 4) can be generated in the spot region for a relatively small s, 0.7–0.9, in contrast to ionization in the plasma jet, as predicted previously for copper cathodes.

Method of Matched Asymptotic Expansions Versus Intuitive Approaches: Calculation of Arc Cathode Spots

A model of steady-state solitary spots with a step-function dependence of the density of the energy flux from the plasma to the cathode surface on the surface temperature is suggested. While being simple and clear cut, this model, nevertheless, reflects essential features of the physics involved. It is found that a solution obtained by means of the method of matched asymptotic expansions is close to exact numerical results, in contrast to a solution obtained with the use of Steenbeck's principle of minimum power.