S. Shalev

Fast deposition of metallurgical coatings and production of surface alloys using a pulsed high current vacuum arc

A preliminary investigation was conducted on the metallurgical treatments produced on a planar workpiece connected as an anode in a pulsed 1 kA vacuum arc. Generally a combination of deposition and heating effects could be produced. Very high deposition rates (approximately 25 μm s-1) are calculated on the basis of the ion flow, and even higher rates (exceeding 100 μm s-1) are observed if part of the macroparticle flow is incorporated into the coating. The concurrent heating of the workpiece can, depending on the arcing conditions, promote interdiffusion of the coating and substrate materials forming a metallurgical bond, can melt the substrate surface in the presence of the plasma flow forming surface alloys and can induce martensite formation near the surface of steel workpieces.

Velocities and emission rates of cathode‐produced molybdenum macroparticles in a vacuum arc

Time‐ and space‐resolved measurements of the axial and transverse velocity components of Mo macroparticles produced in a 1‐kA peak current, 30‐ms duration vacuum arc were performed using laser‐Doppler anemometry (LDA). Individual macroparticle velocity components in the range of 10–750 m/s were detected and average velocity components in the range of 100–400 m/s were determined. Using the results of the measurements of individual velocity components we obtain that a typical rms value of the macroparticle speed is in the range 400–700 m/s. Comparing the average values of the radial and axial components indicates that typical emission angles are at approximately 30°±5° with respect to the cathode surface. The relative time‐resolved macroparticle emission rate was measured by using the LDA apparatus as a particle counter. Peak macroparticle emission occurred at a time midway between peak current and peak average cathode surface temperature. This result, together with the observation that the macroparticle em...

A Model of the Multicathode-Spot Vacuum Arc

A model is proposed for the multicathode-spot (MCS) vacuum arc. A zero-order model is filrst constructed, whereby the interelectrode plasma is produced by the multitude of cathode spots, and flows to the anode upon which it condenses. The electron density is calculated by assuming that the plasma is uniform within a cylinder bounded by the electrodes and using expenmental data for the ionic velocities and ion current fraction obtained in single cathode spot arcs. The electron density thus obtained is proportionate to the current density, and is equal to 5 × 1020 m-3 in the case of a 107-A/m2 Cu arc. The model predictions are a factor of 3-4 lower than measured values. First-order perturbations to the zero-order model are considered taking into account inelastic electron-ion collisions, plasma-macroparticle interactions, the interaction of the self-magnetic field with the plasma and electric current flows, and the interaction with the anode. Inelastic collisions tend to increase the ionicity of the plasma as a function of distance from the cathode, in agreement with spectroscopic observations. Macroparticles are heated by ion impact until they have significant evaporation rates. The vapor thus produced is ultimately ionized, and most probably accounts for the discrepancy between the zero-order prediction of electron densities and the measured values. Constrictions near the anode in both the plasma and electric current flows have been calculated. An overabundant electron current supply forces the anode to assume a negative potential with respect to the adjacent plasma.

Macroparticle Dynamics during Multi-Cathode-Spot Vacuum Arcs

Macroparticle dynamics in multi-cathode-spot (MCS) vacuum arcs were studied by utilizing laser Doppler anemometry (LDA) methods for in situ measurement of the cathodic macroparticle velocities and relative emission rates. Arc current pulses having peak values of 1-2 kA at either 6 or 1 ms after arc initiation were investigated. Systematic dependence of the macroparticle dynamics (i.e., speed and direction of flight) on cathodic thermophysical properties, location of the measurement probe in the interelectrode region, instantaneous value of the arc current, arc current waveform, and macroparticle size was determined. It was found that the macroparticle velocity increased with the melting temperature of the cathode metal, distance from the cathode surface, and the instantaneous value of the arc current, and decreased with macroparticle size and the rise time of the current waveform. All the above dependencies may be understood as direct indications of the plasma-macroparticle interaction during the discharge. The measured instantaneous relative emission rates were found to peak later than the arc current but before the peak average cathode surface temperature, which was estimated using a semi-empirical model. This result may be an indication of the dependence of cathodic erosion in the form of molten metal droplets on the average cathode surface temperature.

Anode Melting in a Multicathode-Spot Vacuum Arc

Melting of the anode surface in a multicathode-spot vacuum arc is expected when the incident energy flux is not balanced. The anodic energy influx is proportional to the arc-current collected by the anode and melting of the anode should be observed when peak arc-current exceeds a critical value. In this work, the critical peak arc-current Ipt was measured, and its dependence on anode and cathode materials was determined. The arc was sustained between two parallel cylindrical electrodes, 14 mm in diameter and spaced 4 mm apart. The almost critically damped current pulse lasted for 30 ms with a 6-ms rise time to peak value. Peak currents were in the range of 500-2300 A. In most of the experiments the anode material differed from that of the cathode. In the runs where the cathode-anode materials were Cu-Al or Mo-Cu, respectively, the time dependence of a spectral line intensity radiated by the anode atoms located in the plasma near the anode surface was recorded. We found that Ipt depended on both the anode and cathode materials. Thus for an Al anode and Al and Cu cathodes, Ipt equaled to 1100 and 900 A, respectively. In arcs with a peak current larger or equal to Ipt, a sudden jump of the spectral line intensity was observed. In all experiments, even when strong melting of the anode was observed, the arc-voltage stayed quiescent and in the range 15-35 V, suggesting that no anode spot was formed.

Laser Doppler anemometry: a tool for studying macroparticle dynamics in a vacuum arc

A method, based on laser Doppler techniques, for measuring macroparticle velocities and emission rates in vacuum (metal vapour) arcs is presented. A real-fringe forward-scattering system was employed, excited by a He-Ne laser, producing an ellipsoidal probe volume. Time domain analysis of individual scattering events was used to determine the macroparticle velocity components, while counting of the individual events was used to yield macroparticle emission rates. The experimental method enabled a study of macroparticle dynamics as a function of space and time in pulsed vacuum arcs of 1 and 2 kA peak current, 5 and 30 m s-1 duration, sustained between 12-14 mm electrodes, spaced 4 mm apart and fabricated from either Cd, Zn, Al, Cu, or Mo. Broad velocity component distributions, ranging from about 10 m s-1 up to about 800 m s-1, and dependent on cathode metal, arc current waveform, time after arc initiation and spatial location in the discharge volume, were measured. The macroparticle emission rate was found to reach its peak a few milliseconds after the occurrence of peak current.

Excited-state densities in a multi-cathode-spot Cd vacuum arc

Abstract An experimental study of Cd I and Cd II excited-state densities in the interelectrode plasma of a multi-cathode-spot Cd vacuum arc is presented. Current pulses of 1.2 kA peak, 0.65 ms HAFW were passed between 12 mm diameter Cd electrodes spaced 4 mm apart and located in a vacuum chamber. Absolute time and space resolved line intensities were measured using a calibrated lens-monochromator-photomultiplier system. Peak excited-state densities ranging from 10 16 to 10 19 m -3 were observed. Excited-state densities generally decreased as a function of distance from the cathode. The time at which the peak excited-state density occurs increased as a function of distance from the cathode, and decreased as a function of excitation energy. Cd II peaks occurred before Cd I peaks. Cd I excited-state densities and most Cd II excited-state densities could be related empirically through the Boltsmann distribution with distribution temperatures of 0.6–0.9 and 1.1–1.5 eV, respectively, though some Cd II states (6–11g 2 G) sometimes displayed inverted populations.

Absorption effects of the Cd II 4416 Å line in a cadmium vacuum‐arc plasma

The absorption of the 4416 A He‐Cd laser line (a2D5/2 →5p 2P3/2) by a cadmium vacuum‐arc plasma, and its dependence on time from arc initiation, spatial position in the interelectrode region, electrode separation, and the peak of current waveform, were determined. The arc was sustained between two cylindrical electrodes of 12 mm diameter. The current pulse lasted for 1.7 ms with peak current at 0.3 ms. The derived relative absorption of the laser line is found to be as high as 70% for electrode separation of 4 mm and peak current of 1.2 kA. We find that the time to peak absorption does not coincide with time to peak current. Furthermore, the absorption increases with increasing peak current or decreasing electrode separation. The measured optical depth of the vacuum‐arc plasma is used for the calculation of the arc plasma self‐absorption at 4416 A, the absorption‐corrected population density of the a2D5/2 level, and the estimation of the Cadmium ions velocity spread parallel to the optical observation axi...