Arc Current Pulse Research Articles

Preparation of multilayered film structures by laser arcs

The new method of laser-sustained arc evaporation combines the advantages of the laser pulse vapour deposition (LPVD) technique with the technologically utilized vacuum arc evaporation method. The laser arc may be effectively and precisely controlled like LPVD but its energetic efficiency and the mechanical properties of the deposits equal those obtained by vacuum arcs. In the present paper the latest results of the technological development of the laser arc method are presented. Investigations of the arc ignition process and the influence of arc current pulse shape on target erosion and film deposition were carried out for different materials. On the basis of these results a controlled deposition of definite multilayered structures may be carried out. As an example a Ti/TiC multilayer system with 25 nm single layers is described. Results of structural and chemical analysis by means of Auger electron spectroscopy are presented. Deposition rates of 1–5 nm s −1 may be achieved even with the present experimental arrangement. A scheme for a full-scale technological device for multilayer preparation by laser arcs, which is currently under construction, will be presented.

Charge state distribution studies of the metal vapor vacuum arc ion source

We have studied the charge state distribution of the ion beam produced by the MEVVA (metal vapor vacuum arc) high current metal ion source. Beams produced from a wide range of cathode materials have been examined and the charge state distributions have been measured as a function of many operational parameters. In this article we review the charge state data we have accumulated, with particular emphasis on the time history of the distribution throughout the arc current pulse duration. We find that, in general, the spectra remain quite constant throughout most of the beam pulse, as long as the arc current is constant. There is an interesting early-time transient behavior when the arc is first initiated and the arc current is still rising, during which time the ion charge states produced are observed to be significantly higher than during the steady current region that follows.

Electric arc discharge in the tokamak TV-1

The basic task of this investigation was to clear up the nature of the emf causing arcing in the course of minor-disruption MHD instability. The arc current can flow through the limiter, between the limiters and between the limiter and the liner. The time correlation between the local poloidal magnetic field ( B pol ) and the arc current pulses and their propagation along the torus was found. Heavy arcing was observed at the final stage of the disruption when the plasma current sharply drops down. We assume that the interaction of the magnetic island with the limiter reduces the plasma current and generates an inductive emf.

Detection of radio-frequency signals emitted by an arc discharge

We analyze the nature of radiation signals detected from electrical arcing events originating on highly charged dielectric surfaces. Starting from a simple model of the arc current pulse, we calculate the electromagnetic radiation signal incident on a distant radio-frequency receiver. The receiver is tuned to some central frequency and operates in a given bandwidth. Because the receiver responds to the arc current derivative rather than the arc current itself, the detected radiation pulse profile can differ substantially from the current pulse profile, even when the receiver bandwidth is large compared to the reciprocal pulse width. This signal distortion significantly affects the interpretation of arc radiation data. To illustrate the point, we show that at high frequencies the detected pulse width carries very little information on the arc itself, and that even a scan of pulse width versus frequency may provide a misleading picture of possible arc dynamics. Finally, we argue that arc radiation data from radio-frequency measurements are most useful at low frequencies, when the receiver frequency and bandwidth are both comparable to the reciprocal arc duration.

Dielectric Recovery of Vacuum Arcs after Strong Anode Spot Activity

Recovery of dielectric strength and post-arc currents after diffuse and constricted vacuum arcs were measured for filat OFHC-Cu contacts (D = 25 mm, d = 7.5 mm) enclosed in a bakable UHV chamber. The arc current pulse had a trapezoidal shape of 5.5-ms duration with peak values up to 11 kA. In comparison with the fast recovery of diffuse arcs, the recovery of constricted arcs with gross melting is considerably retarded. Post-arc currents are simulated using the Andrews-Varey model extended to include the effects of secondary electron emission due to ion bombardment of the cathode and loss of the plasma due to thermal motion. The flow of charge carriers to the anode and the shield, which is at the anode's potential, are registered separately. The amount and decay of the residual plasma is evaluated from the measurements of post-arc current. The decay times of a few tens of a microsecond give evidence of ions with energies below 1 eV. The origin and effect of slow ions on recovery is discussed.

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.

Behavior of a High Current Vacuum Arc between Hollow Cylindrical Electrodes in a Radial Magnetic Field

Experimental observations were conducted on the behavior of a high current vacuum arc on cylindrical electrodes in a radial magnetic field. The arc was sustained between the ends of two cylindrical Cu electrodes, 54-mm diam and 1.5-mm wall thickness separated by 5 mm. Arc current pulses with peak values in the range 4-15 kA with a half amplitude full width (HAFW) duration of 8 ms were investigated with radial magnetic fields proportional to the instantaneous current with proportionality constants of 4.0 and 6.5 × 10-6 T/A. The arcs were photographed simultaneously with a streak camera and by a high speed framing camera and the arc voltage was recorded on a digitizing transient recorder. The results indicated that the arc in this geometry, both with and without an imposed radial magnetic field, can be characterized by three development stages: a) arc formation, b) diffuse arc along the electrode perimeter, and c) simultaneous existence of several concentrated arc columns. When a radial magnetic field was imposed two changes were noted: 1) the arc appeared somewhat more distributed in that a greater number of constricted columns were observed, and they were distributed more evenly; and 2) the constricted columns moved in the J- × B- direction with velocities in the range 5-35 m/s.

Effect of arc vibration and current pulses on microstructure and mechanical properties of TIG tantalum welds

Abstract The effect of the electromagnetic vibration of the arc and the influence of current pulses on the microstructure of welded tantalum sheets were investigated. Optimum conditions of arc vibration and of current pulses for refined microstructure in the fusion zone and in the heat-affected zone (HAZ) were determined. At these conditions equiaxed grains in the fusion zone and a decreased grain size in the HAZ were obtained. The effect of the welding rate on the microstructure was also investigated. Mechanical properties of the welds with improved microstructure obtained by the above techniques were compared with the properties of welds obtained by conventional TIG techniques and with those of the parent metal. Slight improvement in yield stress and tensile stress and significant improvement in elongation were observed in the improved welds, compared to welds obtained by conventional techniques.

Anode surface radiance from microsecond vacuum arcs

Measurements of anode surface radiance as a function of time for microsecond-duration vacuum arcs have been performed using an infrared-sensitive solid-state detector. Anode surface temperatures have been inferred from the measured radiance values using information concerning the surface spectral emittance. Particular attention has been given to the measurement of anode spot temperatures as a function of time. The anode materials investigated are Al, Cu, and Kovar. In addition, surface temperatures due to plasma heating of Al2O3 (alumina) and BeO (beryllia) ceramics surrounding the active anode area have been determined. Values of the average anode current density ranged from 6×103 to 5×104 A/cm2, with arc current pulse lengths of 1–12 μsec. Anode spot temperatures between 2600 and 3300°C for Cu, and between 2000 and 2400°C for Al were observed. The maximum surface temperatures of the ceramic insulators were between 2900 and 3500°C for Al2O3 and between 2000 and 2300°C for BeO. Calculations based on an integral equation solution to the heat-flow equation suggest that the input power density to the surface falls between 106 and 107 W/cm2 for Al, Cu, Al2O3, and BeO. Based on these values, estimates of the anode spot current density for Al and Cu fall between 105 and 106 A/cm2.

The deionization time of thyratrons: a new method of measurement

A description is given of a method of measuring the effective deionization time in thyratrons, i.e the time that must elapse, after arc current has ceased, before the grid regains control. In this method, voltage is applied to the anode at a known rate and at any desired interval after the passage of an arc-current pulse. The duration of the pulse, a few microseconds, is negligible compared with the deionization times being measured. The interval before the anode voltage is reapplied is reduced until the loss of grid control indicates that insufficient time has been allowed for deionization. A study of the decay of ionization, by observations of the ion current to the negatively charged grid, shows that grid control may be regained before deionization is quite complete. The method is illustrated by the results of comparative tests showing the variation of deionization time with changes in grid design, and in the nature and the pressure of the gas or vapour filling.