Characteristics of Macroparticle Emission from a High-Current-Density Multi-Cathode Spot Vacuum Arc

Macroparticle mass transport, size distribution, and spatial distribution were studied in a 6.5-MA/M2 25-ms Cu multi-cathode spot (MCS) vacuum arc. The macroparticle erosion rate was determined to be 105 ?g/C, and together with ionic emission, accounted for most of the cathodic erosion. The number of macroparticles emitted decreased exponentially with macroparticle diameter, with 20-80-?m macroparticles carrying the bulk of the mass transport. Macroparticles are emitted preferentially at an angle of 20° with respect to the cathode surface. In comparison to previous investigations, higher macroparticle erosion rates, a larger proportion of large macroparticles, and a higher emission angle are observed, and the differences are attributed to the large current density used in the present experiment.     

Conditions of the Anode Spot Formation in a Vacuum Arc

A critical analysis of available experimental data and models of an anode spot formation shows their insufficiency for developing a clear-cut physical model of anode processes in a high-current vacuum arc. Based on new results of studying an anode medium- and low-pressure arc region, a qualitative physical model of an anode spot formation in a vacuum arc is proposed. The main idea of the model is that a change of the sign of the anode voltage drop (from negative to positive) is a necessary condition for an anode spot formation. Experimental data are qualitatively discussed from the point of view of the proposed model.     

Measurement of the Current Distribution at the Anode in a Low-Current Vacuum Arc

The distribution of the current collected by a multiring anode is measured in a diffuse vacuum arc for several values of the arc current and the electrode separation. It is found that the distribution widens as the electrode separation increases. Comparison with previous measurements suggests an influence of the electrode geometry on the results.     

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.     

Discharge Modes at the Anode of a Vacuum Arc

Five possible discharge modes can exist at the anode of a vacuum arc. The two most common anode modes are a low current mode, where the anode is basically inert; and a high current mode with a fully developed anode spot. This anode spot is very bright, has a temperature near the boiling point of the anode material, and is a copious source of vapor and energetic ions. Three additional anode modes can occur in appropriate circumstances. A low current vacuum arc with electrodes of readily sputterable material will emit a flux of sputtered atoms from the anode. At intermediate currents, an anode footpoint can form. This footpoint is luminous, but much cooler than a true anode spot. Finally, a high current mode can exist where several small anode spots are present instead of a single large anode spot.     

Anode Sheath Growth and Collapse in a Hollow-Anode Vacuum Arc

Experimental diagnostics in a hollow-anode vacuum arc device have shown that the repetitive growth and collapse of an anode sheath is responsible for observed tens-of-kV voltage spikes. A dynamic model of device operation based on circuit effects and a time varying quasi-Child-Langmuir sheath is presented. The scaling of the repetition rate with respect to cathode material is discussed. The device is being investigated for use as a current limiter for ~10-MW pulses.     

Ion Flux from the Cathode Region of a Vacuum Arc

This paper reviews the properties of the cathode ion flux generated in the vacuum arc. The structure and distribution of mass erosion from individual cathode spots and the characteristics of current carriers from the cathode region at moderate arc currents are described. An appreciable ion flux (～10% of total arc current) is emitted from the cathode of a vacuum arc. This ion flux is strongly peaked in the direction of the anode, though some ion flux may be seen even at angles below the plane of the cathode surface. The observed spatial distribution of the ion flux is expressed quite well as an exponential function of solid angle. The ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply-charged ions. The average ion potential and ion multiplicity increase significantly for cathode materials with higher arc voltages, but decrease with increasing arc current for a particular material. The main theories concerning ion acceleration in cathode spots are the potential hump theory (PH), which assumes that all ions are created at the same potential, and the gas dynamic theory (GD), which assumes that all ions are created with the same flow velocity. Experimental data on the potentials and energies of individual ions indicates that these theories in their original forms are not quite correct, however extensions or modifications of the PH and GD theories seem very likely to be able to predict correct values for the charge states, potentials, and energies of individual ions.     

An Investigation of Regular Patterns of the Anode Spot Glow in a High-Current Vacuum Arc by the Method of High-Speed Photography

Russian Physics Journal - The evolution of an anode spot is investigated in the course of burning of a high-current vacuum arc. The evolution process is recorded with a high-speed video camera at a...     

Investigations of the Cathode Spot Dynamics in a Vacuum Arc Coating Process

Influence of cathode materials (Ti, Al, Cu, TiN), ambient gases (Ar, N₂, p = 0.1-1 Pa) and the arc current itself on the motion and the velocity of cathode spots in an arc coating process have been investigated with the help of a new high speed framing camera. It was found, that the cathode material causes different spot currents but in general the spot arrangement and the motion on the surface are similar. Surface contaminations due to ambient gases affect this dynamics in several ways. Insulating layers like AIN can drastically increase the instantaneous spot velocity, for example from <5 m/s on Al up to 170 m/s on AIN contaminated areas. TiN layers with a high conductivity increase the spot mobility at first. But at nearly completely contaminated surfaces (simulated by a TiN cathode), the mobility is strongly decreased. The values change from an average velocity of 6.3 m/s with a diffusion constant of 54 cm²/s (Ti, 0.01 Pa) to 2 m/s and 6.4 cm²/s at TiN. The course of the instantaneous spot velocity during the spot splitting phase was investigated too. The instantaneous spot velocity of each of the two new spots originated from the starting spot is relatively high (30–40 m/s) within the first 50 μs. The cathode material and the ambient gases are of slight influence in this phase. The movement is directed. In the further development the instantaneous spot velocity is decreasing to values under 5–10 m/s. The motion is now more and more random. Additionally it could be proved, that the lower stability limit for a stable discharge is strongly connected with the spot current, which depends on discharge conditions.     

Investigations of the Current Density in the Cathode Spot of a Vacuum Arc

A short review is given on the problem of the current density in the cathode spot of a vacuum arc, theoretical models and experimental results are discussed. A new measuring method is presented which is directly related to the current carrying area of the spot. It leads to current densities of 10¹² A/m² at clean surfaces (comparable with results obtained from crater measurements) and 10⁹?10¹⁰ A/m² with contaminated surfaces.     

脉冲激光烧蚀过程中靶材表面熔融前温度分布与激光功率密度分布的研究

用较为符合实际的高斯分布表示了脉冲激光输出功率密度分布，讨论了脉冲激光功率密度分布函数形状变化对烧蚀过程中靶材表面熔融前温度分布的影响。建立了考虑热源项的热传导方程，并给出了相应的边界条件。以Si为例，用有限差分方法模拟了温度随时间、位置的变化规律，模拟过程中强调了对边界条件的处理，使整体截断误差保持最小。通过改变脉冲激光功率密度分布函数的形状，分析了温度分布的变化。结果表明，相比恒定脉冲功率密度输出，功率密度高斯分布的激光束与靶材作用时高温阶段的温度变化率变大，靶材表面熔融时刻热扩散距离增加；当激光器上升沿变陡时，在有效作用时间内温度上升得更快，对加工区域周围热效应的影响明显减弱，而热扩散距离变小。     

Arc Root Attachment on the Anode Surface of Arc Plasma Torch Observed with a Novel Method

The arc-root attachment on the anode surface of a dc non-transferred arc plasma torch has been successfully observed using a novel approach. A specially designed copper mirror with a boron nitride film coated on its surface central-region is employed to avoid the effect of intensive light emitted from the arc column upon the observation of weakly luminous arc root. It is found that the arc-root attachment is diffusive on the anode surface of the argon plasma torch, while constricted arc roots often occur when hydrogen or nitrogen is added into argon as the plasma-forming gas.     

Structure and Properties of Silumin Surface after Vacuum Arc Plasma-Assisted Deposition of Coatings Irradiated by Low Energy High Current Pulsed Electron Beam

Russian Physics Journal - The paper presents the results of modification of the silumin surface layer using a multicycle processing technique which combines the formation of the film...     

Expansion Model of the Neutral Component in Vacuum Arc Spot Plasmas

Plasma generated by cathode spots in vacuum arcs expands towards the anode and the walls. In spite of a high degree of ionization (mostly), this plasma contains also a small but noticeable constituent of neutral atoms. Starting from the known analytical solution of a hydrodynamic model describing the plasma ions and electrons, the corresponding motion of neutrals is treated in a similar way. The results show: 1) The neutrals take part in the general plasma expansion; however, their final kinetic energy is lower (by about 50%) than the final energy of the ions. 2) The temperature of the neutrals also is lower than the ion temperature and decreases during expansion. 3) The acceleration of neutrals is caused partly by the pressure gradient, partly by interaction with the ions (friction); the forces diminish rapidly with increasing distance. Finally, the limitations of this approximate solution are discussed.     

Current and Voltage Distribution in the Diffuse Vacuum Arc

On the basis of extensive measurements, a model is developed of the diffuse plasma of the high-current vacuum arc. The model shows that the current constriction and the voltage distribution in the diffuse vacuum arc prior to anode-spot formation are caused by the pressure source to which the charged and the neutral particles contribute.     

Time Dependence of Anode Spot Formation Threshold Current in Vacuum Arcs

The variation of threshold current for the transition between the low current quiescent vacuum arc mode, and the high voltage noisy mode associated with anode spot formation, was measured as a function of peak current, current waveform frequency, and electrode separation on fixed diameter (25 mm) Cu and Ni electrodes. At current waveform frequencies of about 60 Hz on Cu electrodes, the threshold current depends mainly on electrode spacing, as has been observed by other investigators. However, at higher waveform frequencies, the threshold current becomes a strong function of peak current as well. At 347 Hz on 25 mm. diam. Cu electrodes separated by 10 mm, the threshold current rose from approximately 2 kA to 5.5 kA, as the peak current rose from 2 kA to 6 kA. At 543 Hz on 25 mm diam Ni electrodes separated by 9 mm, a saturation in threshold current at about 7.5 kA was observed for peak currents greater than 9 kA. Simultaneous anode temperature measurements indicated that the Ni anode surface temperature immediately prior to transition rose from about 1550° K to 2250° K with variations of peak current from 5 kA to 13 kA. Predictions of the variation of threshold current based on random transitions, and on cathode spot migration over the edge of the cathode, are compared with the experimental data.     

Influence of a Pulsed Axial Magnetic Field on a High-Current Arc of a Vacuum Circuit Breaker

Russian Physics Journal - The effect of a relatively short external axial magnetic field on the characteristics of a vacuum arc discharge in a vacuum circuit breaker is studied. A pulsed magnetic...     

Influence of a Non-Stationary Magnetic Field on the Charge Composition and Energy Distribution of Ions of the Cathode Plasma of a Vacuum Arc

Russian Physics Journal - The influence of an external non-stationary axial magnetic field on the charge composition and energy distribution of ions of various charge states in the cathode plasma...     

Magnetic Field Effect on the Volt‐Equivalent of Arc Spot Heat Flux

The results of an experimental study of the arc spot heat flux on the copper cathode of a coaxial Electric Arc Heaters (EAH), with magnetically driven arc, in air are presented. The three rings method was used for indirect separation of arc spot heat flux from total heat entering the electrode. As a result of joint generalizing new and previously published data, the linear dependence of the volt equivalent of arc spot heat flux on magnetic field was obtained for the range B = 0.01 – 1 Tesla and atmospheric pressure. A method, which allows to improve the measurement of the volt equivalent in highly unsteady arc spots, is proposed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Application of the High-Speed Photography Technique to Study Pulsed Vacuum Arc Plasma in a Short Gap

Physics of Atomic Nuclei - We studied the process of switching a short vacuum gap, initiated by an auxiliary discharge along the surface of the dielectric by registration images using an...