Microplasma generation in a microscale short vacuum arc

Usually multicomponent ions are generated from cathodic vacuum arc plasma with compound cathodes. In this paper, a novel method to produce multicomponent ions by simultaneous anodic and cathodic vacuum arc plasmas with pure electrodes has been studied. The vacuum arc discharge has been operated with Al anode and W cathode. To characterize the plasma plume generation, a combination of high-speed camera and time-of-flight mass spectrometer has been used to measure the spot phenomenon and multicomponent ions, respectively. Luminous spots have been simultaneously found on the anode and cathode to produce multicomponent ions, which has been verified by the examination of electrode erosion using SEM. The ion charge state distribution and multicomponent ions have also been measured. For comparison, the characteristic of vacuum arc discharge operated with the exchange of electrodes has been studied, which exhibits obvious discrepancies. This behavior of vacuum arc is considered to be controlled by the difference of cohesive energy, because the heat flow to the electrodes is sufficient to evaporate the electrode, and the ionization process by the plasma occurs in the nonequilibrium layers near the evaporated surface in cathode and anode regions. This interesting result is proposed to use for the synthesis of compound films.

As the main source of the vacuum arc plasma, cathode spots (CSs) play an important role on the behaviors of the vacuum arc. Their characteristics are affected by many factors, especially by the magnetic field. In this paper, the characteristics of the plasma jet from a single CS in vacuum arc under external axial magnetic field (AMF) are studied. A multi-species magneto-hydro-dynamic (MHD) model is established to describe the vacuum arc. The anode temperature is calculated by the anode activity model based on the energy flux obtained from the MHD model. The simulation results indicate that the external AMF has a significant effect on the characteristic of the plasma jet. When the external AMF is high enough, a bright spot appears on the anode surface. This is because with a higher AMF, the contraction of the diffused arc becomes more obvious, leading to a higher energy flux to the anode and thus a higher anode temperature. Then more secondary plasma can be generated near the anode, and the brightness of the ‘anode spot’ increases. During this process, the arc appearance gradually changes from a cone to a dumbbell shape. In this condition, the arc is in the diffuse mode. The appearance of the plasma jet calculated in the model is consistent with the experimental results.

This work is concerned with devising a method of evaluation of electron emission in the framework of the Murphy-Good theory, which would be as simple and computationally efficient as possible while being accurate in the full range of conditions of validity of the theory. The method relies on Padé approximants. A comparative study of electron emission from cathodes of arcs in ambient gas and vacuum arcs is performed with the use of this method. Electron emission from cathodes of arcs in ambient gas is of thermionic nature even for extremely high gas pressures characteristic of projection and automotive arc lamps and is adequately described by the Richardson-Schottky formula. The electron emission from vaporizing (hot) cathodes of vacuum arcs is of thermo-field nature and is adequately described by the Hantzsche fit formula. Since no analytical formulas are uniformly valid for field to thermo-field to thermionic emission, a numerical evaluation of the Murphy-Good formalism is inevitable in cases where a unified description of the full range of conditions is needed, as is the general case of plasma-cathode interaction in vacuum arcs, and the technique proposed in this work may be the method of choice to this end.

The anode spot in vacuum arcs is discussed; presently available experimental work and desirable areas for future work are examined. The following subjects are discussed: the transition of the arc into the anode spot mode; the temperature of the anode before, during, and after formation of an anode spot; and anode ions. Under appropriate experimental conditions formation of an anode spot may be preceded by the formation of an anode footpoint. This footpoint is luminous, but much cooler than a true anode spot. A fully developed anode spot has a temperature near the atmospheric boiling point of the anode material and is a copious source of vapor and energetic ions. A need exists for temperature measurements on anode spots in dc arcs. No measurements have been made on anode spots in arcs with rectangular current pulses whose durations are in the millisecond range, so measurements on any parameters of anode spots in such arcs would be desirable. 2 figures, 9 tables, 34 references.

A mathmatical model of electromagnetic processes occurring in the 'arc column – anode region – evaporating anode' system is presented. The anode region of electric arc with an evaporating metallic anode is described by a model, under which the non-equilibrium near anode plasma containing atoms and ions of the evaporated metal, along with atoms and ions of the ambient (inert) gas, is subdivided into a space charge layer immediately adjoining the anode surface and ionisation region adjacent to the arc column. This model allows determining the potential drop between welding arc column plasma and anode surface depending on the current density and plasma temperature near the anode, as well as upon the temperature of its surface.

This paper reports about experimental investigations on high-current vacuum-arc phenomena, especially anode-spot formation, arc states, and motion. The presented work was stimulated by lack of information about the transition process from the diffuse low-current mode to the high-current mode characterized by anode spot(s). Optoelectronic measurements, streak photographs, high-speed movies, and correlated arc voltage/current records yielded remarkable results on power-frequency vacuum arcs. Three different high-current vacuum arc modes can be observed beyond a certain threshold current. Which mode appears depends mainly on the momentary electrode distance. The modes are characterized by different anode-spot behavior and interelectrode phenomena. The transition between different arc modes is continuous. The arc modes observed on ring electrodes producing a magnetic blast field are the same as those appearing on butt-type electrodes. Anode-spot formation is preceded by congregations of cathode spots and may be initiated by thermal overload of the anode surface opposite to these cathode-spot clusters.

The objective of this paper is to compare the electron temperature and electron density in diffuse vacuum arc mode and anode spot mode. The experiment was based on the butt type copper electrodes, with contact diameter D=40mm, and gap distance g=10mm. The vacuum arc was ignited by a needle tungsten electrode, which was embedded in the cathode. A high-voltage pulse with a rise time of 8ns, and aptitude of 24kV was applied on the needle electrode. The arc current varied from 2.8kA to 7.5kA (rms). The arc current was measured by Hall sensor and the arc voltage was measured by Tektronix P6015A, respectively. The arcing process was recorded by a high-speed charge-coupled device (CCD) video camera. The emission intensities of the interelectrode plasma of the vacuum arc were collected by an optics spectrograph. The Bolzman plot method was used to calculate the electron temperature and the Saha equation was used to determine the electron density of diffuse vacuum arc mode and anode spot mode. The electron temperature was 1.09eV in diffuse arc mode and it was 1.22eV in anode spot mode. The electron density was increased from 7.6 × 1015/cm3 to 2.1 × 1015/cm3 when the arc mode changed from diffuse arc mode to anode spot mode.

Optical investigations of plasma phenomena using thermographic and high-speed video camera technology and spectroscopy are established and common methods for the analysis of plasma behavior in the experimental vacuum interrupter research. In this paper, plasma and anode spot investigations during high-current switching tests are performed with camera systems and spectrometers. The arc activity is investigated using butt type contacts. A high-speed camera records the switching contacts from the side and provides information on the present plasma mode and arc position. Different setups are applied to investigate the thermal emission from the anode. On the one hand, spectrometers are positioned either towards the anode surface in order to record the anode radiation or in parallel to the electrode surface in order to record the plasma activity. On the other hand, the anode surface is observed by a high-speed camera with a filter to register the thermal emission around 890 nm. In addition, the anode surface is recorded with a fast thermography camera (1500–1700 nm) to determine the surface temperature over time. The recordings are only possible shortly before and after current zero crossing. The anode temperature obtained in different spectral ranges are compared and the influence due to emission from the plasma is discussed.

An investigation into the failure of ceramic insulators that are used in a surface discharge switch (SDS) was conducted. The materials analyzed are Al/sub 2/O/sub 3/-25% SiC, MTF (modified alumina titanate), and CZA 500 (zirconia-alumina composite) ceramics. These insulators were subjected to high-current ( approximately 300 kA) surface discharges in atmospheric air and nitrogen. Energy-dispersive X-ray surface analysis was performed on the insulator surfaces in order to determine the contaminants that are present and the possible failure modes associated with the plasma arc environments mentioned above. Electrode erosion rates have been measured as a function of total charge transfer (up to 50 C/shot) for several in-situ materials including Cu-Nb, Cu-Nb+LaB/sub 6/, and Cu-Ta. Results from comparisons with standard Cu and CuW materials indicate that the in-situ materials represent an efficient method of retaining the copper in the bulk until it vaporizes and thus yield significantly lower erosion rates at high Coulomb transfer rates. >

<div class="htmlview paragraph">A new type of ignitor, the railplug, shows promise of extending the dilution limits for spark ignition engines. While much of the effort expended in our study of railplugs has focused upon demonstrating their effectiveness, it is recognized that railplug durability is presently not acceptable for production engine applications. The goal of the present study was to examine the factors that affect durability. The results of two types of investigations are reported. The effects of rail materials, pressure, delivered energy, and voltage at constant delivered energy on electrode erosion rates were studied for repeated firings in air at constant pressure. Railplug durability in a four-stroke SI engine was also evaluated, including examination of the effects of delivered energy, current pulse characteristics, and materials. This research was not intended to demonstrate improved railplug durability, but rather was intended to develop information to guide future efforts to improve railplug durability. The erosion rate studies showed 1) that several materials are promising candidates for center rails, and 2) that use of a high voltage firing circuit should significantly improve durability. The engine tests used a low voltage firing circuit and 1) indicated that future durability tests should be done using the capacitance that results in fastest combustion, 2) reaffirmed that decreased delivered energies increase durability, and 3) isolated two separate failure mechanisms that depend upon the delivered energy, with the division appearing to be a delivered energy in the range of 1.0-1.4 J. It is concluded that future efforts to improve railplug durability should focus upon 1) additional studies of center rail materials, 2) use of a high voltage firing circuit, 3) the lower delivered energy levels, and 4) most importantly, improved railplug geometries.</div>

Our new vacuum arc control technology SADE doubles the high current interruption capability of our conventional axial magnetic field technology. First, we describe the vacuum arc motion behavior recorded by a high speed charge-coupled device video camera. This arc behavior is closely related to axial magnetic field intensity. In particular, it depends on the profile of the externally generated axial magnetic field. The anode spot is likely to move to the highest magnetic field intensity. Second, we describe analytical results for concentration of vacuum arc at the anode side contact surface. This analysis implies the possibility of an ideal magnetic field profile and intensity for vacuum arc control. Finally, we describe experimental results for vacuum arc control compared with the physical and theoretical results mentioned above, and we show a practical electrode configuration for vacuum interrupters and its application in a high current interruption experiment.

Factors influencing the formation of anode spots in vacuum arcs have been investigated. For anode diameters of 1.25 and 5 cm, copper vapor arcs have been drawn to electrode spacings of 2.5 cm, and spot formation threshold currents of 400 and 2100 A have been determined for the small- and large-diameter anodes, respectively. Comparison of the arc voltage vs current characteristics, together with probe observations of potential distribution, reveals that an anode voltage drop is established prior to spot formation. The magnitude of this drop increases with both electrode spacing and arc current, and decreases with increase of anode area. By analogy with phenomena in mercury arcs, anode voltage drop formation is associated with vapor starvation. Photographs reveal that, even in the presence of an anode spot, high-voltage arcs can exist with a multiplicity of cathode spots and a diffuse arc column. Delays in the onset of the spot phenomena indicate that a temperature rise of the anode surface to the melting temperature precipitates the vapor release necessary for spot formation.

The objective of this article is to determine the anode surface temperature during and after current zero (CZ) with single and dual vacuum arc columns. Both the alternating current (ac) and pulsed direct current (dc) waveforms were applied with a peak value of 5–16 kA. A butt-type electrode and a horseshoe-type electrode were utilized to generate single and dual vacuum arcs. In the draw arc experiment, three spectrometers with working wavelengths of: 1) 200–1000 nm; 2) 880–1700 nm; and 3) 1600–2400 nm, a fast thermography camera, and two high-speed cameras were used for optical analysis. Based on the experimental results, it was certified that spectroscopic analysis with the Planck fitting procedure is an effective method to determine the anode surface temperature after CZ. The most preferable wavelength of the spectrometer in anode temperature determination was 800–1900 nm. The temperatures of anode spot 1, anode spot type 2, and footpoint mode were 3200–3300, 3800–4000, and 3000–3100 K, respectively. Two kinks appeared in the anode temperature cooling curve. The corresponding temperature was the liquidus temperature of the electrode material and the melting point of copper. Besides, two hot areas on the anode surface were observed for the horseshoe-type electrode. The anode temperature tended to be homogeneous at about 0.8 ms after CZ.

Anode melting and crater formation significantly affect interruption of high-current vacuum arcs. The primary objective of this paper is to theoretically investigate the mechanism of anode surface crater formation, caused by the combined effect of surface heating during the vacuum arc and pressure exerted on the molten surface by ions and electrons from the arc plasma. A model of fluid flow and heat transfer in the arc anode is developed and combined with a magnetohydrodynamics model of the vacuum arc plasma. Crater formation is observed in simulation for a peak arcing current higher than 15 kA on 40 mm diam. Cu electrodes spaced 10 mm apart. The flow of liquid metal starts after 4 or 5 ms of arcing, and the maximum velocities are 0.95 m/s and 1.39 m/s for 20 kA and 25 kA arcs, respectively. This flow redistributes thermal energy, and the maximum temperature of the anode surface does not remain in the center. Moreover, the condition for the liquid droplet formation on the anode surfaces is developed. The solidification process after current zero is also analyzed. The solidification time has been found to be more than 3 ms after 25 kA arcing. The long solidification time and sharp features on crater rims induce Taylor cone formation.

Cathode properties of arcs are investigated for ambient gas pressures of 10−3−725 Torr. Copper, silver, and carbon electrodes are used in controlled ambients of either nitrogen, argon, or helium. Experimental techniques include pressure-dependent observations of the ionized cathode vapor detected at cylindrical collectors surrounding the electrodes, arc photography, and measurements of the cathode erosion rate for copper at 100 A using the weight loss method. At pressures approaching atmospheric, it is shown that the measured erosion rate is reduced from that in vacuum by about an order of magnitude. Increasing the ambient pressure also decreases the distance from the cathode spots at which the 8% ion current can be detected. Data extrapolation shows that cathode spots at atmospheric pressure, particularly in the lighter gases, are still associated with the fundamental ion current. Comparison between the measured reductions in both the erosion rates and the ion currents indicates that vapor and ion redeposition on the cathode contribute to the decreased erosion. It is concluded that the cathode spots of low current arcs at pressures approaching atmospheric are basically similar to vacuum arc cathode spots. The final section of the paper compares current-dependent erosion data for copper arcs in vacuum and air, and shows the importance of collective cathode spot heating.