Metal Vapor Density Research Articles

Stepwise behavior of free recovery processes after diffused vacuum arc extinction

The objective of this paper is to gain insight into free recovery processes in vacuum interrupters after diffused vacuum arcs extinction. A sub-microsecond voltage impulse, which peaked at 90 kV with a rising time of 150 ns) (rate of rise 480 kV/μs), were electronically controlled to apply upon a switching gap of a vacuum interrupter at a predetermined interval after a power frequency current zero. Based on the breakdown voltages at different intervals after arc extinction, we can obtain the free recovery behavior of the vacuum interrupter. The contact materials under study included Cu, CuCr25 and CuCr50. The contacts were butttype, and the contact diameters were 12 and 25 mm, respectively. The arc current peaked at 2.1 kA with a frequency of 50 Hz and the arcing time was ~9 ms. The results show the free recovery behavior stepped from ~28 kV to ~70 kV at ~4 μs after arc extinction for the three kinds of contact materials with the two contact diameters given. By calculation of the dissipation of metal vapor density and ion density, we conclude that the decay of ions, rather than metal vapor, may be the main reason for the stepwise behavior of the free recovery process after diffused vacuum arcs extinction.

Temperature gradients in diode-pumped alkali lasers

Abstract : Localized heating in diode-pumped alkali lasers induces spatial variations in metal vapor density, leading to a reduction in laser power.

Modelling of metal vapour in pulsed TIG including influence of self-absorption

Pulsed TIG (tungsten inert gas) welding is used to improve the stability and speed of arc welding, and to allow greater control over the heat input to the weld. The temperature and the radiation power density of the pulsed arc vary as a function of time, as does the distribution of metal vapour, and its effects on the arc. A self-consistent two-dimensional model of the arc and electrodes is used to calculate the properties of the arc as a function of time. Self-absorption of radiation is treated by two methods, one taking into account absorption of radiation only within the control volume of emission, and the other taking into account absorption throughout the plasma. The relation between metal vapour and radiation power density is analysed by calculating the iron vapour distribution. The results show that the transport of iron vapour is strongly affected by the fast convective flow during the peak current period. During the base current period, the region containing a low concentration of metal vapour expands because of the low convective flow. The iron vapour distribution does not closely follow the current pulses. The temperature, iron vapour and radiation power density distributions depend on the self-absorption model used. The temperature distribution becomes broader when self-absorption of radiation from all directions is considered.

Experimental Investigation of Anode Activities in High-Current Vacuum Arcs

It is well known that the melting of electrodes (mainly anode melting) in vacuum arc can increase the metal vapor density around current zero and even lead to interruption failure. In order to clarify the anode activities and their influence on arc appearance and interruption capacity, series experiments of cup-shaped axial magnetic field copper electrodes were conducted. Obvious anode melting was detected; the liquid copper flowed on the contact plate of anode and formed a clockwise swirl flow. The appearance of anode melting is likely to correlate to the transition of arc mode from high-current diffuse mode to high-current diffuse column mode. The melting of anode was severer than cathode and was influenced by the distribution of cathode spots. Various kinds of copper particles at macroscopic level can be seen in arc column. Even at the interruption limit, the majority of melted copper of anode sputtered out of gap in form of liquid droplets or was pressed into the cup of anode, the copper vapor evaporated into arc column only accounted for a few portion and no obvious anode jets was found due to large plasma pressure in arc column.

Use of multiwavelength emission from hollow cathode lamp for measurement of state resolved atom density of metal vapor produced by electron beam evaporation

State resolved atom population of metal vapor having low-lying metastable states departs from equilibrium value. It needs to be experimentally investigated. This paper reports the use of hollow cathode lamp based atomic absorption spectroscopy technique to measure online the state resolved atom density (ground and metastable) of metal vapor in an atomic beam produced by a high power electron gun. In particular, the advantage of availability of multiwavelength emission in hollow cathode lamp is used to determine the atom density in different states. Here, several transitions pertaining to a given state have also been invoked to obtain the mean value of atom density thereby providing an opportunity for in situ averaging. It is observed that at higher source temperatures the atoms from metastable state relax to the ground state. This is ascribed to competing processes of atom-atom and electron-atom collisions. The formation of collision induced virtual source is inferred from measurement of atom density distribution profile along the width of the atomic beam. The total line-of-sight average atom density measured by absorption technique using hollow cathode lamp is compared to that measured by atomic vapor deposition method. The presence of collisions is further supported by determination of beaming exponent by numerically fitting the data.

Influence of the lifetime of the laser-excited level and the laser pulse duration on the saturated LIF signal during the prebreakdown phases of a pseudospark discharge

Ground state copper vapor number density, sputtered from the cathode of an experimental pseudospark switch is measured using laser-induced fluorescence (LIF) from the copper transition 4/sup 2/P/sub 3/2/-4/sup 2/D/sub 5/2/ at 510.55 nm. Measurements are made in the hollow cathode at several axial positions in the time interval between the trigger pulse and the voltage breakdown. At voltage 1 kV, this interval, referred to as a prebreakdown phase, has a random and variable shot to shot duration, but discharges for which this duration is the same reveal similar properties and can be arranged in groups. The presented results are for prebreakdown duration of 1 /spl mu/s. The metal vapor density is determined from the LIF signal by using both its maximum and integral. This approach follows from a widely used LIF model, which is examined in detail to reveal a less familiar feature, namely the possibility to obtain an estimate of the lifetime of the laser-excited level, which has been considered perturbed by collisions during the discharge. A comparison between this estimate and the lifetime, obtained by the exponential decay of the LIF signal, reveals good agreement.

A novel diagnostic for time-resolved spectroscopic argon and lithium density measurements

A new diagnostic technique for determining neutral gas and metal vapor densities over a wide pressure range with high time resolution is described. A compact, current-stabilized hollow cathode/anode discharge is use to ionize the gas and/or trace impurity to be analyzed. We report proof-of-principle operation of the diagnostic in a well characterized argon discharge in the pressure range 6.7–265Pa. The intensity ratio of neutral and singly ionized argon lines (IArI/IArII) is shown to increase monotonically with the neutral argon density. In a second experiment, absolute neutral lithium vapor densities have been measured using this diagnostic.

Influence of the cathode and anode jets on the properties of a high-current electric arc

A study is made of the effects related to the formation of electrode jets in discharges in hydrogen and air at a current of 105–106 A, a current growth rate of 1010 A/s, an initial pressure of 0.1–4.0 MPa, and a discharge gap length of 5–40 mm. After secondary breakdown, jets are observed in a semitransparent discharge channel expanding with a velocity of (4–7)×102 m/s. The formation of shock waves in the interaction of the jets with the ambient gas and the opposite electrode is observed by the shadowgraphy method. Seventy microseconds after the beginning of the discharge, the pressure of the metal vapor plasma near the end of the tungsten cathode amounts to 177 MPa. The brightness temperature in this case is T=59×103 K, the average ion charge number is \(\overline m = 3.1\), and the metal vapor density is n=5.3×1019 cm−3. After 90 µs, the average ion charge number and the metal vapor density near the anode end are \(\overline m = 2.6\) and n=7.4×1019 cm−3, respectively. Based on the experimental data, possible reasons for the abnormally high values of the total voltage drop near the electrodes (up to ∼1 kV) are discussed.

Optical determination of alkali metal vapor number density in the vicinity (∼10^-5 cm ) of cell surfaces

We describe a method for determining the number density of the alkali metal vapor by use of an evanescent wave in a single total internal reflection. The method is simple to use, and it has the following useful features: It is most suitable for determining the number density of dense (>1013 cm-3) alkali metal vapor and it permits the measurement of the alkali metal vapor number density in the vicinity (∼10-5 cm) of cell surfaces. Using this method, we measured the number density of Rb metal vapor in the vicinity (∼10-5 cm) of the cell wall in cells that contain only Rb metal and no buffer gas; we found that there is no systematic difference between the Rb number density near the surface and that in the bulk.

The spatial distributions of spectral intensity and temperature in the cross section of an arc column between separating Pd contacts

The authors have measured the distributions of spectral intensities of PdI-447 nm and PdI-540 nm and the distribution of the temperature in the cross section of a Pd arc column near the cathode contact between separating Pd contacts in circuits of DC 50 V/3.3-5.0 A, As a result, the distributions of spectral intensities in the cross section of an arc column were not symmetrical from ignition to extinguishing of a breaking arc, and the cross-sectional area of an arc column became large past the middle of a breaking arc. The total amounts of measured spectral intensities decreased slightly in case of long arc or remained constant in the case of short arc past the middle of a breaking arc. The peak value of the distributions of spectral intensities in the cross section of an arc column was strong at the beginning of the breaking arc and became weaker with development of the breaking arc. These intensities were greater in the case of a long arc than in the case of a short arc. The radial distributions of temperature in the cross section of the arc column were constant. The temperature in the position of the peak value of spectral intensity in the cross section of the arc column rose at the beginning of a breaking arc and remained constant at about 6000 K. Judging from the time dependence of spectral intensities and arc temperatures near the cathode contact, the arc duration was long when metal-vapor quantity was large and metal-vapor density was high between contacts at the beginning of a breaking arc.

Ground-state depletion mechanisms in pulsed barium vapor lasers

Depletion of the metal vapor density from the central region of metal vapor laser tubes has recently been identified as an important factor limiting laser output. We have studied the mechanisms that deplete the ground-state metal density profile by measuring radially resolved population histories of neutral and ionic species during a “burst” (i.e., a pulse train of 20–30 excitation pulses) in a barium vapor laser (BVL). The observed spatiotemporal density behavior during the afterglow of the first shot in the burst agrees well with a simple model for diffusion and recombination, which we have used to show that the primary depletion mechanism during the establishment of steady-state conditions arises from the ambipolar diffusion of ions to the tube wall, and that gas-heating effects are secondary. Analysis of the steady-state (i.e., late-burst) afterglow behavior further reveals that the depletion is significantly greater than that expected from the measured barium ionization, particularly when operating at low barium densities at the wall; this we attribute to a large radial ambipolar field induced by the presence of ionized buffer–gas atoms on axis. The results show that it is important to reduce the time-averaged ionization in order to minimize ground-state depletion. The implications for power scaling of the BVL and other metal vapor lasers are discussed.

A MODEL OF THE MAGNETICALLY CONFINED SPUTTERING DISCHARGE USED FOR METAL VAPOR LASERS

A model is presented for the magnetically confined sputtering discharge which is being examined to be used in metal vapor lasers.The current-density and spatial dependences of the metal vapor density are described.The predictions are found to be in agreement with the- experiments.

The anode surface temperature of CuCr contacts at the limit of current interruption

This paper estimates anode surface temperature at the current interruption limit by measuring the melting time of a contact material after current interruption and simulating the anode surface temperature. As a result, the minimum anode surface temperature of CuCr(50/50) contact material was about 1750 K. We also calculated the metal vapor density between electrodes with a simplified model. The calculation results showed that the critical vapor density was about 3/spl times/10/sup 20/ atoms/m/sup 3/. This vapor density is equivalent to the averaged pressure of 8 Pa, which is close to the value of the Paschen minimum.

Robust high-temperature sapphire cell for metal vapors

We describe a simple vacuum cell that successfully contains high-temperature and high-density alkali-metal and alkaline-earth vapors. The transparent cell, made from sapphire parts joined by melted glass powders, operates without a buffer gas. Temperatures over 700 °C and metal-vapor densities greater than 1017 cm−3 have been achieved. Such an optical cell is useful for many atomic physics experiments where collisions with buffer gas atoms are problematic or where a spatially uniform distribution of high-density, corrosive atoms is desired.

Investigation of high-current interruption of vacuum circuit breakers

The physical processes occurring during recovery of a vacuum interrupter after high-current interruption are surveyed. New results have been obtained by the application of modern diagnostic techniques like laser shadow imaging, laser-induced fluorescence of atoms and of metal vapor ions, Mie scattering of droplets, and high-resolution recording of voltage and current on digital oscilloscopes. The density of metal vapor produced by the arc determines the kind of breakdown processes occurring after current zero. Liquid droplets and residual plasma play a minor role. The commonly used contact material, i.e., copper-chromium, is compared with pure copper, and its advantage is explained by the surface structure of the melt.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-current metal ion beam extraction from a multicusp ion source

Improvements have been made in a multicusp ion source, which made it possible to produce metal–vapor plasma and extract a high-current metal ion beam. In the discharge chamber, double radiation shields were set and the inner shields were heated to 1860 K. Therefore, it became possible to maintain enough metal–vapor density to produce plasma without the use of support gas. For the extraction of a high-current metal ion beam, we used multislit electrodes in the accel–decel electrode structure. Experiments of the metal ion beam extraction were performed with several kinds of metals (Al, Cr, Si). For example, the extracted Al ion beam current reached 65 mA. As a result of mass analysis, it was found that more than 90% of an Al ion beam is Al+. Furthermore, stable operation could be achieved. For Al (58 mA) and Si (28 mA) ion beam extraction, the temporal drift of each ion beam current was within ±5% for 3 h.

Development of high current metal ion beam source

A magnetic multipole ion source with some modifications to sustain high density metal vapour plasma has been developed and investigated. The following results are experimentally obtained for aluminium plasma: (1) the inside of the ion source is heated up to about 1500 K to sustain the metal vapour density necessary for a discharge; (2) the high purity aluminium plasma is produced; (3) the ion current density is over about 10 mA/cm 2; and (4) the radial uniformity of ion current density is about ± 5% over a radius of 3 cm.

High temperature fluorescence cell for laser spectroscopy of Na in noble gas atmospheres: design and performance

The design and performance of a simple fluorescence cell for laser-spectrometric studies of metal vapors at elevated temperatures (T </=1000 K) are described. The cell is operated with Na as a radiator in a heat bath of noble gas perturbers. The gas temperature and metal vapor density in the fluorescence zone are obtained using a variant of a well-known light-scattering technique. For operation with sodium we achieved an optically thin vapor at gas temperatures between 700 and 1000 K.

Optical determination of alkali metal vapor number density using Faraday rotation

We present a convenient and reliable method for measuring the number density of the alkali metal vapor. The method is based on the diamagnetic Faraday rotation of the plane of polarization of nearly resonant light by the alkali metal vapor, and it is suitable for optically thick vapor where the conventional integrated absorption technique fails.

Measurement of the Neutral Copper Vapor Density around Current Zero of a 500-A Vacuum Arc Using Laser-Induced Fluorescence

The technique of laser-induced fluorescence was applied to measure the time dependence of the copper vapor density from 280 μs before to 400 μs after current zero of a vacuum arc driven by a sinusoidal current of 500-A rms. Atomic densities between 5 × 1017 m-3 and 5 × 1014 m-3 were observed at the center of the gap with a local resolution of better than 1 mm3. The observed densities were more than an order of magnitude smaller than those predicted by model calculations according to Rich and Farrall [16], and the density decay after current zero occurred much more slowly than expected. Both findings indicate that the correlation of recovery of dielectric strength to metal vapor density after current zero is different from the recovery criterion based on the ratio of the mean free path for atomic collisions to the gap length.