Low-current Vacuum Arc Research Articles

Validity conditions for complete and partial local thermodynamic equilibrium of nonhydrogenic level systems and their application to copper vapor arcs in vacuum

Validity conditions for complete and partial local thermodynamic equilibrium (CLTE and PLTE) of homogeneous, time-dependent, and optically thin plasmas are derived. For Cu I, electron densities of n/sub e/>or=(5*10/sup 22/-5*10/sup 23/) m/sup -3/ are required for the establishment of CLTE. For Cu I and Cu II, n/sub e/>or=(5*10/sup 21/-5*10/sup 21/-5*10/sup 22/) m/sup -3/ is necessary for PLTE (for electron temperatures of 1-2 eV). Application to low-current copper vapor arcs in vacuum shows that CLTE can be expected for r<200-600 mu m (r=distance from the cathode spot). A further limitation follows for temperatures of 2 eV or higher if diffusion effects are taken into consideration. Consequently, the use of the LTE formulas in plasma spectroscopy of low-current vacuum arcs is very limited.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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.

Stability of low-current vacuum arcs

A detailed analysis of current and voltage 'noise' in a low-current vacuum arc is presented. The occurrence of typical instabilities in arc current is shown to be correlated to the DC arc lifetime. Influence of the parameters of the feeding circuit on the instability behaviour is investigated and explained qualitatively. A model of the interaction between circuit and arc is proposed on the basis of which arc extinction seems to be a result of an accumulation of instabilities at low currents (I<20 A).

Electron Temperature and Probe Floating Potentials in a Plasma of the Low-Current Vacuum Arc

Measurements of the electron temperature and of floating potentials are presented, as obtained utilizing Langmuir probes situated in the plasma of a low-current (120-360-A) vacuum arc on copper electrodes. The ring-shaped probes were located coaxially to the axis of the electrode assembly at different angles ψS from the cathode surface normal. Probe characteristics were obtained at varying probe bias voltage, generated from the 100-Hz half-sinusoidal voltage source. The electron temperature, far away from the cathode spot, in the region of direct radial plasma flux, is 2.1 eV for 120 A and 2.8-3.5 eV for 230 A. Floating potentials are functions . Uf = f(ψS) and diminish distinctly if ψS < ψA, where ψA is the semi-angle of the anode. The analysis was carried out assuming a shifted Maxwellian ion velocity distribution. Characterristic parmeters vio and α were evaluated from the derived floating potential equation.

A model for the cathode mechanism in low-current metal vapour arcs

An analytical model is proposed for the cathode spot of a metal vapour arc. A number of physical parameters at the cathode surface and at the boundary of the cathodic plasma such as macroscopic velocity, temperatures, fluxes, particle densities and electric field can be determined from equations for conservation of electric charge, mass, momentum and energy, the equation for ion current to the cathode, bipolar space charge equation, Saha equation, electron emission equation, and evaporation equation. The cathode drop voltage and the current density serve as independent variables. The multiply charged ions are taken into account, and the jumps of some quantities over the first mean free path-due to non-equilibrium evaporation-are incorporated into the model. Numerical calculations are carried out for low-current copper vacuum arcs. The results are compared with experimental data.

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.

Vacuum Arc Anode Phenomena

This paper briefly reviews anode phenomena in vacuum arcs, specially experimental work. It discusses, in succession, arc modes at the anode, anode temperature measurements, anode ions, transitions of the arc into various modes (principally the anode spot mode), and theoretical explanations of anode phenomena. The two most common anode modes in a vacuum arc are a low current mode where the anode is basically passive, acting only as a collector of particles emitted from the cathode, and a high current mode with a fully developed anode spot. Characteristically this anode spot has a temperature near the atmospheric boiling point of the anode material and is a copious source of vapor and energetic ions. However, other anode modes can exist. A low current vacuum arc with electrodes of readily sputterable material may emit a flux of sputtered atoms from the anode. Usually this sputtered flux will have little effect upon the vacuum arc, but in certain circumstances it could be significant. A vacuum arc doesn't always transfer directly from a low current mode to the anode spot mode. In 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. Finally, (again in appropriate circumstances) several small anode spots may form instead of one large anode spot. With sufficient increase in arc current or arcing time these will usually combine to form a single large active spot.

A study of the erosion rate of vacuum arcs in a transverse magnetic field

The cathode mass loss due to the action of low-current vacuum arcs under the influence of a high transverse magnetic field has been measured for titanium, stainless steel and graphite. The effects of magnetic field, arc current and cathode temperature on the erosion rate are investigated. The measured erosion rate decreases marginally with the magnetic field, the total variation being less than 20% and the rate of decrease is much lower at higher fields. Within the experimental accuracy only very small or no increase of erosion rate is observed with the arc current for currents up to 70 A. For measurements made at higher cathode temperatures (upto 500°C) there is a considerable increase in erosion rate for the stainless steel cathode and the results are discussed for titanium and graphite. The importance of clean cathode surfaces in the erosion studies is demonstrated by observing the cathode spot motion using an Image Converter Streak Camera. This method has proved to be an effective diagnostic tool in judging the surface condition before any measurements are made. Prolonged glow discharge cleaning in hydrogen is found to provide an acceptably clean surface. The earlier reported cathode spot velocity measurements are extended to higher magnetic fields.

Voltage-current characteristics of a diffusion vacuum arc

The arc voltage for a low-current vacuum arc between copper electrodes has been measured in dependence on current (in the 50–340 A range), electrode separation (4.5–36 mm) and two anode diameters (11 and 20 mm). The results show that the anode-cathode voltage contains a dc component U o on which the oscillation component Δ U a is superimposed. It is found that a critical solid angle ω cr subtended by the anode at the cathode center exists which divides the voltage characteristics into two regions. At large solid angle, if ω > ω cr , the magnitude of the U o component is approximately constant and the voltage fluctuation component has a low amplitude. For ω < ω cr variations of U o with solid angle have been found and Δ U a exhibits large amplitude fluctuations. The results can be explained on the basis of two criteria representing the mechanism of phenomena in the anode regions, i.e. the critical solid angle and the critical power density on the anode.

Experimental Investigation of Low-Current Vacuum Arc Instabilities

The instabilities of 50-Hz low-current vacuum arcs and chopping phenomena were investigated in a test circuit where current was limited either by a resistor or an inductance. Employing a special test method which was developed for statistical evaluation, current zeros were measured oscillographically with a time sweep of 50 to 100 ns/div for durations of ⩽ 60 div. The effects observed are quantitatively better to assess than with other techniques. It was found that each chopping process is initiated by a partial arc extinction (which as a rule occurs within less than 10 ns). One of the dominant factors of the whole process is the speed of recovery of the contact material. Copper contacts showed a much faster recovery and thus higher chopping currents than contacts of copper-tungsten. Furthermore, that current value at which the first instability occurs was measured as weli as the corresponding chopping current. Both values were evaluated statistically. In agreement with previous findings the occurrence of the first instability turned out to be independent of the capacitance parallel to the switching gap. However, the chopping current showed a significant dependence on the capacitance, as is well known from earlier investigations.

Study of the motion of vacuum arcs in high magnetic field

An investigation into the motion of low current vacuum arcs in a high transverse magnetic field is described with regard to arcing impurities in tokamaks. Cathode spot velocity measurements using streak camera studies are presented. The effects of magnetic field, arc current, material and surface finish are reported in some detail. The arc tracks are subsequently analysed using optical and scanning electron microscopes. Arc erosions observed are similar to those found in DITE at Culham Laboratory. Surface cleanliness of the electrodes seems to play an important role in establishing the type of arc. Prolonged glow discharge cleaning of the electrodes in hydrogen is found to drastically reduce the arcing tendency.

3875. A theory for the cathode mechanism in low-current vacuum arcs, with application to the calculation of erosion rate. (USA)

3875. A theory for the cathode mechanism in low-current vacuum arcs, with application to the calculation of erosion rate. (USA)

A theory for the cathode mechanism in low-current vacuum arcs, with application to the calculation of erosion rate

A theory has been proposed which seeks to explain the cathode mechanism in low-current vacuum arcs. It is shown that the high current density at the cathode surface has been successfully discussed by relating the energy-balance equation, space-charge equation, electron-emission equation, current equation, MHD equation of motion, and the minimum principle of joule heating. When four equations and two limiting conditions combine the five dependent variables, which are temperature of the arc root, electric field at the cathode, total current density, current density carried by electrons, and the radius of the arc root, the current density and temperature at the arc root are obtained as a function of the arc current with the cathode-fall voltage as a parameter. It is also shown that the relationship between the arc current and the evaporation rate from the arc cathode root can be calculated. The theoretical results are in good qualitative agreement with the experimental data of the current density and the erosion rate.

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.

1341. Cathode spot structure and dynamics in low-current vacuum arcs: B E Djakov and R Holmes, J Phys D: Appl Phys, 7 (4), 1974, 569–580

1341. Cathode spot structure and dynamics in low-current vacuum arcs: B E Djakov and R Holmes, J Phys D: Appl Phys, 7 (4), 1974, 569–580

Cathode spot structure and dynamics in low-current vacuum arcs

A study has been made of cathode spot structure and dynamics in copper vacuum arcs for currents between 25 and 100 A using a high-magnification optical technique. The cathode spot consists of one or more electron emission zones or fragments with areas fluctuating between 10−10 m2 and 5×10−8 m2. The spot generally exists in one of two types. In the second type the average current density is about 5×1010 A m−2 and in the first type it is about (0·5-1)×1010 A m−2. The cathode spot undergoes continuous rebuilding in the form of fluctuations in the number and size of the fragments such that transitions in spot type, from type 1 to type 2 and vice versa, occur on average every 100 μs.A possible explanation of the observations in terms of a cathode spot model based on temperature-field electron emission is discussed.

Voltage Hash in Low-Current DC Vacuum Arcs

The nature and mechanism of the high-voltage pulses in the anode region of copper-vapor arcs have been investigated. Arcs are drawn between a 1.3-cm-diameter anode and a 5-cm-diameter cathode in the dc range of 30-400 A. Prior to anode spot formation and at constant current, the arc voltage exhibits HF pulses superposed on the dc voltage. The amplitude and frequency of repetition (= 500 kHz) of these pulses is highly erratic, and the total instability is evidenced as voltage hash. The hash amplitude increase with both arc current and contact spacing and, at 2.5 cm, varies from several tens of volts at 30 A to several hundreds of volts at 400 A. The hash is attributed to fluctuations in vapor and plasma density in the anode region. Although these fluctuations appear unrelated to the gross motion of the cathode spots, streak photographs indicate a time-variant cathode evaporation that may contribute to the hash phenomenon. A second contributory mechanism is suggested from experience of hash in the anode region of fluorescent lamps, and involves quasi-periodic emission from the anode.