Interelectrode Region Research Articles

The effect of electrode shape on the voltage of hall/heroult cells—I. carbon anodes/wetted cathodes

Two-dimensional profiles assumed by carbon anodes suspended over shaped, wetted cathodes in the Hall process for aluminum production and the cell voltage of each configuration were calculated. Cathodes shaped as square waves, sine waves and triangular waves were used. Designation of a region of lower conductivity adjacent to the anode simulated the presence of carbon dioxide bubbles. The profile of the anode did not penetrate the shape of the cathode. A dimensionless group characterizing the response of the anode profile to a wave sculpted into the cathode is ▪ where s| is the stoichiometric coefficient, p is the reaction order, n is the number of electrons transferred, b o is the amplitude of the cathode wave, h o is the minimum separation of anode from the cathode, and γ o is the wavelength of the cathode wave. Cell voltages in all cases were greater than the voltage of a cell having the same amount of gas spread uniformly over a flat plane. Voltage reduction can be achieved if the shape cut into the anode aids removal of gas from the interelectrode region.

The Current Distribution and the Magnetic Pressure Profile in a Vacuum Arc Subject to an Axial Magnetic Field

The steady-state electric-current distribution and the magnetic pressure in a uniform conducting medium, flowing in a cylindrical configuration between two circular electrodes, was determined by solving the magnetic field transport equation with a superimposed axial magnetic field. This medium models the interelectrode plasma of the diffuse mode metal vapor vacuum arc. The results show the following. a) The electric current and the flux of the poloidal magnetic field are constricted at the anode side of the flowing plasma. Most of the constriction takes place within a boundary layer, with a characteristic length of 1/Rme, where Rme is the magnetic-Reynolds number for axial electron flow. b) The electric-current constriction inversely depends on K?, where K? is the azimuthal surface current density which produces the axial magnetic field. c) The magnetic-pressure profile shows a radial pinch force in most of the interelectrode region, but in the anode boundary layer it is axially directed, thus retarding the plasma flow. d) The peak of the magnetic pressure is at the anode, and its amplitude directly depends on K?. As K? increases, the peak location moves toward the anode center.

Evaluation of AC Stabilization of a Dc Arc Lamp for Spectroscopic Applications

ABSTRACT Spectroscopic applications of the recently described method of arc lamp stabilization by the addition of a small (<2v) alternating current (AC) to a direct current (DC) arc lamp are discussed. A possible explanation for the improved arc stability is presented. Evaluation of arc formation and spatial behavior is monitored by a series of still photographs of the arc image. Observation of the electrode surfaces in similar photographs provide insight into the distribution of heat in the inter-electrode region. A photodiode array imaging device is used to accurately detect lateral movement of the arc image over prolonged time periods. Advantages of AC stabilized lamps in spectroscopic applications are also presented.

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.

Electric Field Distributions in Askarel Subjected to DC Stress between Plane Parallel Electrodes

The results of electric field measurements in a mixture of pentachlorobiphenyl and trichlorobenzene using the Kerr effect are presented. Field non-uniformity results from the accumulation of heterocharges at both electrodes. The effects of dividing the interelectrode region with solid spacers and variation of electrode material and temperature have been investigated.

Asymptotic Analysis of the Steady-State Current Flow in a Uniform Multicathode-Spot Generated Vacuum-Arc Plasma Flow

The current distribution in a model multicathode-spot vacuum-arc generated uniform plasma flow is analyzed analytically by studying the asymptotic behavior of the magnetic transport equation in the limit of large electron-flow magnetic Reynolds number. It is found that the radial distribution of the axial current flow imposed at the cathode is maintained throughout most of the interelectrode region, and that a strong constriction of the current flow occurs in a narrow boundary layer adjacent to the anode surface. The results support the general trend observed in previous numerical solutions.

The sputtering of field electron emitters by self-generated positive ions

The trajectories of positive ions formed in the interelectrode region of a field emitter are calculated. This calculation is used to determine the proportion of ions formed in the interelectrode region which strike the emitting area of the cathode and also to determine their energy distribution. Only ions formed near to the axis of the emitter (with zero initial energy) strike the emitting part of the tip and these have modal energy of the order of 10% of the interelectrode potential, which means that they originate close to the emitter. Experimental results based on depth of damage measurements confirm this energy computation. Ions formed when the electron beam strikes the anode can also cause sputtering but this is generally a less important factor.

Computational models of liquid metal ion sources

This work defines a computational model of the motion of ions from a liquid metal ion source in the interelectrode region. An Algol 68 program has been written which calculates the Laplace potential in an axially symmetric region by a variable mesh finite difference method, for arbitrary shaped boundaries, and the results have been compared with those of the finite element method. The effect of space charge can be important for these sources due to the large current densities near the emitter and a method for determining these effects by an iterative solution of Poisson's equation is described. Finally, the liquid motion on the needle is considered, which has been shown to be described, for a wide variety of different liquid metals and needle sizes, by the linear equations of slow viscous flow.

Sharpening of metal sheets during electrochemical polishing. Theory and experiments

The electrochemical polishing with simultaneous shape changes of anodes was studied. A theory was derived based on the knowledge of basic electrochemical parameters and the solution of the Laplace equation. To this purpose, the finite element method and the finite difference method with a double transformation of the inter-electrode region were employed. Only the former method proved well and can therefore be recommended for different geometries.

Performance of a Vacuum Arc Commutating Switch for a Fault-Current Limiter

A vacuum arc device with a hollow cylindrical anode and a button cathode located axially at the bottom of the anode has been evaluated theoretically and experimentally for use in a fault-current limiter as a commutating switch with an applied axial magnetic field. An optimally designed commutating switch of this type should meet the following conditions: 1) inductance of the parallel capacitor should be minimized, 2) time rate of increase for the applied magnetic field should be within an optimum range, 3) a slot in the anode is needed to provide for fast penetration of the magnetic field into the interelectrode region, and 4) anode size (radius and height) should be large enough to give a particle density and a commutation voltage of suitable magnitude for the duty expected.

Infrared laser optogalvanic spectroscopy of molecules

Infrared laser optogalvanic spectra of portions of the NH3 ν2 band at 9.5 μm and the NO2 ν3 band at 6.2 μm have been recorded using cw tunable diode lasers to probe dc electrical discharges in pure NH3 and an NO2/He gas mixture. Two contributions to the optogalvanic signal are identified: one which corresponds to an increase in discharge impedance and is seen only for irradiation of the negative glow region; and a second which corresponds to a decrease in discharge impedance and is seen for irradiation of all other discharge regions and the volume outside the interelectrode region. Mechanisms by which infrared laser irradiation may cause impedance changes in the discharge are proposed.

Basic Characteristics of Vacuum Arcs Subjected to a Magnetic Field Parallel to Their Positive Columns

As vacuum arcs subjected to a magnetic field parallel to their positive column (an axial magnetic field) spread uniformly over all the electrodes and burn in the interelectrode region, arc voltages of these arcs are low and quiescent. When the magnetic field strength decreases, however, the arc voltage develops a large noise component and electrode melting occurs. Experiments were conducted to investigate the condition of these transition phenomena. As a result of these experiments, it was found that these two phenomena do not always occur simultaneously and that a new explanation for the mechanism of anode spot formation should be considered.

The Interaction of Vacuum Arc Ion Currents with Axia I Magnetic Fields

We summarize a series of experiments in which we measured the distribution of ion currents leaving the interelectrode region of a vacuum arc with Cu electrodes. Ion currents were collected by an arrangement of cylindrical collectors surrounding the arcing space. A Helmholtz coil arrangement surrounding the arcing chamber generated the axial magnetic field. Arc currents ranged from 70 to 2400 A dc.

The Influence of the Self-Magnetic Field on the Steady-State Current Distribution in an Axially Flowing Conducting Medium

The steady-state electric current distribution in a multicathode-spot vacuum arc was determined by a solution of the magnetic transport equation subject to various boundary conditions. The inter-electrode region of the arc is modeled as a uniform plasma flowing from the cathode to the anode. Dimensional analysis shows that three parameters determine the magnetic field, and hence the current density which is derived from it: AR-the ratio of the electrode separation to the electrode radius, Rmm-magnetic Reynolds number of the axial material flow, and Rme-magnetic Reynolds number of the axial electron flow. While the anode side of the conducting medium is described as an equipotential surface, the following three cases of boundary conditions for the cathode side are examined: 1) a known current density distribution is assumed over the entire cathode side of the plasma surface; 2) the cathode side is an equipotential surface; and 3) the current is allowed to cross the cathode surface only through a finite number of ring shaped regions. Numerical solutions of the nonlinear magnetic transport equation show a constriction of the current at the anode side for all boundary conditions mentioned. On the other hand, the current moves to the perimeter of the cathode for boundary condition 2). When AR, Rmm, and Rme equal 0.72,-0.16, and 1.73, respectively, and a uniform current density flows at the cathode side, the on-axis current density at the anode is six times larger than its value at the cathode.

Interface state density in the interelectrode region of overlapping polycrystalline silicon gates

Constant capacitance deep-level transient spectroscopy was applied to analyze the interface state density in the interelectrode region of metal-oxide-semiconductor (MOS) structures having over-lapping polycrystalline silicon gates. A dual-gate structure fabricated on an integrated circuit chip by the double polycrystalline silicon technology was used to vary the extent of the space-charge region in the boundary. The interface state density D*it per boundary length was determined in MOS structures on p-type silicon to D*it =3×106 cm−1 eV−1 at the energy EV +0.99 eV. The interface state density Dit per unit area increases from a value Dit ≊109 cm−2 eV−1 under the bulk gate to Dit ≊1011 cm−2 eV−1 in the boundary region.

Absorption effects of the Cd II 4416 Å line in a cadmium vacuum-arc plasma

The absorption of the 4416 Å He-Cd laser line (a2D5/2 →5p 2P3/2) by a cadmium vacuum-arc plasma, and its dependence on time from arc initiation, spatial position in the interelectrode region, electrode separation, and the peak of current waveform, were determined. The arc was sustained between two cylindrical electrodes of 12 mm diameter. The current pulse lasted for 1.7 ms with peak current at 0.3 ms. The derived relative absorption of the laser line is found to be as high as 70% for electrode separation of 4 mm and peak current of 1.2 kA. We find that the time to peak absorption does not coincide with time to peak current. Furthermore, the absorption increases with increasing peak current or decreasing electrode separation. The measured optical depth of the vacuum-arc plasma is used for the calculation of the arc plasma self-absorption at 4416 Å, the absorption-corrected population density of the a2D5/2 level, and the estimation of the Cadmium ions velocity spread parallel to the optical observation axis. The estimated full width half-amplitude (FWHA) of the ion velocity distribution is in the region of 0.7–33×103 m/s.

New Analytical Expressions for Calculating Unipolar DC Corona Losses

Using the bipolar coordinate system and assuming the condition of a constant current within the interelectrode region, a rigorous expression for the voltage-current relation for cylinder-to-plane configuration for the case of unipolar dc corona was obtained. From the general integral formula established, simple practical formulas, introducing sufficiently small errors, have been derived.

Ionizing waves of potential gradient in a transversely excited N_2 laser cavity

The positions of maximum intensity in the beam cross section of transversely excited N(2) lasers have been measured at different gas pressures for three types of laser channel. The results show that there is a nonuni-formity in the population inversion in the interelectrode region, which is discussed in terms of ionizing waves of potential gradient.

A thin-film capacitance humidity sensor

A thin-film capacitance humidity sensor, fabricated using typical planar MOS technology, is described. The device responds to reversible changes in the surface conductance of a thin dielectric film deposited over planar interdigitated electrodes. Interelectrode capacitance measurements in the frequency range 1 – 100 kHz showed an accuracy of 7% of relative humidity over the range 17% – 65% relative humidity, and better accuracy at higher humidities, the accuracy being limited primarily by hysteresis. A theoretical model of device operation based on distributed RC transmission lines, in which the moisture-dependent surface conductance serves as the only conductive path, is presented. It is found from experiment, and confirmed with the model, that the interelectrode region is less useful for moisture meaurement than the region of the dielectric surface directly over the electrodes. This observation is then used as the basis of modified sensor designs, both for improved incremental sensitivity and for increased range of useful operation.

Measurements of Pulsed Corona in Coaxial Electrode Structures

A low-inductance pulse generator is used to deliver a high-voltage short-duration pulse to an air-filled coaxial electrode configuration. When the voltage is below the sparkover value, the pulse energy is delivered to a diffuse corona which visually appears to fill the Interelectrode region. Energy transfer, current distribution, and pulse shape are measured for variations in electrode diameters, air temperature, and voltage. Over a range of voltages up to sparkover, the entire stored energy is delivered to the corona. Pulse rise times are approximately 0.2 μs, the maximum peak voltage is 92 kV, peak currents equal approximately 400 A, and pulse energies range up to 1.6 J.