Low-current Arc Research Articles

Modeling of hydrogen-rich gas production by plasma reforming of hydrocarbon fuels

The work is devoted to simulation of plasma reforming of hydrocarbon gases by means of a kinetic approach. It is shown that the use of standard kinetic schemes, without inclusion of specific plasma processes, is sufficient for simulation of hydrogen production in discharges with hot plasmas, in particular, in low-current arc discharges. Results of simulation of methane and octane reforming are compared with data of experiments with low-current arcs available in the literature. An amount of hydrogen produced in octane reforming is calculated for a wide range of mixture compositions, temperatures at the reactor entrance, and discharge powers.

Modelling of discharges in a flow of preheated air

A model of discharges in a fast longitudinal flow of preheated atmospheric-pressure air is described. The model is based on a kinetic scheme developed recently for the simulation of low-current arc discharges in air. Results of calculations of plasma parameters are given and compared with experimental data.

Hydrodynamic effects in liquids subjected to pulsed low current arc discharges

Hydrodynamic phenomena in liquids associated with low current electrical discharges were analysed theoretically and the radial plasma column expansion was considered as an arbitrary discontinuity propagation. For assumed initial discharge channel radii of R0 = 10−6 and 10−5 m, the initial temperatures in the discharge channel was found to be T0 = 1.4 × 106 and 6.5 × 104 K for a discharge current of I = 1 A, and T0 = 6.5 × 106 and 3 × 105 K for I = 10 A. The temperature decreases during the initial ∼10−8 s, due to radial plasma column expansion. During the initial 10−8 s the velocity of the plasma column expansion exceeds the speed of sound in the undisturbed water (by 5–15 times), creating a shock wave in the water with a pressure discontinuity of ≈105 atm, and a relaxation length of 10−4 m.

Proposal for a modified fowler-milne method to determine the temperature profile in TIG welding at low currents

It is possible to assess the thermal efficiency of welding shielding gases by means of the arc temperature field analysis. Since this approach opens a remarkable study field to assess different shielding gases, giving support for dealing with advantages and disadvantages of commercial gas mixtures, there is a research line under development, which goal is to find techniques to measure arc temperatures. This work describes a proposed method containing different procedures to quantify plasma jet temperature profiles from experimental data. This method was applied on data taken from TIG welding arc, at low current (40 A). This low current was selected to contrast to and amplify the current literature focus, i.e., high currents. The experiment was conducted using emission spectroscopy, picking punctual luminescence from the plasma through an optic system. The TIG arc was stroked on a water-cooled copper plate and shielded by pure argon. The temperature field was determined through the modified Fowler-Milne method. The introduced modification aimed to overcome the limitation that this method has for low radiation intensity presents in low current arcs: the Fowler-Milne method has an intrinsic threshold of 10,000 - 25,000 K. For a 40-A arc, the lower 10,000-K limiting isotherm is reached close to the cathode, restricting the analysis field, especially for the anode region. The proposed modification suggests a linear distribution of the particle density instead of a Maxwellian one, at temperatures below 12,500 K. The experimental temperature field was compared to a previous publication that deals with numerical simulation and the results were found in good agreement, what indicates the supposition of a linear distribution it is not far from the reality.

Near-cathode region of a free-burning arc: a spectroscopic investigation

The cathode as an electron emitter possesses different modes of operation to maintain the current forced upon the discharge. We investigated spectroscopically the near-cathode region of a free-burning low-current argon arc operated at atmospheric pressure in order to measure the plasma parameters of that mode of operation that we call the blue-core mode. For better interpretation of the experimental data we applied a simple two-temperature model. As a result, we can describe the arc plasma up to 100 µm near the cathode surface.

Effect of process parameters of micro-plasma arc welding on morphology and quality in stainless steel edge joint welds

The effects of the process parameters of micro-plasma arc welding (micro-PAW) on the morphology and quality of stainless steel edge joint welds were investigated. Micro-PAW was applied on type 304 stainless steels to produce an edge joint weld. Welding experiments were carried out for various combinations of arc current, welding speed, arc length, shielding gas, and clamp distance, with all other operating conditions held constant. The experimental results indicated that the collimated shape of the low current plasma arc was mainly responsible for the low sensitivity of the weld morphology to variations in the nozzle standoff distances. The arc voltage increased with increasing quantity of added hydrogen in the argon shielding gas. It was also found that satisfactory edge joint welds can be formed using a clamp distance of 0.35 mm, and that the edge joint penetration on a 0.1 mm thickness stainless steel is about 60% of the clamp distance.

Soft x-ray generation after arc extinction of a short vacuum gap operated with low-current, repetitive discharge

Soft x-ray emission from a short vacuum gap, operated with a repetitive low-current arc with a period of ∼2 µs and peak current of 200 A, was investigated. Electrons are accelerated by an electric field in the cathode sheath that is formed after arc extinction, and soft x-rays were emitted in the period between arc extinction and subsequent breakdown. The bulk of the radiation was detected in the 1–3 keV soft x-ray range when a tungsten anode was employed. The dimension of the point-like x-ray source in the radial direction was less than 170 µm. The current density at the time of x-ray emission was ∼107 A m−2.

Numerical simulation of high-current vacuum arcs with an external axial magnetic field

Numerical simulations are presented for physical behavior and heat flux to the anode of high-current diffuse of arcs as found in vacuum interrupters. The magnetohydrodynamic approach is applied. Of importance is the consideration of energy balance. Heat flux densities to the anode are predicted in the right order of magnitude and essential physical details of the high-current vacuum arc are disclosed. Only at low or no axial magnetic field superimposed externally and low-arc currents, the anode-directed flow of plasma of diffuse arcs reveals supersonic conditions. Otherwise, subsonic conditions exist. In supersonic diffuse arcs, the anode-directed plasma flow is decelerated and highest pressures appear in front of the anode. At subsonic conditions the highest pressure prevails in the cathode region and the pressure gradient drives the flow to the anode. The transition from diffuse to diffuse columnar arc seems to occur when the evaporation rate of metal vapor from the contact surfaces approaches the emission rate of plasma from the body of cathode spots. Diffuse columnar arcs have moderate pressure variations from cathode to anode. With rising plasma density, the energy loss from the emission of electromagnetic radiation increases and can no longer be neglected.

Experimental characterization of vacuum arc instabilities for different electrode metals

Arc voltages and instabilities of low-current, dc vacuum arcs, less than 30 A, ignited by the opening of electrodes were investigated for eight kinds of electrode metals. The low-current dc arcs were always unstable, but the dc components of the arc voltages showed peculiar values to the metals. The intensity of the instability differed depending on the electrode material, and it was confirmed that the waveform of the spike voltage was the same with the voltage drop of the inductive resistor of the circuit. The spike voltage excites an oscillation of the arc current as a result of inductance and capacitance resonance of the circuit. The oscillations were observed by adding a parallel capacitance and a series inductance to the circuit, and the waveforms of the oscillation agreed with simulated waveforms.

An Analysis of the Instability Phenomena of a Low-Current Vacuum Arc for Silver Cathode-A Comparison of Electron Temperature between Probe Measurement and Analysis-

In order to clarify the instability phenomena of a low current vacuum arc, characterized by noise on the current trace prior to the actual current chopping, an analysis of silver cathode spot has been made. For the analysis, cathode spot region is recognized as the collisionless space charge sheath connected with singly ionized collisional plasmas.It is proposed that a current level below that no real solution exists is unstable current and this current corresponds to the point at which a vacuum arc turns into unstable. It is found that there is no real solution when the current is below 19.4A. The physical explanation is considered that the electrons returning to the sheath region from the plasma one dominate over positive ions and, consequently, the electric field at the cathode surface is F02<=0 below 19.4A. Similar results have already been obtained for copper cathode. In order to examine the analytical result for its validity, the probe measurements of electron temperature in the diffused arc plasma has been performed. The electron temperature-arc current characteristics were found to be similar between experimental and analytical results.

Effect of Arc Current on Wear Properties of CrN Film Deposited onto Tool Steel by Arc Ion Plating Method

The wear properties of CrN mm has been investigated as a function of arc current which is one of the processing parameter of arc ion plating (AIP) method. CrN films were coated onto high-speed tool steel substrate under four arc currents of 50, 100, 150 and 200 A. CrN films were also examined using X-ray diffraction. The results obtained in the present study were summarized as follows. The dynamic micro hardness was hardly influenced by the arc current. For SEM observation, many droplets existed on the CrN film, which were generated by the dropping out of the macro-particle of non-evaporated Cr. The distribution of droplet depended markedly on the arc current. The wear weight was also dependent on the arc current. CrN film, which was synthesized at low arc current of 50 A markedly improved the wear resistance. For CrN film coated under the high arc current of 200 A, the wear resistance was less than that of film of 50 A. By the successive observation of wear track using optical microscope, the large adhesive matter was observed on the wear track. For opposite ball, the wear weight increased with decreasing the arc current. It was also seen from SEM observation and EDX that for the wear track, the adhesive matter was opposite ball. The area of adhesion of wear track for CrN film was less for 50 A than for 200 A. For 50 A CrN film, the decreases of adhesion contributed to increases in the wear resistance.

Effect of “overshooting” in the transitional regimes of the low-current gliding arc discharge

A linear stability analysis of the low-current gliding arc discharge in the transitional regime is performed. It is shown that the gliding arc remains stable during the evolution and gradually transforms into a more nonequilibrium one. The low-current arc discharge can propagate with the effect of “overshooting” at which the gliding arc extinguishes long after its maximum power has been achieved. Analytical and numerical solutions explain the general behavior of the low-current gliding arc and are in a good agreement with our experiment.

The influence of arc currents on the properties of carbon nitride films deposited by vacuum cathodic arc method

Carbon nitride films have been deposited by vacuum cathodic arc method at various arc currents while keeping the flow of Ar/N 2 at a constant value. The films were characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. During the deposition, the total pressure kept the falling tendency upon the increasing of the arc currents. The N content of the films varied over the range of 15–32 at.%. The films mainly consisted of three types of bonds: CC bonds, sp 2 CN bonds and sp 3 CN bonds besides CO and NO bonds. The sp 2 CN bonds dominated in the films relative to sp 3 CN bonds. Raman spectra and XPS indicated that the films showed different properties according to the arc currents. At 20 and 30 A, the films demonstrated the high N concentration and the characteristic of polymer-like material. With the increase of arc currents, the films became more and more disordered due to the energy enhancement of species in the atmosphere and transformed from the polymer-like microstructure to the type similar to amorphous carbon at 40 A and the sp 3 CN ratio also kept the rising tendency. Above 40 A, some new Raman peaks were observed, which suggested some new microstructure appear and be embedded in the amorphous matrix. Meanwhile the N content dropped relative to that at low arc currents and both the sp 3 CN ratio and the N content kept a little higher value at 60 A than those at 50 and 70 A due to the dominance of the new microstructure.

真空アーク小電流領域の不安定性と裁断現象

真空アーク小電流領域の不安定性と裁断現象

Photocatalytic decomposition of NO 2 on TiO 2 films prepared by arc ion plating

The structure of TiO 2 prepared by arc ion plating was changed by RF substrate bias from anatase (0 V) through amorphous (−50 V) to rutile (or rutile+anatase) (>−100 V). This tendency was almost independent of arc current (50–80 A). As insulating TiO 2 deposited on the chamber wall, irregular discharge occurred in the chamber. This irregular discharge was enhanced by the substrate bias so that the growth rate was decreased from 1 to 0.6 nm/s and the substrate temperature was decreased from 350°C to 150°C. The oxygen content was increased with decreasing arc current, but the crystallinity at a low arc current of 50 A was low. The decomposition efficiency of NO 2 gas by TiO 2 photocatalyst prepared at arc current of 60 A was high, which was due to the low oxygen deficiency. In particular, anatase structure showed the highest efficiency. Therefore, it is said that structure, composition and crystallinity of the TiO 2 films affect the photocatalytic efficiency.

Laser-Monitor-Assisted Investigation of the Regions of Closure of Current on the Electrodes of an Atmospheric-Pressure Low-Current Carbon Arc

The results are given of observations of regions of closure of current on the electrodes of an atmospheric-pressure low-current carbon arc, performed using a laser monitor immediately during the time of burning of the arc. It is found that the time of existence of a cathode crater on a graphite cathode is 0.2–0.3 s. An analysis of images of the region of closure of current on a pyrographite cathode reveals evidence of melting of pyrographite. A comparison of images of the region of closure of current on the electrodes of a carbon arc with the available literature data on the conditions of closure of current on the anode and cathode enables one to establish the origins of the various details of images of the region of closure of current on the cathode and anode of a low-current carbon arc. The results of this comparison further lead one to conclude that a low-current electric arc is an arc with liquid cathode and anode spots, and that the melting process is a stabilizing factor as regards the current density in those spots.

Energy and density of ions in vacuum arcs between axial and radial magnetic field contacts

In vacuum circuit breakers, two different contact types are used to overcome the consequences of arc constriction, which sets in when currents of several kiloamperes are exceeded. Radial magnetic fields (RMF) force the constricted arc to rotate and distribute its power more evenly on the contact surface. Axial magnetic field (AMF) contacts prevent the arc from becoming constricted up to higher thresholds. To improve the interruption capability of vacuum circuit breakers of both types, it is essential to know about the processes and properties of the vacuum plasma ("vacuum arc") around current zero, such as plasma density and its decay and the energy of the plasma species. In this work, the energy distribution of ions in the vacuum arc plasma during the last 3 ms before current zero has been investigated by means of a retarding field analyzer up to arc currents of /spl ap/9 kA RMS, and significant differences could be observed between RMF and AMF contacts. For currents above 5 kA the distribution in both cases resembles a Maxwellian distribution, characteristic for a collision-determined plasma. On lower currents, i.e., when current zero is approached, RMF arrangements show ions with strongly directed motion, while the energy distribution for AMF contacts seems to be more influenced by collisions. There are also indications of the v/spl times/B ion rotation in the AMF field. Furthermore the post-arc charge as an indication of the plasma density and its free decay after current zero has been investigated. With AMF contacts, the initial density at current zero lies higher, especially on lower arc currents. The first decay time constant grows slightly with the arc current, and lies higher for larger shield diameter, i.e., higher ratio between plasma volume and shield surface for recombination.

Low current arc modes of short length and time: a review

Until the middle of the 20th century the electric arc was known solely as a steady, self sustaining gas discharge phenomenon requiring certain minimum values of current and voltage, depending on the electrode material and the gas environment. Special investigations on material transfer in contacts switching low direct currents, however, revealed step by step that there were certain till then unknown unstable arc modes appearing in extremely short electrode gaps only but requiring minimum current and voltage values distinctly lower than those of the classical steady arc discharge in air. Low current break arcs have to pass one or more of these transient modes whether or not they may eventually attain the steady gas arc mode.

Application of vacuum arc cathode spot model to graphite cathode

A model of near-electrode processes is applied here to describe the behavior of cathode spots on graphite cathode in vacuum arc. The physical model is based on a kinetic treatment of cathode evaporation, electron emission from the cathode, and plasma production. The model consists of physical assumptions and a system of equations that are formulated in the paper. Spot parameters, such as cathode erosion rate, cathode potential drop, cathode surface temperature, current density, electric field, and plasma density, temperature, and velocity in the near-electrode region are calculated numerically. The calculation includes the dependence of spot parameters on spot current and spot lifetime. The variation of spot parameters as a function of spot lifetime are very strong at lifetimes shorter than 10 /spl mu/s. The calculations indicate that Joule heating in the cathode body is significant, and may exceed cathode heating by the ion heat flux. Calculated spot parameters are compared with the corresponding experimental data for relatively low arc currents (<100 A) and their agreement is discussed.

Observations of the anode boundary layer in free-burning argon arcs

In order to make clear the physical grounds of the potential drop in front of the anode, namely, the anode fall in atmospheric free-burning argon arcs, the results of experimental measurements of the laser-scattering method and Langmuir-probe method are presented. The experimental results show that the anode boundary layer at low arc currents such as 50 A remarkably deviates from local thermodynamic equilibrium (LTE), whereas the boundary layer at higher arc currents such as 150 A preserves a similar state to the LTE. The Langmuir-probe measurements also show that the anode fall for 50 A is positive, whereas that for 150 A is negative. From these results, an assumption regarding the physical state of the anode boundary layer in the free-burning argon arcs is presented synthetically and it is also concluded that the sign and magnitude of the anode fall in the arcs relate vary closely to the thermal state of the anode boundary layer and that the thermal state should be influenced strongly by the arc current density, namely, the electron number density.