Free-burning Argon Arc Research Articles

Theoretical study of the melting of the cathode tip of a free burning arc in argon for various conical angles

Theoretical predictions, based on a unified arc-cathode theory, are made regarding the melting of the tips of thoriated tungsten cathodes for free burning arcs in argon. Theoretical results with cathode conical angles of 30 degrees , 45 degrees and 60 degrees are in fair agreement with experimental results. The current density, and thus the degree of melting, increases strongly with the sharpness of the cathode at its extreme tip. The effect of melting is to modify the sharpness of the tip so that, for 200 A arcs, the tip shape for conical angles of 60 degrees and less becomes approximately the same with a radius of curvature of approximately 0.25 mm.

Thomson scattering diagnostics of an argon arc plasma and spectral resolution of the ion feature with a narrow bandwidth dye laser

Thomson scattering diagnostics of a free burning argon arc are reported. The electronic plasma parameters are measured with the aid of a periodically pulsed Nd:YAG laser with second harmonic generation. With arc currents in the 100 A range, the electron temperature and density on the axis are Te approximately=1.7 eV and ne approximately=5*1016 cm-3, respectively; and the scattering parameter alpha is close to 1.5. The ion feature is resolved with the aid of a narrow-bandwidth dye laser tuned relative to a spectrally fixed detection channel. Electron and ion temperatures are in satisfactory agreement.

Numerical simulation of a free-burning argon arc with copper evaporation from the anode

A free-burning, high-intensity argon arc at atmospheric pressure was modeled during the evaporation of copper vapor from the anode to study the impact of the vapor to the entire plasma region. A uniform and a Gaussian radial velocity distribution are adopted for the copper vapor at the anode boundary with a net mass flow rate known from the experiment. The effect of both velocity distributions on the temperature, mass flow, current flow, and Cu concentration was studied for the entire plasma region. The cathode region is not affected by the evaporated copper, and the Cu vapor concentration in the arc core is negligible.

Departures from equilibrium in a free-burning argon arc

A magnetogasdynamic arc model due to Kovitya and Lowke has been combined with a collisional-radiative model of the argon atom to show how intense radiation from an arc can produce excited-state densities in the boundary of the arc which are far in excess of the densities corresponding to thermodynamic equilibrium. Quantitative predictions based on this model have been compared with measurements recently reported by Farmer and Haddad. There are encouraging similarities between theory and experiment, although it is suggested that the theory could be further improved by including the effect of resonance radiation which is scattered in the arc boundary.

Rayleigh scattering measurements in a free-burning argon arc

Rayleigh scattering of argon-ion laser radiation has been used to obtain number densities in the outer regions (T<9000 K) of a free-burning arc. Excellent agreement is found between the measured argon atom densities and those predicted by theoretical models (e.g. Kovitya and Cram, 1986). A considerable discrepancy exists between the number densities derived from these scattering measurements and those obtained from argon line emission measurements. This difference is shown to be consistent with the non-equilibrium theory of Cram and co-workers for the outer regions of these arcs.

Temperature distributions in a free-burning arc. IV. Results in argon at elevated pressures

For pt.III see Appl. Phys. Lett., vol.45, p.24-5 (1984). Measured temperature profiles are presented for free-burning argon arcs at pressures from 1 to 5 atm and comparisons are made with prediction based on the theoretical models of Kovitya and Cram and Hsu and Pfender. Evidence is presented for departures from local thermodynamic equilibrium (LTE) in the region near the cathode of the arc in agreement with previous results and it is also suggested that the assumption of LTE in the outer regions of the arc may not be valid.

Synthesis of emission line profiles from free-burning arcs

Theoretical emission line profiles have been evaluated by solving the radiative transfer equation and using physical parameters predicted by applying a two-dimensional model of a free-burning arc in argon at atmospheric pressure. Quantities describing the Stark shift and broadening of an ArI Line (λ763.5 nm) have been obtained by adjusting the theoretical profile to agree with a measured profile. A prominent absorption feature observed at the line center in the experimental profile is ascribed to absorption in a relatively cool layer surrounding the luminous arc column. There is strong evidence that this layer is not in LTE.

Two-dimensional analysis of free burning arcs in argon

The authors present a theoretical model of a free burning argon arc. Equations describing the conservation of mass, momentum and energy are solved together with Ohm's law and Maxwell's equation for the magnetic field in a cylindrical coordinate system. The material properties used in these solutions are derived theoretically, using the assumption of local thermodynamic equilibrium (LTE). Calculations are made for an axisymmetric, laminar arc burning between a rod electrode and a plane electrode. Comparisons between predicted and measured temperatures and velocities show good agreement for 100 A, 200 A and 400 A arcs in argon for a pre-specified current density of 108 A m-2 at the rod electrode and a plasma pressure of 1 atm. They describe the consequences of variations in the current density at the rod electrode and in the net radiation losses. They also discuss the importance of energy transfer due to the electron convection current and analyse in detail the various components which contribute to the momentum and energy conservation.

Modeling of a free-burning, high-intensity arc at elevated pressures

A previously established model has been applied to a free-burning high-intensity argon arc at elevated pressures for simulating the situations experienced, for example, in plasma processing or in underwater welding. With the calculated thermodyanmic and transport properties and appropriate boundary conditions, solutions of the entire arc are obtained with exception of the electrode sheath regions. The results show that the arc contracts as the pressure increases. As a consequence of this contraction, the current density, the enthalpy flux, and the voltage drop increase also while the velocity of the induced cathode jet decreases.

Local thermodynamic equilibrium in free-burning arcs in argon

Measurements of normalized radial emission coefficients in a free-burning arc in argon are presented as a function of position in the arc. The variation of these emission coefficients with pressure indicates departures from local thermodynamic equilibrium at 1-atm pressure.

Temperature determinations in a free-burning arc. I. Experimental techniques and results in argon

Spectroscopic techniques have been used to measure the temperature distribution in free-burning arcs in argon at 1 atm pressure. The experiment provides evidence for departures from LTE in the arc, and demonstrates deficiencies in the theory describing continuum emission from high temperature plasmas. This paper describes the apparatus and data reduction procedures and compares the measurements with recent theoretical calculations.

Analysis of the cathode region of a free-burning high intensity argon arc

This paper is concerned with analytical/numerical studies of the cathode region of an atmospheric pressure, free-burning high intensity argon arc. The cathode region is divided into two model zones: the ionization zone and the space-charge zone. The former accounts for the generation of ions while the latter is responsible for the sheath formation. The governing equations for each zone are established by taking the dominant physical processes into account. Employing boundary conditions from the results of previous studies of the flow-affected region, one-dimensional solutions of the governing equations are obtained. The results indicate a net positive space charge in front of the cathode surface and the electron and ion fluxes are mainly driven by the potential gradient. A significant fraction of the total current is carried by positive ions which is important for establishing the energy balance at the cathode surface.

Study of the free-burning high-intensity argon arc

Although the high-intensity, free-burning argon arc has been the object of many studies, modeling of the entire arc has been precluded because of complexities due to the interaction of electric, magnetic, fluid dynamic, and thermal effects, and the associated lack of realistic boundary conditions, in particular, close to the cathode. For establishing the most crucial boundary condition, which is the current density in the vicinity of the cathode, the maximum current density has been determined experimentally by measuring the size of the molten cathode tip (thoriated tungsten) for a given arc current. Calculated temperature profiles for a 100- and 200- A atmospheric pressure argon arc (electrode gap of 1 cm) are in good agreement with spectrometric measurements based on absolute line and continuum intensities. The arc current and arc current distribution are not only responsible for the temperature distribution in the arc, but also for the magnetohydrodynamics (MHD) pumping action in the cathode region, i.e., the arc behavior is mainly controlled by the current. In contrast to the sensitivity of the current density boundary condition on the results, the calculations show that variations of the boundary condition for the flow field are insignificant.

Thermal Nonequilibrium of a Free-Burning Argon Arc Plasma

The axial distributions of the electron temperature and number density of a free-burning atmospheric argon arc plasma at 15 Amps have been measured using a Thomson scattering technique. In addition, the excitation temperature of singly ionized argon has been found spectroscopically by relative line intensity method. The results are presented in terms of the degree of electron temperature nonequilibrium as compared with the calculated Saha equilibrium temperature and with the measured ionic excitation temperature along the axis of the arc. The observed temperatures are discussed with respect to the theoretical ion-like particle temperature and to the effect of electrode geometry.

The effect of ionization on heat transfer to wires immersed in a highly thermallyionized plasma

Results obtained with a previously developed technique for measuring local heat transfer to bare and ceramic coated metal wires from a dense, highly thermallyionized plasma are presented and compared with theoretical predictions. A free-burning arc in argon at atmospheric pressure provides a plasma in local thermodynamic equilibrium. Wire probes are swept through this plasma in planes parallel to the anode. The wires do not draw a net current from the plasma. Experimental evidence indicates that the layer between the undisturbed plasma and the probe surface is not in chemical equilibrium. The data show that heat transfer to the wire in this situation is dominated by the energy transport associated with electron and ion partial currents to the probe surface as determined by the potential of the probe with respect to the plasma. The data obtained using this technique provide an experimentally verified model for the heat transfer to wires immersed in a plasma over a wide range of plasma conditions including a 100 percent variation in the degree of ionization.

New Method for Obtaining Emission Coefficients from Emitted Spectral Intensities Part II—Asymmetrical Sources*

The method given in Part I for obtaining emission coefficients from emitted spectral intensities is generalized here to include asymmetrical sources as well. In this method the emission coefficient is expanded in terms of a complete set of functions which are invariant in form to a rotation of axes and the integral equation relating the emission coefficient to the emitted spectral intensity is used to determine the unknown expansion coefficients. The method has been checked by means of a hypothetical example corresponding to a source whose emission coefficient is a displaced gaussian. This same hypothetical example is used to test the numerical method which was developed for summing the series representation for the emission coefficient in situations where the emitted spectral intensity is given in the form of experimental data. Finally the numerical method is used to obtain the spatial distribution of the emission coefficient corresponding to an atomic spectral line of argon emitted by a free-burning argon arc which is distorted by an external magnetic field.