A unified theory of free burning arcs, cathode sheaths and cathodes

A theoretical method of predicting properties of free burning arcs and their cathodes is presented in a unified treatment. The method combines a one-dimensional model of the non-equilibrium plasma sheath adjacent to the cathode and a two-dimensional model for the arc column and the solid cathode. Two internal boundaries divide the arc-cathode domain into an arc region, a sheath region and a cathode region. The internal boundary conditions are adjusted during the iteration procedure to satisfy the energy conservation and current continuity equations. The effective resistance of the cathode sheath region is obtained from the sheath calculation assuming charge transport using an ambipolar diffusion approximation. No assumptions are made as to the distributions of, current density and temperature at the cathode surface. The model accounts for cathode surface effects and assumes that the cathode is a thermionic emitter. Material functions such as the thermal and electrical conductivities of the arc plasma and cathode are required as input parameters. Predictions are made, for any given arc current and cathode configuration, of the temperature and current density distributions in the arc and the cathode. Information is also provided about sheath properties. The results from a calculation for 200 A arc burning in argon with a thoriated tungsten cathode are in good agreement with the experimental measurements of the arc column and cathode surface temperatures and the arc voltage.

[1]  J. Mostaghimi,et al.  Impact of cathode evaporation on a free-burning arc , 1989 .

[2]  G. Haddad,et al.  Temperature determinations in a free-burning arc. I. Experimental techniques and results in argon , 1984 .

[3]  IańM Richardson,et al.  The physics of welding, 2nd ed.: Edited by J. F. Lancaster. Pp. 340. Pergamon Press, Oxford. 1986. Hardcover £32.00 ($55.00), Flexicover £16.25 ($28.00) , 1988 .

[4]  R. D. Richtmyer,et al.  Difference methods for initial-value problems , 1959 .

[5]  S. Kou,et al.  Heat transfer and fluid flow in welding arcs produced by sharpened and flat electrodes , 1990 .

[6]  R. S. Tankin,et al.  Measurement of Emission and Absorption of Radiation by an Argon Plasma , 1967 .

[7]  John J. Lowke,et al.  Two-dimensional analysis of free burning arcs in argon , 1985 .

[8]  J. Dutton,et al.  A survey of electron swarm data , 1975 .

[9]  C. Chen.  Collisional–Radiative Electron–Ion Recombination Rate in Rare‐Gas Plasmas , 1969 .

[10]  T. Fujimoto.  Kinetics of Ionization-Recombination of a Plasma and Population Density of Excited Ions. III. Recombining Plasma–High-Temperature Case , 1980 .

[11]  Lawrence E. Cram,et al.  Statistical evaluation of radiative power losses from thermal plasmas due to spectral lines , 1985 .

[12]  Y. S. Touloukian.  Thermal conductivity: metallic elements and alloys , 1971 .

[13]  E. Pfender,et al.  Study of the free‐burning high‐intensity argon arc , 1983 .

[14]  L. Cram.  A model of the cathode of a thermionic arc , 1983 .

[15]  R. S. Devoto.  Simplified Expressions for the Transport Properties of Ionized Monatomic Gases , 1967 .

[16]  J. Waymouth.  Electric Discharge Lamps , 1971, Nature.

[17]  R. Wienecke.  über das Geschwindigkeitsfeld der Hochstromkohlebogensäule , 1955 .

[18]  T. J. Moratz.  A theoretical investigation of the cathode fall in a neon glow discharge , 1988 .

[19]  E. Pfender.  Thermal plasma processing in the nineties , 1988 .