Two forms of attachment of an atmospheric-pressure direct-current arc in argon to a thermionic cathode

Comparative investigation of two forms of attachment of a dc (20 < I < 200 A) atmospheric-pressure arc in argon to a thermionic cathode made of pure tungsten is carried out. The current-voltage characteristics of the arc, axial distribution of the cathode rod surface temperature (except for the site of arc attachment), and plasma temperature axial distribution in the cathode region are measured, and the current density on the cathode surface is estimated. The measurements of current-voltage curves shows that the voltages of the arc with different forms of cathode attachment differ distinctly (but not too much) one from another, the curves for two modes iontersect. This confirms the results of theoretical analysis carried out earlier by M.S. Benilov who has showed that an existence of different forms of attachment is associated with the presence of branching points in a solution to the cathode heat balance problem. The point of intersection should be viewed as one such point. Optical measurements disclose that the temperature and its distribution over the cathode rod surface differ greatly for the two forms of attachment considered. The plasma temperature in the cathode region of the contracted attachment far exceeds that in the diffuse attachment, exceeding 3 eV in the immediate vicinity of the cathode surface. The maximal temperature of the plasma in the contracted attachment does not depend on the current. Analysis of erosion prints shows that the current density on the cathode does not depend on the current for both forms of cathodic attachment. For the contracted attachment, the current density is roughly four times higher (～10⁴ A/cm²) than for the diffuse form. The experimental data are in good agreement with present-day calculations of the cathode plasma parameters and temperature conditions.