Heat flux at the refractory cathode of a high-current, high-pressure arc (two modes of cathode spot attachment)

Abstract

Calorimetric measurements of a refractory (pure and thoriated tungsten) cathode in a high-current (100–500 A) high-pressure (1–5 atm) arc in nitrogen were performed. The measurements confirmed the existence of a ‘high-current’ mode of cathode spot (CS) operation—the mode we observed by a footprint method in our work (2003 J. Phys. D: Appl. Phys. 36 3007). In this mode, the heat load of a cathode does not depend on the pressure and is directly proportional to the arc current as opposed to the ‘low current’ mode where it is inversely proportional to the square root of pressure and directly proportional to the square root of current.The current density at the cathode surface calculated from the measured heat flux to the cathode is approximately half of the current density obtained by a footprint method. Both densities exhibit the same current and pressure dependences.The radial distribution of the cathode temperature inside the CS is discussed. We hypothesize that the temperature has its maximum at the spot periphery. In the case of the thoriated tungsten cathode, this maximum may be so high that, at this location, the cathode is virtually thorium free.