Anode discharge mode and cathodic plasma state in high-current vacuum arcs

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Abstract
Anode surface states and traces of the cathode spots after arcing of various anode discharge modes in vacuum have been studied in the current range of 5–20 kA. Spectroscopic measurements of the radial electron temperature profile in the cathode region, which is related to the distribution of the current density, have also been performed to investigate the cathodic plasma state of the arcs in the various anode modes. The arc of a half‐wave sinusoidal current of 60 Hz was burned between a 60‐mm‐diam anode and a 30‐mm‐diam cathode spaced 4 mm apart. The electrodes were made of oxygen‐free, high‐conductivity copper. The cathode spots spread uniformly over the entire cathode and a uniform current density profile was obtained in the diffuse‐arc mode. It was found, however, that the cathode spots bunch together and the current density profiles peak at the center of the discharge in the anode‐spot mode. The cathode‐spot bunching and the nonuniformity in the current density profile become significant with a sufficient increase in arc current to cause severe erosion of the anode. Another finding was that the cathode spots spread almost uniformly over the cathode while a nonuniform current density profile appears in the diffuse arc associated with the presences of the arc‐voltage fluctuation and a slight anode melting, i.e., in which the transition to the foot‐point mode has begun. The occurrence of current constriction in the arc without cathode‐spot bunching will be briefly discussed.