Anode phenomena in vacuum arcs

Arc modes at the anode, anode temperature measurements, anode ions, transitions of the arc into various modes (principally the anode spot mode), and theoretical explanations of anode phenomena are reviewed. Characteristically this anode spot has a temperature near the atmospheric boiling point of the anode material and is a copious source of vapor and energetic ions. With sufficient increase in arc current or arcing time these will usually combine to form a single large active anode spot. The dominant mechanism controlling the transition of the vacuum arc into the anode spot mode depends upon the electrode geometry, the electrode material, and the current waveform of the particular vacuum arc being considered. However, the most probable explanation of anode spot formation is a combination theory, which considers magnetic constriction in the plasma together with the thermal, mechanical, and electrical effects of the anode in analyzing the interaction between the gap plasma near the anode and the anode itself.