Mechanism and current balance of unipolar arcs and implications for vacuum arcs

A unipolar arc is a form of an arc discharge that ignites and burns between the surface of a conducting, but floating, plasma-facing component and the plasma. Phenomenological similarities between unipolar arcs and vacuum arcs suggest that the spots are cathode spots, which, in the case of unipolar arcs, require a ring-like area around the spot that acts as an anode. All models of unipolar arcs seek to explain how a sufficiently high electron return current can be achieved. Building on elements of various existing models, it is shown that the combination of high density of expanding spot plasma and high electron temperature in the spot vicinity enables high return current. Taking the traveling double layer of an expanding plasma into account, it follows that the electron return current may be even larger than the net arc current in the case of the vacuum arc, while the ion current to the surface plays only a minor role. The similarity between vacuum arcs and unipolar arcs suggests that the vacuum arc has effectively two anodes, the one connected to the power supply, and the ring-like area around each cathode spot. The arguments, underpinned by analytical formulas and experimental data from the literature, could serve as a blueprint for future simulations and targeted experiments.