A novel anode attachment mode in argon-helium free-burning arcs at atmospheric pressure

Highlights: • Both arc and anode attachment are characterized by demixing. • Annular dark region on the anode appears under high arc current and high helium ratio. • Anode attachment with annular dark region may be a self-organized mode. • Carbon vapors from anode evaporation promote the formation of annular dark region. -- Abstract: An argon-helium free-burning arc that operates on a graphite anode is investigated. Images of the arc and anode attachment under different arc currents and different helium ratios are exhibited. Experimental results show that both the arc and anode attachment are characterized by demixing. Moreover, an annular dark region surrounding the anode center is observed in the anode attachment region under the conditions of a higher arc current and a higher helium ratio, which is similar to the self-organized anode attachment mode in glow discharges. Preliminary analysis indicates that the arc demixing, the thermophysical properties of the mixture gas, and the carbon vapors from anode evaporation contribute to the formation of this novel anode attachment mode.