Wall ion current in high-current vacuum arcs

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Abstract
Ion currents collected at the metal wall connected to the cathode in a high-current (2 to 20 kA) vacuum arc are experimentally studied. Arcs are burned by exploding a Cu wire between a 60 mm diameter anode and a 30 mm diameter cathode made of various materials in a demountable chamber. The wall draws a larger ion current from a Cu cathode than from a Zn cathode and a smaller current than from a Mo cathode. The anode melting is caused for an 8 kA arc in a 4 mm gap between the Cu anode and the Mo cathode. A high-speed movie shows that the diffuse-arc mode appears in the same gap configuration between the Cu anode and the Zn cathode up to 15 kA. The anode-mode transition is found to occur when the collected ion current attains a certain value independent of the cathode material. The behaviour of the wall ion current associated with the ion generation between the anode and the arc plasma is briefly discussed, proposing a new idea for modelling the anode spot formation.

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