Free or confined arc model relevant to the quench hazard of large superconducting coils

This arc model was motivated by the need to model arcs during the failure of a large superconducting magnet following an unmitigated quench. During a quench, resistive heating raises the conductor and insulator temperature. Electrical and mechanical properties change and inline and bypass arcs can form. The arcs are sustained by the massive (gigaJoule) stored magnetic energy. As windings are bypassed by shorts, the inline arc current and hence, arc diameter, increases. Cable-in-conduit conductors limit the maximum arc column diameter and when limited, the arc properties change rapidly as the arc changes from a free arc to a confined arc. For arc current 10-100 kA we calculate the arc column electrical properties and temperature, by solving a set of equations describing the arc physics. The equations describe the arc column heating, gas ionisation, heat loss and electrical properties. By constraining the maximum arc column diameter in the solution, the transition between free and confined arc can be calculated.