Evaluation of AC axial magnetic fields needed to prevent anode spots in vacuum arcs between opening contacts

The influence of AC axial magnetic (AM) fields on the development of half-cycle drawn vacuum arcs is described. A Helmholtz coil provided an independent AC AM field B_ax^ac in the opening gap between flat profile, butt type, Cr-Cu contacts. For peak arc currents I_p up to 20 kA, critical AM fields B_crit ^ac were determined, which prevented anode involvement. The dependence of B_crit^ac on I_p is compared with the analogous dependence using constant AM fields B_ax ^DC. These results extend the usefulness of the B_crit parameter to the design of contact structures for producing self-generated AM fields from the AC arcing current. A similar direct transition to the high-current diffuse arc mode can be maintained over the half-cycle without an imposed B_ax if I_p<~7 kA