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 area 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 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 magnetic fields B_ax^dc. The results are used to extend the usefulness of the B_crit parameter to the design of contact structures which produce self-generated axial magnetic fields from the AC arcing current. A similar direct transition to the diffuse arc mode occurs without an imposed B_ax for separation currents I_s <7 kA and I_p<10 kA