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/sub ax//sup ac/ in the opening gap between flat profile, butt type, Cr-Cu contacts. For peak arc currents I/sub p/ up to 20 kA, critical fields B/sub crit//sup ac/ were determined which prevented anode involvement. The dependence of B/sub crit//sup ac/ on I/sub p/ is compared with the analogous dependence using constant magnetic fields B/sub ax//sup dc/. The results are used to extend the usefulness of the B/sub 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/sub ax/ for separation currents I/sub s/<7 kA and I/sub p/<10 kA.