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 h alf-cycle drawn vacuum arcs is described. A Helmholtz coil provided an independent ac AM field B(ax)ac in the opening gap between flat profi le, 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 i nvolvement. The dependence of B(crit)ac on I(p) is compared with the a nalogous dependence using constant AM fields B(ax)dc. These results ex tend the usefulness of the B(crit) parameter to the design of contact structures for producing self-generated AM fields from the ac arcing c urrent. A similar direct transition to the high-current diffuse arc mo de can be maintained over the half-cycle without an imposed B(ax) if I (p) < approximately 7 kA.