Cathode spot behavior on tungsten-copper contacts in vacuum and the effect on erosion

Arc visualization experiments on W-Cu electrical contacts in vacuum connect the cathode spot motion to the previously observed erosion behavior. After contact separation at high current, a transition mode forms between the initial bridge column and the diffuse arc. The transition mode consists of a concentrated region cathode spots slowly expanding with time to cover the contact surface. The expansion on W-Cu is much slower than the comparable expansion on Cu-Cr contacts. This arc mode increases the current density on a localized region of the contact. The higher current density drives enhanced erosion of the cathode, and increases material transfer to the anode. The transition mode in subsequent arc operations tends to form over a limited area of the contact. The transition mode behavior can explain the unidirectional contact erosion for W-Cu seen for operations at a single polarity of AC current. A cathode crater and anode pip formed during 11,000-52,000 contact opening operations.