Splitting and Motion of Diffuse Arc Cathode Spots in an Axial Magnetic Field

By high-speed photography, diffuse vacuum arcs between opening contacts in axial magnetic fields were found to have, in general, a concentrated cathode spot at the beginning of arc initiation. The spot then begins to split into two or more fragments when sinusoidal arc current and axial magnetic fields rise with time. It was found that at instantaneous current the value for the first splitting of the cathode spot decreases with increasing axial magnetic fields, and the current density of the cathode spots increases with increasing arc current. Experimental results showed that the group of cathode spots under the influence of axial magnetic fields was nearly uniformly distributed in dynamic form within a definite area on the cathode surface. It was also shown that the cathode spot-covered area reached a maximum value which lagged slightly behind the peak arc current. The group of the cathode spots expanded and contracted at the speed of the so-called "bi-hump" characteristics, i.e., the spots expanded twice at high speed before the maximum value of the spot-covered area was reached. These experimental results are discussed.