Electrode Material Adhesion between Anode and Cathode in Spark Conditioning in Vacuum

We discuss the electrode material adhesion between cathode and anode during spark conditioning process in vacuum, and its effect on dielectric strength in terms of breakdown (BD) charge. We focus on the conditioning effects under rod-plane electrodes with different combination of electrode materials. The material of the rod (cathode) is Cu and that of the plane (anode) is stainless steel (SUS), and their gap length is 1 mm. We observe the electrode surface before and after conditioning experiments. As a result, when BD charge is smaller than 2.0 mC, the anode material (SUS) is found on the cathode (Cu) surface and the dielectric strength becomes high. With increasing BD charge, the adhesion ratio of anode material on the cathode surface decreases, and the dielectric strength also tends to decrease. In the case where BD charge is larger than 2.0 mC, on the contrary, the cathode material (Cu) adheres to the anode (SUS) surface and the dielectric strength is low, which is consistent with the conditioning characteristics when both the cathode and anode are made of Cu. These results indicate that up to a certain BD charge, the anode material adheres to the cathode surface, and above this BD charge the cathode material adheres to the anode surface. When the cathode (Cu) surface is excessively melted, i.e., damaged, the cathode material Cu can adhere to the anode SUS surface. In other words, too large BD charge can not only fails to improve the dielectric strength due to the adhesion of the anode material SUS to the cathode Cu surface, but also causes a critical decrease in the dielectric strength, or deconditioning.