Melting and Generation of Micro-Protrusions on Cathode Surface During Spark Conditioning Process in Vacuum

Spark conditioning is an effective method to increase the dielectric strength in vacuum by melting and removing micro-protrusions on the cathode surface. We aim to optimize the conditioning effect in vacuum for the application of vacuum circuit breakers (VCBs) to higher voltage levels. To obtain high breakdown (BD) voltage, the control of BD charge in the conditioning process is required. When a BD occurs, we have suggested that micro-protrusions can be generated on the cathode surface by particles from the anode. For the suitable control of BD charge, the meltability of generated micro-protrusions and their size are important. In this article, we focus on the generation of micro-protrusions on the cathode surface in conditioning process. First, we conduct spark conditioning by repetitive impulse voltages of rod–plane electrodes made of OFHC Cu in a vacuum chamber with different BD charges. Later, we observe the surface of rod cathode with a digital microscope in order to count the number of visible protrusions and measure their radii. By considering the meltability of micro-protrusions, we quantitatively reveal the existence of residual protrusions that cannot be completely melted by BD. Next, we estimate the maximum size of generated micro-protrusions by BD charge. In consideration of the collision of ions into the anode accelerated by the anode sheath voltage, we estimate the meltable amount of anode material by the BD charge. We confirm that the radii of experimentally measured protrusions are close to the estimated ones. Moreover, we consider the difference in dielectric strength due to the difference in the initial surface condition of the electrodes. We clarify that the dielectric strength is improved by applying acid wash and heat treatment to the electrodes before conditioning, which can be optimized by the BD charge and the initial surface condition of the electrode.