A thorough numerical model of cathode spots challenges conventional wisdom

The interaction of plasma clouds with the electrode surface in cathode attachments of vacuum arcs has long been conjectured to produce explosive electron emission events, but past attempts at modeling the physics of cathode spots have been incomplete. For example, some models simplified the hydrodynamic aspects or neglected them altogether, while other models ignored the plasma produced in the spot. Recently, researchers from Universidade da Madeira and Instituto de Plasmas e Fusão Nuclear (Portugal) and Corporate Technology of Siemens AG (Germany) constructed a numerical model that addresses the aforementioned defects. Armed with the new model, the researchers observed the formation of a crater, a molten metal jet, and the ejection of droplets in the absence of thermal explosions. They report their findings in the Journal of Applied Physics.

Helena Kaufmann, the primary author of the study, explains that a major challenge in deriving the result was to build a model that accounts for all the relevant physical phenomena in a reasonably accurate way but is still tractable. Another highly challenging task was to weave together multiple numerical methods that do not easily remain numerically stable, a numerical-analytic property of computational methods that guarantees solutions with small errors even after many iterations. The solid-liquid phase transition in the cathode body was modeled using the enthalpy-porosity method. The deformation of the molten cathode surface on a fixed Cartesian grid was tracked via the level-set method.

According to Kaufmann, the researchers are working to apply the developed model to fusion devices and suggest an application of the work in the investigation of other types of gas discharges.

Source: “Detailed numerical simulation of cathode spots in vacuum arcs: Interplay of different mechanisms and ejection of droplets,” by H. T. C. Kaufmann, M. D. Cunha, M. S. Benilov, W. Hartmann, and N. Wenzel, Journal of Applied Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4995368 .