The effect of electrode energy balance on variable polarity plasma arc pressure

The plasma arc evolution and the physical mechanism associated with variable polarity plasma arc (VPPA) remain unclear, slowing down its process development and optimization. In this work, we revealed its increasing phenomenon in the initial segment of electrode negative (EN) phase through the experimental measurement of VPPA pressure. The current increasing enlarged the pressure difference between EN and electrode positive (EP) phase. A unified numerical simulation model was developed coupling tungsten electrode, constricting nozzle, plasma arc and workpiece for clarifying the mechanism of pressure evolution. In this model, the electrical conductivity of gas adjacent to melting region in tungsten electrode is considered to be superior than that of gas adjacent to solid region. Through the evolution of temperature field and energy transfer, the electrode was gradually cooled mainly by thermionic electron emission at the initial segment of EN phase, because of which the current attachment shrank to the electrode tip. Thus, the electromagnetic force depending on the current density increased in EN phase, leading to an increase in the plasma arc pressure. Our results clarified aspects of energy balance physics behind VPPA, which is critical for its development. (C) 2019 Elsevier Ltd. All rights reserved.