Instability and dynamic behavior of arc attachments on electrodes and the effect on electrode erosion

Abstract

Plasma interacting with electrodes is one of the most challenging issues in many industrial applications, such as power-interruption and plasma-metal erosion. Because of the concentration of arc attachments (root) and the voltage drop across the plasma sheath layer, the arc roots consume great amount of energy, which subsequently will increase the local temperature and erode the electrodes. Due to the nonequilibrium condition at plasma sheath, it is very difficult to quantitatively estimate the arc root temperature profile. The recognition of arc roots behavior, like instability and pattern formation, is important to estimate the electrode erosion. The potential drop arising through the sheath (double layer) is nonuniform. Due to thermionic field emission, the strong flux of charge carriers through the sheath will cause instability of the double layer, which weakens the inner potential gradient. As a result, the strong current dependent potential drop features a negative resistance. The existence of negative resistance causes the instability of arc attachments in the forms of immobility and constriction. Their interdependence between local current density and potential drop gives rise to the arc root formation that concentrates the energy into a small spot. Owing to the negative resistance, any perturbation will cause the current density in the sheath to grow to approximately infinity or decay to vanish, namely arc root formation or extinction. Thereby, the arc root instability provides the basis for the dynamic behavior of arc attachments and detachments on the electrodes, which will help to understand electrode erosion and avoid the damage from the arc plasma in engineering applications.