Secondary plasma in the gap of high-current vacuum arc: origin and resulting effects

This paper deals with the conditions for steady burning of a high-current vacuum arc (HCVA) as well as the conditions and causes for the development of an instability that is specific for the anode region of high-current arcs and results in contraction of a freely burning HCVA in the near-anode region. The steady burning of the HCVA requires that electrode processes are efficient enough to generate plasma-forming matter. This perspective is used to analyze the cathode processes and to show that the cathode spots (CSs)-the primary plasma sources-do not generate sufficient plasma for the steady burning of the HCVA. Therefore, the generation of the secondary plasma (SP), whose existence in the HCVA is evident from results of many experiments, is important to study. Analysis is done on the efficiency of a variety of possible sources of SP. The basic SP source is shown to be the anode-emitting atoms that are efficiently ionized by the plasma electrons in its vicinity. The anode emits atoms under the action of fast primary ions of plasma jets flowing out of the CS. Therefore, the dynamics of the CS under the action of the intrinsic magnetic field of the arc defines the state of both the anode region and the arc as a whole, since it defines the flux of the primary ions to the anode. A dynamic HCVA model that includes SP generation is proposed and substantiated. The arc contraction near the anode and the formation of an erosion anode spot are assumed to result from the development of instability of the evaporation-ionization (EI) nature that is specific to the anode region of high-current arcs. The HCVA loses stability with respect to EI processes because of an increase in the lateral loss of the primary ions and the corresponding drop in the efficiency of the SP generation. The increase in the lateral loss of the primary ions is a corollary fact of the CS dynamics. The conclusions that follow immediately from the dynamic model of HCVA have been shown to be in satisfactory agreement with the results of various experiments. The consideration carried out in this paper is based on the analysis of the vast amount of experimental material obtained by different researchers while investigating both the electrode processes and those in the HCVA gap.