# An Investigation into the Behavior of Cathode and Anode Spots in a Welding Discharge
## 1 Introduction  
Many industries widely use fusion welding technologies for metals and alloys, which use an electric arc as a source of heating and melting [1]. At the same time, many physical phenomena occurring in the cathode and anode regions of the welding arc remain undiscovered [1, 2], in particular, the physical nature of the formation, existence, and quenching of cathode and anode spots [3, 4, 5]. The development of the work has shown [6, 7, 8, 5] that the experimental study of these phenomena is very difficult since one has to deal with a whole complex of complex interrelated physical processes, the course of which strongly depends on the state of the electrode surface. For the same reason, the theoretical description of the cathode and anode regions turned out to be no less difficult [4, 5, 6, 7, 8]. Numerous theoretical and experimental studies have shown that the thermal history in the welding arc is closely related to thermal, hydrodynamic liquid and electromagnetic phenomena occurring during electrode melting and in the welding bath, as well as in the arc heat source [2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. Various reviews and monographs [1; 4; 5; 19] summarize the results of numerous studies conducted since 1900 on the processes occurring in the cathode and anode regions of an arc discharge. Refs. [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19] present the use of atmospheric plasma in the field of material processing, modeling the cathode and anode layers, arc processes in relation to vacuum switches, and surface cleaning from the oxide layer. To study the behavior of cathode and anode spots in a welding arc, vacuum arc, or plasma arc, various methods are used using high-speed cameras and streak cameras [5; 7; 10; 11; 22; 23] to obtain high-resolution images of cathode and anode spots and to record spatial and temporal details of spot formation, including the lifetime of the spot. However, the process of obtaining and interpreting recorded images is quite complex and limited, especially in terms of spatial resolution. Moreover, mainly, the use of optical methods gives us information indirectly--by the glow of the plasma above the spot, i.e., a certain optical image. As a physical material object, it is not yet possible to fix the spot itself (cathode or anode). The autograph method is known [7; 24], when the property of a cathode or anode spot, size, and shape are judged by the size of the imprint left on the surface of the material. The difficulties of experimental studies are significantly aggravated by the small size of cathode spots and the transience of the processes occurring in them [25]. The literature [7; 8; 21; 22; 23; 26; 27; 28; 29; 30; 31] presents the types of cathode spots and problematic issues of a theoretical and experimental nature related to obtaining new information. Cathode spots (KS) are complex, nonstationary, self-organizing objects that have microdimensions and exist on microsecond time scales [21]. The KS consists of an overheated, boiling, and exploding volume of metal and a cloud of superdense erosive plasma covering it, the pressure of which reaches tens of atmospheres [5; 6; 7; 8]. KS is the source of a supersonic jet of dense, strongly ionized plasma. The current between the metal and the plasma is closed through the KS, which provides the huge energy release density necessary for the existence of the KS [25]. The released energy is absorbed during heating and in the phase transition from solid to liquid and gaseous states, and is also spent on heating, the ionization of this vapor, and the acceleration of erosive plasma and droplets. The lifetime of the KS is determined by the time at which the thermal conductivity in the metal \"does not work\" [7; 8; 25]. This is the time during which the heat wave manages to move away along the metal at distances of the order of the spot size.