Fast video registration of transition processes from diffuse mode to anode spot mode in high-current arc with copper-chromium electrodes

Transition process from a diffuse to anode spot mode in high-current vacuum arc discharge with Cu-Cr electrodes has been studied experimentally. Images of the arc were recorded with a frequency of about 25000 frames per second synchronously with the acquisition of current and voltage waveforms. Experiments clarify that after the current reaches a certain threshold value (4 to 5 kA for bulk-type, 2-cm electrodes), the arc discharge passes from diffuse mode to the mode with anode spot and voltage increased by ~20 V. In this mode, bright anode jet appears; cathode micro spots are reduced in number and grouped in front of the anode jet with the formation the cathode macro spot. In such a threshold regime, the discharge can change to the anode-spot mode and back to diffuse mode many times, with transition time in the microsecond range. With increasing current amplitude, anode spot mode becomes more stable, cathode and anode macro-jets are coming closer together forming a quasi-continuous radially-constricted discharge channel. The constriction of discharge is accompanied by increasing power density on both electrodes. In the case of TMF type electrodes, the constricted plasma channel moves azimuthally along the electrode lobe.