Validated two-dimensional modeling of short carbon arcs: Anode and cathode spots

Abstract

In order to study the properties of short carbon arcs, a self-consistent model was implemented into a CFD code ANSYS-CFX. The model treats the transport of heat and electric current in the plasma and electrodes in a coupled manner and accounts for gas convection in the chamber. Multiple surface processes at the electrodes are modeled, including the formation of space-charge limited sheaths, ablation and deposition of carbon, and emission and absorption of radiation and electrons. The simulations show that the arc is constricted near the cathode and anode front surfaces, leading to the formation of electrode spots. The cathode spot is a well-known phenomenon, and mechanisms of its formation were reported elsewhere. However, the anode spot formation mechanism discovered in this work was not previously reported. We conclude that the spot formation is not related to plasma instability, as commonly believed in the case of constricted discharge columns, but rather occurs due to the highly nonlinear nature of heat balance in the anode. We additionally demonstrate this property with a reduced anode heat transfer model. We also show that the spot size increases with the arc current. This anode spot behavior was also confirmed in our experiments. Due to the anode spot formation, a large gradient of carbon gas density occurs near the anode, which drives a portion of the ablated carbon back to the anode at its periphery. This can consequently reduce the total ablation rate. Simulation results also show that the arc can reach the local chemical equilibrium state in the column region, while the local thermal equilibrium state is not typically achieved for experimental conditions. It shows that it is important to account for different electron and gas temperatures in the modeling of short carbon arcs.

Analytical model of low and high ablation regimes in carbon arcs

“…The lower is the distance between the electrodes, the higher is the ablation rate. Numerical simulations of carbon arc discharges at similar conditions 24 showed that the near-anode layer in plasma is quite narrow, considerably smaller than 0.5 mm. Therefore, if the gap width is kept above 0.5 mm, its variation should affect neither plasma parameters in the vicinity of the anode nor the heat flux from the plasma to the anode which can be determined as t 8\\ • w. The solutions plotted in Fig.…”

“…If there is a background gas, it will impede the flow of the ablated material away from the ablating surface 9,23 leading to its accumulation near the surface. This will create a returning flux of the ablated material back to the evaporation surface reducing the net ablation flux 24,25 :…”

“…2(b)). In electric arcs most of the ablated material deposits at a closely located cathode 18,24 . In laser-ablation systems, the temperature rapidly decays below the saturation point = −% /( )* ⁄ + (on a scale of about 1mm 8 ) leading to rapid carbon vapor condensation.…”

“…Here, is the temperature of the cathode front surface equal to 3400 K to support thermionic emission 24,29 .…”

“…It is known that the presence of background gas can substantially limit the outflow of the evaporated material from the evaporation surface 9,23 , thereby reducing the ablation rate. Our latest model 24 has a more accurate expression for the ablation rate that accounts for the presence of the background gas, however, the results were obtained for the low ablation regime only.…”