A numerical analysis of the mechanism of anode spot generation in short-gap vacuum arc discharges

Abstract Anode vacuum arc ion sources play a crucial role in electric propulsion and thin-film deposition systems. Short electrode gaps facilitate the transition of anode vacuum arc discharge modes, but insufficient understanding of anode spot formation mechanisms limits device performance improvement. This study established a plasma-sheath-anode coupling theoretical model by analyzing anode surface temperature variations during short-gap vacuum arc discharges, revealing the mechanism of anode spot formation. The results demonstrated that the transition in anode spot formation mechanism is closely related to the breakdown delay time. With a short breakdown delay time, electrons from the cathode thermo-field emission heat the anode under the gap electric field, but their heating energy is insufficient to cause a significant rise in the anode surface temperature. At this stage, cathode plasma is generated. The electrons from this plasma then become the dominant heat source, heating a limited anode region and leading to intense local vaporization, thereby forming an anode spot. With a long breakdown delay time, the energy flux density from cathode-emitted electrons increases significantly, causing anode surface temperature to rapidly rise and spot formation. Subsequently, impact ionization occurs between electrons and evaporated metal atoms, generating anode plasma that expands toward the cathode. The anode plasma electrons then dominate the anode heating process, sustaining the anode spot until discharge termination. Furthermore, this study also observed that titanium electrodes—with lower thermal conductivity than aluminum electrodes—exhibit a higher tendency for anode spot formation. These findings provide important theoretical guidance for optimizing anode vacuum arc ion source designs and enhancing operational efficiency.