Plasma characterization of the atmospheric pressure carbon arc *

Atmospheric pressure anodic carbon arc discharges are a promising method for low-cost, high-volume synthesis of nanomaterials. During arc operation, carbon material is introduced into the arc by the ablation of the graphite anode [1]. The anode ablation depends on the power balance at the anode, which is influenced by whether the anode sheath is electron-repelling (negative anode sheath) or electron-attracting (positive anode sheath) [1–6]. Anodic carbon arcs exhibit a transition between low and high ablation modes; at larger arc currents the ablation rate of the anode grows nonlinearly [1,4,5]. We show the existence of a positive anode sheath in both low and high ablation modes. The electron temperature and density are determined by optical emission spectroscopy and corroborated by a Langmuir probe measurement. The plasma potential is determined with a floating probe. The floating probe potential is related to the plasma potential by assuming ions diffuse through neutrals in the probe presheath. Effects of the positive anode sheath on anode ablation rate are discussed. We also discuss a plausible explanation for the discrepancy in experimentally determined discharge voltage and discharge voltage calculated by recent models of the arc [6].