Self-Organization of Atmospheric Pressure Carbon Arc Discharges

The atmospheric pressure carbon arc in inert gases is an important method for the production of nanomaterials [1]. Typical nanosynthesis arcs operate in a dc mode between a graphite anode, which is consumed, and a cathode which can be made from either graphite or a lower melting point material [2]. In spite of many studies, the basic physical processes in this discharge such as cathode electron emission, evaporation and deposition of the anode material, particle and heat transport, and arc instabilities are still not well understood. A lack of understanding of these processes limits predictive capabilities of existing arc models and their application for nanosynthesis modeling. Our current research involves integrated experimental and modeling efforts aimed at developing an understanding of the plasma processes and their synergy with material processes. In recent arc experiments, measurements of evaporation and deposition rates, electrodes temperatures, arc discharge characteristics, and material characterization of the deposit revealed self-organization of plasma and material processes in the arc discharge [3,4]. In particular, during the arc operation, a carbon deposit is formed on the cathode surface. Electrons emitted from this deposit heat the graphite anode, which evaporates. The carbon ions and atoms travel to the cathode and condense to form the deposit, which is at sufficient high temperature (Fig. 1) for thermionic emission to support the arc current of 50-100 A [4]. Our results suggest that for the same operating conditions (gas, pressure, current), the arc can operate in two different regimes of evaporation and deposition of the anode material. The transition between these regimes is determined by the anode diameter (in our experiments ~ 0.8 cm). For larger anodes, the evaporation and deposition rates are relatively small and independent on the anode diameter. For smaller anodes, both evaporation and deposition increase dramatically as the anode diameter decreases. This regime is favorable for high yield nanosynthesis. It was suggested that the transition to this regime is due to the formation of the positive anode sheath leading to enhanced power deposition on the anode [5]. This regime is also characterized by the enhanced contribution from the latent heat to the cathode energy balance [3]. Future studies will include detailed plasma measurements and numerical simulations of these arc regimes and self-organization which can be important for controlling of nanosynthesis material processes.