DOE Center for Control of Plasma Kinetics Highlight Self-Organization of Atmospheric Pressure Carbon Arc Discharges Yevgeny Raitses, Jonathan Ng, Igor Kaganovich, and Andrei Khodak Princeton Plasma Physics Laboratory (yraitses@pppl.gov) 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 cath- ode 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 nanosynthe- sis modeling. Our current research involves inte- grated experimental and modeling efforts aimed at developing an understanding of the plasma processes and their synergy with material pro- cesses. In recent arc experiments, measurements of evaporation and deposition rates, electrodes tem- peratures, arc discharge characteristics, and mate- rial 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 suf- ficient 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 transi- tion 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 en- hanced power deposition on the anode [5]. This regime is also characterized by the enhanced con- tribution 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. References [1] C. Journet et al., Nature 388, 756 (1997). [2] M. Keidar et al., Phys. Plasmas 17, 057101 (2010). [3] J. Ng and Y. Raitses, “Self- organization processes in the carbon arc” submitted (2014). [4] J. Ng and Y. Raitses, “Role of the cathode deposit in the carbon arc” submitted (2014). [5] A. Fetterman, Y. Raitses, and M. Keidar, Carbon 46, 1322 (2008). Anode Cathode 1.2 cm Deposit b) b) Anode Cathode 1.2 cm Deposit Fig. 1 – Results of IR temperature measurements of the arc electrodes: graphite anode and copper cathode