  Comprehensive non-invasive spectroscopic techniques and electrical
measurements of the carbon arc revealed two distinguishable plasma synthesis
regions in the radial direction normal to the arc axis. These regions, which
are defined as the arc core and the arc periphery, are shown to have very
different compositions of carbon species with different densities and
temperatures. The colder arc periphery is dominated by carbon diatomic
molecules (C2), which are in the minority in the composition of the hot arc
core. These differences are due to a highly non-uniform distribution of the arc
current, which is mainly conducted through the arc core populated with carbon
atoms and ions. Therefore, the ablation of the graphite anode is governed by
the arc core, while the formation of carbon molecules occurs in the colder arc
periphery. This result is consistent with previous predictions that the plasma
environment in the arc periphery is suitable for synthesis of carbon nanotubes.
