  In the atmospheric pressure anodic carbon arc, ablation of the anode serves
as a feedstock of carbon for production of nanomaterials. It is known that the
ablation of the graphite anode in this arc can have two distinctive modes with
low and high ablation rates. The transition between these modes is governed by
the power deposition at the arc attachment to the anode and depends on the gap
between the anode and the cathode electrodes. Probe measurements combined with
optical emission spectroscopy (OES) are used to analyze the voltage drop
between the arc electrodes. These measurements corroborated previous
predictions of a positive anode sheath (i.e. electron attracting sheath) in
this arc, which appears in both low and high ablation modes. Another key result
is a relatively low electron temperature (~ 0.6 eV) obtained from OES using a
collisional radiative model. This result partially explains a higher arc
voltage (~ 20 V) required to sustain the arc current of 50-70 A than predicted
by existing simulations of this discharge.
