  Atmospheric pressure arcs have recently found application in the production
of nanoparticles. Distinguishing features of such arcs are small length and hot
ablating anode characterized by intensive electron emission and radiation from
its surface. We performed one-dimensional modeling of argon arc, which shows
that near-electrode effects of thermal and ionization non-equilibrium play
important role in operation of a short arc, because the non-equilibrium regions
are up to several millimeters long and are comparable with the arc length. The
near-anode region is typically longer than the near-cathode region and its
length depends more strongly on the current density. The model was extensively
verified and validated against previous simulation results and experimental
data. Volt-Ampere characteristic (VAC) of the near-anode region depends on the
anode cooling mechanism. The anode voltage is negative. In case of strong anode
cooling (water-cooled anode) when anode is cold, temperature and plasma density
gradients increase with current density resulting in decrease of the anode
voltage (absolute value increases). Falling VAC of the near-anode region
suggests the arc constriction near the anode. Without anode cooling, the anode
temperature increases significantly with current density, leading to drastic
increase in the thermionic emission current from the anode. Correspondingly,
the anode voltage increases to suppress the emission - and the opposite trend
in the VAC is observed. The results of simulations were found to be independent
of sheath model used: collisional (fluid) or collisionless model gave the same
plasma profiles for both near-anode and near-cathode regions.
