Langmuir Probe Measurements in the Anode Boundary Layer of High Intensity Arcs

Abstract

Summary form only given. Anode boundary layers of high pressure, high intensity arcs are characterized by large gradients of temperature, charged particle density and gas velocity. In this study, a planar Langmuir probe has been used to study the anode boundary layer of an atmospheric pressure, high intensity transferred argon arc with lateral cold gas flow. The measurements were performed with different plasma gas flow rates (15 slpm or 18 slpm) and different arc currents (80 A or 100 A). Deviations from local thermal equilibrium (LTE) and negative anode falls were observed in all experiments, which is consistent with previous studies. The electron temperature was higher with higher plasma gas flow rate and lower current. A small increase in electron temperature was observed when the probe was moved into the plasma. When lateral gas flow was present, the arc attachment changed. With increasing argon lateral gas flow, the electron temperatures remained constant initially, then slightly increased and finally decreased. The synchronized high speed images showed the arc going from a stable attachment, to a one-sided constricted attachment on the upstream side and then to a deflected attachment. When the lateral gas was nitrogen, the electron temperature increased significantly initially and then remained constant. This increase coincided with a constriction of the attachment. The increase in electron temperature during constriction of the arc can be explained by the increase in the electrical field strength. The current density at the probe position was calculated by interpolating the probe characteristic curve, and the resulting values show the same behavior as the electron temperature. A simple method was used to estimate the electron density at the sheath edge and the measured electron density was in the order of 1020 m-3 to 5*1020 m-3