Langmuir probe measurements in the anode boundary layer of a high intensity arc

Summary form only given. The anode boundary layer of a thermal arc is still poorly understood even though its control is crucial for the performance in numerous thermal plasma applications. The boundary layer is defined as the region between the arc and the cold anode surface where the heavy particle temperature is low and where there is little emission of light from excited states. In this study, the boundary layer thickness has been controlled by using a wall constricted arc with an anode perpendicular to the arc axis, a 14 mm gap between the constrictor and the anode, and by introducing a cold gas flow parallel to the anode. Argon is used as the plasma gas with a flow rate of 15 slpm, while argon or nitrogen are used as the laterally injected gases with flow rates between 0 and 30 slpm. The arc current is kept at 100 A. Langmuir probes are imbedded in the anode surface and probe characteristics are obtained by biasing the probe in pulse mode and recording the probe current. Simultaneously, high speed video observation of the anode attachment allows determination of the boundary layer appearance. Electron temperatures are derived from the measured probe characteristics. For increasing lateral gas flow rates an increasingly thicker dark layer between the arc and the anode surface is noticeable as well as a deflection of the arc. The distribution of the electron characteristics, such as temperature and density in this dark region is presented.