Anode attachment modes and their formation in a high intensity argon arc

The anode region of a high intensity argon arc has been studied experimentally. As the cold ambient gas enters, the arc-anode attachment can appear in four different modes. Besides the well-known diffuse and constricted anode attachment modes, two transition modes have been observed, namely a lift-up mode and a multiple-attachment mode. The electron temperature and the electron density distributions for these modes have been obtained with a laser Thomson scattering system, which can probe the plasma 50 mu m in front of the anode surface. The measurement results show good agreement with previous Langmuir probe results. The electric field distributions for these modes have been obtained for the first time by solving the charge continuity equation. The results indicate that the entrainment of the ambient cold gas first generates a ring in the fringes of the arc, which has low electron temperatures, low electron densities and an increased electric field strength accelerating electrons towards the anode. These characteristics encourage the electron overheating instability to develop, and lead to the formation of the multiple-attachment mode. One of the constricted multiple attachments grows and takes over most of the current as more ambient gas enters to form an anode jet and a constricted mode. Moreover, directly from the electric field distributions, the anode potential fall has been calculated. A negative anode fall exists in the centres of diffuse attachments, while a positive anode fall exists in the centres of constricted attachments.