Transport of a vacuum arc produced plasma beam in a magnetized cylindrical duct

The transport of a vacuum arc produced plasma beam along a magnetized cylindrical duct was studied experimentally. The plasma source consisted of a Sn or an Al cathode and a 17 mm i.d. annular copper anode through which the plasma beam entered into the 160 mm diameter and 500 mm length cylindrical duct. Arc current, I.,,, was in the range of 30-100 A. Three magnetic coils positioned co-axially with the duct axis produced an approximately axial guiding magnetic field, B_g≤20 mT in the duct. A 130 mm diam. movable planar disk probe, positioned normal to the duct axis, was used to measure the ion saturation current, I_probe, along the duct. The ion current to the duct-wall, I_duct, and the probe and duct floating potentials, φ_probe and φ_duct, respectively, were measured as functions of B_g and the axial distance of the probe from the anode, L. Generally, I_probe decreased while I_duct increased with L, and the sum I_probe+I_duct was approximately independent of L. For an Al arc, I_probe and I_duct initially increased as a function of I_arc, reached a maxim at I_arc=40-45 A, and then decreased by a factor of 2-2.5 relative to their maximal values. Both φ_probe and φ_duct were negative relative to the grounded anode. φ_probe became more negative as L or B_g were increased, while φ_duct depended only weakly on L.