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/sub g//spl les/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/sub probe/, along the duct. The ion current to the duct-wall, I/sub duct/, and the probe and duct floating potentials, /spl phi//sub probe/ and /spl phi//sub duct/, respectively, were measured as functions of B/sub g/ and the axial distance of the probe from the anode, L. Generally, I/sub probe/ decreased while I/sub duct/ increased with L, and the sum I/sub probe/+I/sub duct/ was approximately independent of L. For an Al arc, I/sub probe/ and I/sub duct/ initially increased as a function of I/sub arc/, reached a maxim at I/sub arc/=40-45 A, and then decreased by a factor of 2-2.5 relative to their maximal values. Both /spl phi//sub probe/ and /spl phi//sub duct/ were negative relative to the grounded anode. /spl phi//sub probe/ became more negative as L or B/sub g/ were increased, while /spl phi//sub duct/ depended only weakly on L.