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 internal diameter annular copper anode through which the plasma beam entered into the 160-mm diameter and 500-mm length cylindrical duct. The arc current I/sub arc/ was in the range of 30-100 A. Three magnetic coils positioned coaxially with the duct axis produced an approximately axial guiding magnetic field, B/sub g/<20 mT in the duct. A 130-mm-diameter 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, (phi)/sub probe/ and (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 maximum at I/sub arc/=40--45 A, and then decreased by a factor of 2-2.5 relative to their maximal values. Both (phi)/sub probe/ and (phi)/sub duct/ were negative relative to the grounded anode. (phi)/sub probe/ became significantly more negative as L or B/sub g/ increased, while (phi)/sub duct/ depended only weakly on L and B/sub g/.