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

The transport of a vacuum-are 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-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-g less than or equal to 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-probe along the duct. The ion current to the duct wall, I-duct, and the probe and duct floating potentials, phi(probe) and phi(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 arcq I-probe and I-duct initially increased as a function Of I-arc, reached a maximum at I-arc = 40-45 A, and then decreased by a factor of 2-2.5 relative to their maximal values. Both phi(probe) and phi(duct) were negative relative to the grounded anode. phi(probe) became significantly more negative as L or B-g increased, while phi(duct) depended only weakly on L and B-g.