Transport of a vacuum arc plasma beam through the aperture of an annular anode

The plasma beam produced by a vacuum arc plasma source was injected into a cylindrical duct through an annular anode aperture. The plasma source consisted of a frustum cone-shaped Cu cathode, and either a 20-mm-thick annular Cu anode with aperture diameter D of 10, 17, 30, 40, or 50 mm, or 35 mm thick and D=40 or 50 mm. Magnetic coils positioned coaxially with the duct axis produced an approximately axial magnetic field guiding the plasma in the duct. The arc current, I/sub arc/, was in the range of 30-100 A. A 130-mm-diameter negatively biased planar disk probe, positioned normal to the duct axis at a distance of 150 mm from the anode exit, was used to measure ion saturation current I/sub p/. I/sub p/, as well as the ion saturation current to the duct wall I/sub d/, the arc voltage V/sub arc/, and the probe and duct floating potentials with respect to the anode /spl phi//sub p/ and /spl phi//sub d/, were measured as functions of D, I/sub arc/, and the axial magnetic field B. Generally, I/sub p/ and I/sub d/ increased with D and I/sub arc/. For D=10 mm, I/sub p/ was /spl sim/0.4% of I/sub arc/, while with D=50 mm, I/sub p/ reached /spl sim/9.5% of I/sub arc/. However, with large D, the probability of the arc extinguishing increased. Both /spl phi//sub p/ and /spl phi//sub d/ were negative relative to the anode. /spl phi//sub p/ became increasingly negative with increasing D, and approximately linearly depended on D. With a small D, I/sub p/ and I/sub d/ increased almost linearly with B, while /spl phi//sub d/ was almost independent of B. However, for a large D, I/sub p/ and I/sub d/ were only slightly affected by B, and /spl phi//sub d/ became less negative with increasing B.