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_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_p. I_p, as well as the ion saturation current to the duct wall I_d, the arc voltage V_arc, and the probe and duct floating potentials with respect to the anode φ_p and φ_d, were measured as functions of D, I_arc, and the axial magnetic field B. Generally, I_p and I_d increased with D and I_arc. For D=10 mm, I_p was ∼0.4% of I_arc, while with D=50 mm, I_p reached ∼9.5% of I_arc. However, with large D, the probability of the arc extinguishing increased. Both φ_p and φ_d were negative relative to the anode. φ_p became increasingly negative with increasing D, and approximately linearly depended on D. With a small D, I_p and I_d increased almost linearly with B, while φ_d was almost independent of B. However, for a large D, I_p and I_d were only slightly affected by B, and φ_d became less negative with increasing B.