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

The plasma beam produced by a vacuum are plasma source was injected into a cylindrical duct through an annular anode aperture. The plasma source consisted of a truncated 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 co-axially 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 diam. 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 increasing D to 50 mm, I_p reached ˜10% of I_arc. However, at a large D the probability of the arc extinguishing increased. Both ϕ _p and ϕ _d were negative relative to the anode, and became increasingly negative with increasing D. With B 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.