Influence of the aperture diameter on plasma transport through the annular anode of a vacuum arc plasma deposition system

The influence of the anode aperture diameter on plasma transport was investigated in a vacuum arc deposition system. The plasma source consisted of a 17-mm diam. frustum cone Cu cathode, and either a 20-mm-thick annular Cu anode with an aperture diameter of D=10, 17, 30, 40, or 50 mm, or a 35-mm-thick anode with D=50 mm. The plasma generated at cathode spots passed through the anode aperture and entered a 160-mm diameter cylindrical duct. Magnetic coils positioned co-axially with the duct axis produced a magnetic field to guide 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-shaped probe, positioned normal to the duct axis at a distance of 150 mm from the anode exit, monitored the ion saturation current, I_p. The ion saturation current to the duct wall, I_d, the arc voltage, V_arc, and the probe and duct floating potentials, φ_p and φ_d, respectively, with respect to the anode, were measured as functions of D and I_arc. Generally, I_p and I_d increased approximately linearly with D. I_p increased almost linearly as function of I_arc for D=50 mm. For small diameter anodes, I_p first increased with I_arc, and then became saturated for D=17 mm, while for D=10 mm, I_p first increased up to a maximum value at I_arc=80 A and then decreased for larger values of I_arc. V_arc increased with D, and decreased with I_arc. Both φ_p and φ_d were negative relative to the anode, and φ_p became increasingly negative with increasing D. It was shown that the ratio I_p/ I_arc increased with D, from 0.4% for D=10 mm to 10% for D=50 mm.