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

Abstract

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.