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, Iarc, 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, Ip. The ion saturation current to the duct wall, Id, the arc voltage, Varc, and the probe and duct floating potentials, φp and φd, respectively, with respect to the anode, were measured as functions of D and Iarc. lly, Ip and Id increased approximately linearly with D. Ip increased almost linearly as function of Iarc for D=50 mm. For small diameter anodes, Ip first increased with Iarc, and then became saturated for D=17 mm, while for D=10 mm, Ip first increased up to a maximum value at Iarc=80 A and then decreased for larger values of Iarc. Varc increased with D, and decreased with Iarc. Both φp and φd were negative relative to the anode, and φp became increasingly negative with increasing D. It was shown that the ratio Ip/Iarc increased with D, from 0.4% for D=10 mm to 10% for D=50 mm.