Ion current distribution produced by a vacuum arc carbon plasma source

A vacuum arc carbon plasma source is described, in which an arc was ignited between a cathode and an anode having an aperture, by bringing the two electrodes into contact, and parting them while current was flowing. The inter-electrode gap length was varied. A focusing magnetic field was applied in the inter-electrode gap, and a toroidal magnetic field was applied to guide plasma through a toroidal duct. The influence of the arc current (I_arc), gap length (L), and focusing magnetic field (B_f) on the cathode spot motion, plasma behavior and output ion current was studied. It was shown that with I_arc=150-250 A and relatively weak B_f, the spots moved on the cathode periphery, causing noisy arcing and fluctuation of the output ion current, while at lower I_arc=50-100 A and strong B_f⩾28 mT the number of the spots on the cathode surface decreased, and they tended to move towards the cathode center, decreasing the arc noise and increasing the average output ion current. With increasing L from 2 to 18 mm, the total ion current at first rapidly increased, and then saturated at L⩾10 mm