Cu ion current measurements in a vacuum arc with a black body electrode configuration

The ion current ejected from a vacuum arc black body assembly (VABBA) with a Cu cathode is studied. In the VABBA, cathode material is emitted into a closed volume formed by a water-cooled cylindrical cathode and a cup-shaped W or Ta anode heated by the arc. If this volume is filled with a dense plasma when anode is hot, it acts as a black body toward macroparticles while a directed plasma jet is emitted through a single small anode aperture or a shower head aperture array. Arc currents are I=150 - 300 A and arc time 120 s. The ion current density J_i is measured by an electrical probe. By inserting the probe through a single 4-mm hole in the Ta anode, it is found that J_i in the closed volume increased with I from ∼ 5.7 A cm² at I=150 A, to 18.4 A cm² at I=225 A. The plasma density is estimated to be ∼1015 - 10-16 cm^-3 inside the closed volume. Outside of a W shower head anode with 250 holes with 1-mm diameter in a 200-A arc, the peak J_i in the axial direction decreased from ∼ 0.3 to ∼ 0.12 A cm² when the distance from the anode to the probe increased from 3 to 50 mm. This result agrees with previous measurements in the hot refractory planar anode vacuum arc.