ION CURRENT COLLECTED AT VARIOUS DISTANCES AND ARGON BACKGROUND PRESSURES IN A COPPER VACUUM-ARC

The ion current collected by a probe biased at the cathode potential and located behind an annular anode of a vacuum arc was measured as a function of distance to the cathode and background argon pressure. The arc was formed between a circular Cu cathode and an annular anode. The cathode diameter was the same as the anode aperture (26 mm). The distance between the cathode and the anode was about 1.5 mm. Arc current was 170 A, and the arc duration was 0.9 s. The arc was ignited by making a momentary contact of a moveable W trigger rod (held at anode potential) with the cathode. The ion current probe was a 18 mm diameter disk mounted parallel to the cathode surface. The distance (d) between the probe and the cathode was varied in the range of 32-142 mm, and the background pressure of Argon (p) was between 5 x 10(-5) to 10 torr. Arc voltage, arc current, and ion current were measured using an analog data acquisition card and a personal computer. Arc voltage and arc current values were stable during the arc and their normalized standard deviation sigmaBAR was less than 0.07. Ion current was noisy and fluctuated during the arc with sigmaBAR that varied from 0.5 at p < 0.1 torr up to more than 1.5 at p > 1 torr. When d was in the range 3 x d(c) < d < d(p), the ion current average value decreased according to 1/d2 . Here, d(c) is the cathode diameter, and d, is a critical distance which decreases with pressure. For d > d(p), the ion current decrease (as function of d) is faster than 1/d2. For d < 3 x d(c), the ion current decrease is slower than 1/d2 . Up to a critical gas pressure (p(c), that can be estimated theoretically for each probe distance, the probe current is relatively independent of p. For p > p(c), the ion current decreases rapidly with increasing p.