A study of vacuum arc ion velocities using a linear set of probes

The most likely velocity of ions moving away from vacuum arc cathode spots was measured using a set of probes along the path of plasma expansion. The goal was to determine how much, if any, change in the ion drift velocity occurs in the expanded plasma. The arc discharge current was perturbed to create plasma density markers whose travel is picked up by the set of probes. It was found that the perturbation with current oscillations did not result in consistent data because ion current maxima and minima are determined not only by the plasma production but also by the transients of the arc pulse and by the asymmetry of the ion velocity distribution function. Perturbation with a short current spike was more conclusive. The most likely ion velocity, which depends on the cathode material but is generally (1-3) x 10(4) m s(-1), was measured to be reduced (about 25%) with increasing distance (0.5 m) from the cathode, which can be explained by collisions of ions with the background of neutrals. The ion velocity was slightly increased when the arc current was increased (about 15% when going from 50 to 400 A), which correlated with enhanced arc voltage and power dissipation. The ion velocity could be enhanced (by about 20%) when the plasma was produced in a non-uniform magnetic field (up to 300 mT).