Ion energy distribution functions of vacuum arc plasmas

Abstract

The velocity distribution function of vacuum arc ions can be measured by a time-of-flight technique. The measuring principle makes use of the well-justified assumption that the ion drift velocity from the cathode spot region to a collector is approximately constant. It is shown that the negative time derivative of the collector current is directly proportional to the ion distribution function provided that the time-averaged emission of ions from cathode spots is constant until the arc is rapidly switched off. In the experiment, arc termination took about 700 ns, which is much faster than the decay of the ion current measured at the collector placed more than 2 m from the cathode. The experimental distribution functions for most cathode materials show one large peak with a tail and one or more small peaks at higher ion velocities. The typical peak position is at about 104 m/s, with the precise values being material specific. The distribution functions for some materials exhibit not one but several peaks. No conclusive answer can be given about the nature of these peaks. Arguments are presented that the peaks are not caused by different charge states or plasma contamination but rather are due to insufficiently averaged source fluctuations and/or acceleration by plasma instabilities.