Plasma fluctuations, local partial Saha equilibrium, and the broadening of vacuum-arc ion charge state distributions

Charge state distributions (CSDs) of Fe, Co, Zr, Nb, Hf, Ag, Pt, Au ions produced at vacuum arc cathode spots are investigated without often-used averaging procedures. Statistical results are used to evaluate the contribution of fluctuations to the broadening of averaged CSDs. Calculations of Saha equations in the Debye-Huckel approximation are often successful in replicating experimental CSDs even when using the simple model of “instantaneous freezing”. This allows the use of charge state spectrometry as a plasma diagnostic tool for the dense plasma near the cathode spot. Typically, total heavy particle (atoms and ions) densities and electron temperatures of 10²⁴-10²⁵m^-3 and 3-4 eV have been measured this way at the transition zone from equilibrium to nonequilibrium, the “event horizon”. It was also found that CSDs for some elements such as Co, Ag and Hf cannot be accurately simulated with the assumption of instantaneous freezing. Here, a more gradual transition to nonequilibrium occurs. This transition can be described by the model of partial local Saha equilibrium. Finally, an axial magnetic field was found to soften the transition to nonequilibrium thus causing broadening of experimental CSDs