Production of neutrals and their effects on the ion charge states in cathodic vacuum arc plasmas

Cathodic arc plasmas are considered fully ionized and they contain multiply charged ions, yet gaseous and metal neutrals can be present. It is shown that they can cause a significant reduction of the ion charge states as measured far from the cathode spots. Several cathode materials were used to study the evolution of the mean ion charge state as a function of time after arc ignition. The type of cathode material, arc current amplitude, intentionally increased background gas, additional surfaces placed near the plasma flow, and other factors influence the degree of charge state reduction because all of these factors influence the density of neutrals. In all cases, it was found that the mean ion charge state follows an exponential decay of first order, $Q¯(t)=Aexp(−t∕τ)+Q¯ss$⁠, where $A$ is a parameter describing the importance of the decay, $τ$ is the characteristic decay time, and $Q¯ss$ is a steady-state value approached for continuous arc operation. The extrapolated values $Q¯(t→0)$ indicate surprisingly high mean charge states as produced at cathode spots and not “skewed” by charge exchange collisions with neutrals.