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

Abstract

Cathodic arc plasmas are considered fully ionized and theycontain multiply charged ions, yet, gaseous and metal neutrals can bepresent. It is shown that they can cause a significant reduction of theion charge states as measured far from the cathode spots. Several cathodematerials were used to study the evolution the mean ion charge state as afunction of time after arc ignition. The type of cathode material, arccurrent amplitude, intentionally increased background gas, additionalsurfaces placed near the plasma flow, and other factors influence thedegree of charge state reduction because all of these factors influencethe density of neutrals. In all cases, it was found that the mean ioncharge state follows an exponential decay of first order, Q(t) = A *exp(t/tau) + Qss, where A is a parameter describing the importance of thedecay, tau is the characteristic decay time, and Qss is a steady-statevalue approached for continuous arc operation. The extrapolated valuesQ(t-->0) indicate surprisingly high mean charge states as produced atcathode spots and not "skewed" by charge exchange collisions withneutrals