Time-dependence of ion charge State distributions of vacuum arcs: an interpretation involving atoms and charge exchange collisions

Experimentally observed charge state distributions are known to be higher at the beginning of each arc discharge. Until know, this has been attributed to cathode surface effects in terms of changes in temperature, chemical composition, and spot mode. Here it is shown that the initial decay of charge states of cathodic arc plasmas may be at least in part due to charge exchange collisions of ions with neutrals. Neutrals gradually fill the discharge volume, and therefore, the effect of charge exchange shows delayed onset after arc initiation. Besides desorbed gases, sources of neutrals may include evaporated atoms from macroparticles and still-hot craters of previously active arc spots. More importantly, atoms are also produced by energetic condensation of the cathodic arc plasma. Self-sputtering is significant, and additionally, ions have a low sticking probability when impacting at oblique angle of incidence. Estimates show that the characteristic time for filling the discharge volume agrees well with the charge state decay time, and the likelihood of charge exchange is reasonably large to be taken into account.