Supersonic metal plasma impact on a surface: an optical investigation of the pre-surface region

Abstract

Aluminium plasma, produced in high vacuum by a pulsed, filtered cathodic arc plasma source, was directed onto a substrate to form a coating. The accompanying ‘optical flare’ known from the literature was visually observed, photographed and spectroscopically investigated with appropriately high temporal (1 µs) and spatial (100 µm) resolution. Consistent with other observations using different techniques, it was found that the impact of the fully ionized plasma produces metal neutrals as well as desorbed gases, both of which interact with the incoming plasma. Charge exchange collisions between doubly charged aluminium ions and neutral atoms lead to a reduction of the flow of doubly charged ions before they reach the surface. Accordingly, some of the neutrals become ionized as they move away from the surface. These plasma–wall interactions are relevant for coating processes such as plasma-based ion implantation and deposition. They are also important for interpreting the plasma properties such as ion charge state distributions. Specifically, ions in cathodic arcs may be more highly charged near their origin, the cathode spots, than when measured far from the spots.