Analysis of the interaction of cathode microprotrusions with low-temperature plasmas

On real cathode surfaces which are in contact with plasmas, micrometre-sized protrusions are abundant. A two-dimensional numerical model has been developed to investigate the interaction of such a cathodic micro-tip with a low-temperature plasma. The model allows integration and analysis of the various physical effects and an evaluation of their specific role in the heating, emission and erosion of cathode surfaces. In particular, the influence of heating due to the intense ion flux from the plasma sheath onto the cathode surface has been investigated in detail. Special attention was paid to the simultaneous action of strong electric fields in the cathode fall region. The plasma-cathode interaction is analysed for a range of parameters typical for high-current-density, low-pressure gas discharges and vacuum arcs. The temporal evolution of the thermophysical properties of the micro-tip, its interaction with the resulting metal vapour and its shape are modelled on a nanosecond time scale. Special cases of the evolution of microemitters have been identified, such as quasi-stationary emission on a microsecond time scale and extremely fast, explosive-like behaviour with emitter lifetimes on a nanosecond time scale. It is found that ion bombardment heating plays an essential part in the inception of emissive and erosive behaviour for a wide range of parameters.