  Vacuum arcing poses significant challenges for high-field vacuum devices,
underscoring the importance of understanding it for their efficient design. A
detailed description of the physical mechanisms involved in vacuum arcing is
yet to be achieved, despite extensive research. In this work, we further
develop the modelling of the physical processes involved in the initiation of
vacuum arcing, starting from field emission and leading to plasma onset. Our
model concurrently combines particle-in-cell with Monte Carlo collisions
(PIC-MCC) simulations of plasma processes with finite element-based
calculations of electron emission and the associated thermal effects. Including
the processes of evaporation, impact ionization and direct field ionization
allowed us to observe the dynamics of plasma buildup from an initially cold
cathode surface. We simulated a static nanotip at various local fields to study
the thresholds for thermal runaway and plasma initiation, identifying the
significance of various interactions. We found that direct field ionization of
neutrals has a significant effect at high fields on the order of 10 GV/m.
Furthermore, we find that cathode surface interactions such as evaporation,
sputtering and bombardment heating play a major role in the initiation of
vacuum arcs. Consequently, the inclusion of these interactions in vacuum arc
simulations is imperative.
