  When an electron emitting tip is subjected to very high electric fields,
plasma forms even under ultra high vacuum conditions. This phenomenon, known as
vacuum arc, causes catastrophic surface modifications and constitutes a major
limiting factor not only for modern electron sources, but also for many
large-scale applications such as particle accelerators, fusion reactors etc.
Although vacuum arcs have been studied thoroughly, the physical mechanisms that
lead from intense electron emission to plasma ignition are still unclear. In
this article, we give insights to the atomic scale processes taking place in
metal nanotips under intense field emission conditions. We use multi-scale
atomistic simulations that concurrently include field-induced forces, electron
emission with finite-size and space-charge effects, Nottingham and Joule
heating. We find that when a sufficiently high electric field is applied to the
tip, the emission-generated heat partially melts it and the field-induced force
elongates and sharpens it. This initiates a positive feedback thermal runaway
process, which eventually causes evaporation of large fractions of the tip. The
reported mechanism can explain the origin of neutral atoms necessary to
initiate plasma, a missing key process required to explain the ignition of a
vacuum arc. Our simulations provide a quantitative description of in the
conditions leading to runaway, which shall be valuable for both field emission
applications and vacuum arc studies.
