  Under strong electric fields, an arc of strong current flowing through plasma
can link two metal surfaces even in ultra high vacuum. Despite decades of
research, the chain of events leading to vacuum arc breakdowns is hitherto
unknown. Previously we showed that a tall and sharp Cu nanotip exposed to
strong electric fields heats up by field emission currents and eventually
melts, evaporating neutral atoms that can contribute to plasma buildup.
  In this work, we investigate by means of molecular dynamics simulations
whether surface diffusion biased by the presence of an electric field gradient
can provide sufficient mass transport of atoms toward the top of the nanotip to
maintain supply of neutrals for feeding plasma. To reach the necessary
timescales and to add electric field in MD, we utilized a novel combination of
collective variable~-driven hyperdynamics acceleration and coupling to a finite
element mesh. In our simulations, we observed biased self-diffusion on Cu
surfaces, that can contribute to the continuous replenishment of
particle-emitting nanotips. This mechanism implies a need to reduce the rate of
surface diffusion in devices that are susceptible to vacuum arcs. Finding
suitable alloys or surface treatments that hinder the observed biased diffusion
could guide the design of future devices, and greatly improve their efficiency.
