Biased self-diffusion on Cu surface due to electric field gradients

TL;DR

This study uses advanced molecular dynamics simulations to show that electric field gradients cause biased surface diffusion on copper, which can sustain plasma-generating nanotips and influence vacuum arc breakdowns.

Contribution

It introduces a novel simulation approach combining hyperdynamics and finite element coupling to demonstrate biased Cu surface diffusion under electric fields.

Findings

Biased self-diffusion on Cu surfaces observed in simulations.

Surface diffusion can replenish nanotips, contributing to plasma formation.

Reducing surface diffusion could improve device resilience against vacuum arcs.

Abstract

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.