Thermal runaway of metal nano-tips during intense electron emission

Abstract

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 finitesize 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.arXiv:1710.00050v3 [cond-mat.mtrl-sci]

Dynamic coupling between particle-in-cell and atomistic simulations

“…Moreover, it was shown that it is sufficient to supply about 0.015 neutrals per electron to initiate the ionization avalanche leading to the formation of a stable plasma. Recent multiscale atomistic simulations on Cu nanotips [12] revealed a thermal runaway process which causes field-assisted evaporation of Cu atoms and nanoclusters at a rate exceeding the threshold found in Ref. [11].…”

DC vacuum breakdown in an external magnetic field

“…A considerably increased electric field and higher emitted currents are observed on metal surfaces exhibiting asperities or some other irregularities. Phenomena accompanying the breakdown (heating, melting, evaporation of asperities with subsequent crater formation in the electrodes) are considered at length in [8][9][10][11].…”

Field electron emission induced glow discharge in a nanodiamond vacuum diode

“…We believe that the precursor rise in temperature can be explained by Nottingham process [30]. According to analytical formulation, supported by the recent computational results [11], the inversion temperature should exist, that balances out the Joule effect and guarantees a stable emission within a certain I-E domain for classical emitters based on tungsten and…”