Mechanism of Spontaneous Surface Modifications on Polycrystalline Cu Due to Electric Fields.

Kristian Kuppart,Simon Vigonski,Flyura Djurabekova,Ye Wang,Alvo Aabloo,Veronika Zadin,Andreas Kyritsakis

doi:10.3390/mi12101178

Kristian Kuppart, Simon Vigonski + Show 5 more

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https://doi.org/10.3390/mi12101178

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Abstract

We present a credible mechanism of spontaneous field emitter formation in high electric field applications, such as Compact Linear Collider in CERN (The European Organization for Nuclear Research). Discovery of such phenomena opens new pathway to tame the highly destructive and performance limiting vacuum breakdown phenomena. Vacuum breakdowns in particle accelerators and other devices operating at high electric fields is a common problem in the operation of these devices. It has been proposed that the onset of vacuum breakdowns is associated with appearance of surface protrusions while the device is in operation under high electric field. Moreover, the breakdown tolerance of an electrode material was correlated with the type of lattice structure of the material. Although biased diffusion under field has been shown to cause growth of significantly field-enhancing tips starting from initial nm-size protrusions, the mechanisms and the dynamics of the growth of the latter have not been studied yet. In the current paper we conduct molecular dynamics simulations of nanocrystalline copper surfaces and show the possibility of protrusion growth under the stress exerted on the surface by an applied electrostatic field. We show the importance of grain boundaries on the protrusion formation and establish a linear relationship between the necessary electrostatic stress for protrusion formation and the temperature of the system. Finally, we show that the time for protrusion formation decreases with the applied electrostatic stress, we give the Arrhenius extrapolation to the case of lower fields, and we present a general discussion of the protrusion formation mechanisms in the case of polycrystalline copper surfaces.

Highlights

The planned Compact Linear Accelerator (CLIC) [1] in CERN uses extremely high electric fields (~100 MV/m) to accelerate electrons and positrons to energies up to 3 TeV.Operation under such conditions leads to vacuum breakdowns in the accelerating structures [2], decreasing the luminosity of the collider and increasing the power loss
We use molecular dynamics (MD) simulations to study the effect of the external stress induced by an electric field on the surface morphology of nanocrystalline copper and we investigate possible surface protrusion growth mechanisms due to the enhanced mobility of atoms on surface grain boundaries
There are 4 different mechanisms contributing to protrusion formation present in our simulations – surface diffusion, surface grain boundary diffusion, diffusion along bulk grain boundaries and intra-grain dislocation activity

Summary

Introduction

The planned Compact Linear Accelerator (CLIC) [1] in CERN uses extremely high electric fields (~100 MV/m) to accelerate electrons and positrons to energies up to 3 TeV Operation under such conditions leads to vacuum breakdowns in the accelerating structures [2], decreasing the luminosity of the collider and increasing the power loss. Emission current measurements in alternating current [3] and direct current [4,5] regimes point to the existence of significant field enhancement in the range of β ≈ 30–150, the physical origins of which is not entirely clear It has been theorized [6] that the source for this field enhancement is rough static features residing on the electrode surface acting as field emitters, surface features with aspect ratio high enough to cause field enhancement of this magnitude have never been observed [5]. Other surface features, such as contaminants, oxides and residual gas are believed to be removed from the electrodes subject to the conditioning process [4] in CLIC

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