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Flyura Djurabekova. News from Helsinki…. Helsinki Institute of Physics and Department of Physics University of Helsinki Finland. Outline. Basic outline of activities at Helsinki Institute of Physics and not only Multiscale/ multiphysics model to describe an arcing event News:

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Flyura djurabekova

Flyura Djurabekova

News from Helsinki…

Helsinki Institute of Physics and Department of Physics

University of Helsinki

Finland


Outline

Outline

  • Basic outline of activities at Helsinki Institute of Physics and not only

    • Multiscale/multiphysics model to describe an arcing event

  • News:

    • Novel view on the “worm”-like features found in high resolution SEM images of electrical breakdown spots

    • The latest simulations coupling the dislocation activity in a stress concentrator and effect of electric field


Problem at hand

Problem at hand

Setup to measure dc sparks on a flat cathode surface (CERN)


Problem at hand1

Problem at hand

Means for solution: a Model?


Multiscale multiphysics computational model to simulate electrical breakdown

…Multiscale/multiphysics computational model to simulate electrical breakdown

Stage 1:Charge distribution @ surface

Method:DFT with external electric field

~few fs

Stage 2: Atomic motion & evaporation

+

Joule heating (electron dynamics)

Method: Hybrid ED&MD model (includes Laplace and heat equation solutions)

~fewns

~ sec/min

~ sec/hours

Stage 3a: Onset of tip growth; Dislocation mechanism

Method:MD, ED+MD, FEM…

Stage 3b:Evolution of surface morphology due to the given charge distribution

Method:Kinetic Monte Carlo

R. Behrisch, Plenum, 1986

B. Jüttner, 1979

Stage 4:Plasma evolution, burning of arc

Method:Particle-in-Cell (PIC)

~10s ns

~100s ns

Stage 5: Surface damage due to the intense ion bombardment from plasma

Method:Arc MD


Behind the model

University of Helsinki, Finland

Behind the model…

Doc. Flyura Djurabekova

Senior scientist

Prof. Kai Nordlund

M Sc AarnePohjonen

Dislocations

M Sc Stefan Parviainen

Field emisssion and neutral atom evaporation

M Sc AvazRuzibaevCharges on surfaces

Dr LottaMether

Plasma simulations

CERN, Switzerland

M Sc Anders KorsbäckExperiments at CERN

Dr. Yinon Ashkenazy

Hebrew University

Israel

Dr. VahurZadin

University of Tartu

Estonia

Inspiration from and constant discussions with CERN, Geneva

… and many other colleagues from CLIC project, KEK and SLAC

Dr. Walter Wuensch

Sergio Calatroni


High resolution sem new features called flowers or spaghetti

High resolution SEM: new features called ”flowers” or ”spaghetti”

  • The halos seen around the breakdown spots were well identified in HR-SEM as the dense network of white lines (in SEM images) of uncertain origin

SEM images here after are courtesy to presentation by Markus Aicheler at “MeVArc-2012”


Fine flowers as plastic response of surface on a sparking strike

Fine ”flowers” as plastic response of surface on a sparking strike

  • The “flowers” spread well beyond the actual breakdown spots


Zoom in one part where the lines look the clearest

Zoom in one part where the lines look the clearest


Vectorized image

Vectorized image

  • Binarize “flowers” to be analyzed by a computer

r


Filtered view

Filtered view

  • Filter out the undistinguishable areas to reduce the unnecessary noise in the analysis


Fit in polygons

Fit in polygons

  • Polygons are fit in the white areas, which of course is not always perfect, but sufficiently close


Angle distribution gives a peak at 60 o

Angle distribution gives a peak at 60o

  • Peak at 60 degree indicates a dislocation origin of the white ”worms” features. The most straightforward thought is that the slip planes intersect each other on Cu {111}-surface at 60o.


News from helsinki

  • Experimental evidence that there is some correlation between the “worms” and grains with 111 oriented surfaces

  • All the experimental analysis of surface structure is courtesy of Markus Aicheler

IPF coloring EBSD data

All Euler coloring EBSD data


Worms against the random walkers

Wormsagainst the randomwalkers

  • We have also ran some 2D random walk simulations , but did not find the regular patterns seen in the “worms”


Electrodynamics molecular dynamic model study of a void under tensile stress

Electrodynamics-molecular dynamic model+ study of a void under tensile stress

  • ED-MD model follows the evolution of the charged surface.

  • The dynamics of atom charges follows the shape of electric field distortion on tips on the surface

  • Wealsostudies the dislocationdynamics on a voidburrowednear the surface in Cuheldunderunilatertensilestress.

Details in F. Djurabekova, S. Parviainen, A. Pohjonen and K. Nordlund, PRE 83, 026704 (2011).

A. Pohjonen, F. Djurabekova, et al., Jour. Appl. Phys. 110, 023509 (2011).


Back to simulation with assumed voids under surface

Back to simulation with assumed voids under surface

  • We have now finished the analysis of the behavior of a void under tensile stress due to the electric field (Simulations now done with the hybrid ED-MD code, where the electric field effect is accounted explicitly)


The gradual growth seen previously is catastrophic in the presence of the field

The gradual growth seen previously is “catastrophic” in the presence of the field

  • The analysis of the protrusion height increase shows an asymptotic character. Once it starts growing, the self-reinforcing effect of the field enhancement around the tip of the protrusion causes the increase of its height in the ”catastrophic” manner


Initial plastic deformation

Initial plastic deformation

  • the top view and a slice of the system at time t = 114 ps when the first bump has formed


Catastrophic growth of a protrusion at the void

“Catastrophic”growth of a protrusion at the void

  • . the top view and a slice of the system at time t = 130 ps when the fully developed protrusion is clearly visible.


Analysis of the shear stress near the void surface

Analysis of the shear stress near the void surface

  • We analyzed more carefully the stresses at the void surface

  • We find that even if the void is burrowed in the bulk the shear stress is more than half of the value of tensile stress at the surface


Deeper view on atomic level stress

Deeperview on atomiclevelstress

  • The distribution of shear stress xz on MD simulations along a line (directed in z-direction) starting from the point of maximum shear on void surface and ending on the material surface for a void located at depth H = 2R. b) A detailed view of the atomic level stress near the void surface shows an alternating pattern caused by the surface stress. c) The difference between the simulation results with external stress and pure surface stress corresponds to the analytical result.


Summary

Summary

  • The model has been actively developed and gave many new insights in the physics of the plasma onset and surface damage

  • The model underlines the importance of mechanical properties of metal surfaces

  • We also noticed these to be important for surface damage

    • Angular analysis showed a dislocation origin of “worm”-like features around breakdown spots

  • The coupling of dislocation model and electric field effect resulted in “catastrophic” protrusion growth, which was not observed previously, but intuitively in line with field emission measurements from flat copper surfaces.


Thank you for attention and valuable feedback

Thankyou for attention and valuable feedback!

Accelerator Laborary, Helsinki


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