In a recent paper, accepted for publication in Nanotechnology, we describe a recent break-trough in the research about how vacuum arcs may form.
Vacuum arcs are electric disharges, a bit like small lightening, that have been observed to appear at very high electric fields. Vacuum arcs have in particular been a problem for the development of high-energy particle colliders, like the CLIC linear accelerator, currently developed at CERN. In these kind of accelerators, the particles (electrons and positrons) are accelerated by high electric fields. Higher fields gives higher accelerations and thus higher collision energies. However, as the arcs will appear above certain fields strengths and disturbe the particle beams, this puts a limit on the maximum energy of the accelerator. This is one of the reasons that vacuum arcs have been in focus for a lot of research for many years.
The main hypothesis for explaining how vaccum arcs appears goes like this:
- Due to the strain caused by the electric field in the copper accelerator tube, dislocations (line-like defects, present in all metals) are set in motions and some of them will penetrate the surface on the inside of the tube and create a small asperity.
- The small asperiety will locally enhance the electric field present in the tube and a process where surface atoms are accumulated at the asperity due to biased diffusion is started. The asperity will thus grow into a nanotip.
- When the nanotip has grown large enough, it will have enhanced the local fields enough to emit electrons and neutral atoms by field emission. This will build up a plasma in the vacuum of the accelerator tube.
- When the The plasma is enough dence, it will burn in the form of a vacuum arc.
- The arc will damage the coppar surfaces and only a crater is left, destroying any traces of any dislocations or nanotips. These craters are large enough to be seen by the naked eye.
So far, all of these points are to a high degree supported by theoretical and experimental works, except for the second point about the growth of nanotips, that until now had not been investigated.
In our new paper, we show using a new simulation model for surface atom diffusion in electric fields that, given high enough fields and temperatures, a small surface asperity may indeed start to grow into a nanotip due to biased atom diffusion (see the animation of the simulation above). It is the first Kinetic Monte Carlo model to take the effect of the electric field into account for atom diffusion. In this work we simulate tungsten atoms on a tungsten surface, as this allowed us to validate our model with a previous experiment, but the mechanism is very likely to be the same for surfaces of other metals, such as copper. The simulation results thus give strong support for the nanotip hypothesis for the formation of vacuum arcs, described above.
Reference:
Ville Jansson, Ekaterina Baibuz, Andreas Kyritsakis, Simon Vigonski, Vahur Zadin, Stefan Parviainen, Alvo Aabloo, Flyura Djurabekova, “Growth mechanism for nanotips in high electric fields” 2020 Nanotechnology https://doi.org/10.1088/1361-6528/ab9327 (E-print: https://arxiv.org/abs/1909.05825)
This work was done using the opensource Kinetic Monte Carlo code Kimocs, together with the field solver from the code Helmod (not opensource). All the details of the code, model and the results are described in the paper (and in the open-access e-print).
Ville Jansson, PhD 