Mechanisms of vacuum arching in high electric field systems

From Intelligent Materials and Systems Lab

CERN has started a new compact electron-positron linear collider CLIC project with the aim of precise measurements of the properties of Higgs boson and to collect new data about fundamental physics topics like supersymmetry, Higgs strong interactions, contact interactions and extra dimensions. The collision energies in CLIC are in range of 0.5-5 TeV, reach by extreme electric fields (in range of 100 MV/m). However, such field values often lead to the electric breakdowns of the system. Controlling the breakdown rate and keeping it under critical level is one of the key issues to ensure successful operation of CLIC.

The electrical breakdown (BD) signifies vacuum arc discharges in accelerating structures. The BD’s taking place under stable operating conditions (not due to surface contamination), can be limited with “conditioning” techniques. The conditioning takes several hundreds of hours, making it inconvenient for large scale production. Instead, materials science based optimized solution, explaining the breakdown mechanisms and causes is needed.

Theories explaining the breakdowns link them to emission currents and spontaneously appearing nanoscale field emitters. However, mechanism responsible of the appearance of such emitters, is not yet understood. The field emission currents are commonly described by Fowler-Nordheim type equations, from where the local electric field enhancement factor β is estimated. Common explanation for the β is the surface roughness, protrusions or other irregularities, with the geometrical aspect ratio in the same order of magnitude as field enhancement. Since the theoretical field strength needed to initiate a vacuum arc in case of copper is in the order of 10 to 11 GV/m, it’s commonly assumed that high aspect ratio field emitters are always present before the breakdown.

The aim of the current work is to understand the mechanisms leading to vacuum arching in high electric field systems. Most important gain is fundamental understanding of processes initiating the vacuum arching, leading to the development of new technologies utilizing high electric fields such as new generation particle accelerators, novel nanofabrication applications, free electron lasers, medical linear accelerators for hadron therapy or design of better RF components for satellite microwave devices.