Electromigration occurrences and its effects on metallic surfaces submitted to high electromagnetic field : A novel approach to breakdown in accelerators

TL;DR

This paper explores how electromigration at metallic surfaces under high electromagnetic fields contributes to breakdown phenomena in accelerator RF cavities, proposing a new scenario involving pre-plasma formation due to atom migration.

Contribution

It introduces a novel perspective on breakdown mechanisms by emphasizing electromigration effects, supplementing classical models focused on explosive electron emission.

Findings

Electromigration significantly influences surface melting and breakdown.

Surface atom collective motion can lead to early surface damage.

A new pre-plasma formation scenario is proposed based on electromigration.

Abstract

The application of a high electrical field on metallic surfaces leads to the well described phenomena of breakdown. In the classical scenario, explosive electron emission (EEE), breakdown (BD) originates from an emitting site (surface protrusion). The conditioning process consists of "burning" the emitting sites one after another and numerous observations exhibit surfaces covered with molten craters that more or less overlap. When dealing with RF cavities for accelerators, where increasingly fields are now sought, one can legitimately wonder if other physical phenomena should also be taken into account. In particular, we believe that electromigration, especially at surfaces or grain boundaries cannot be neglected anymore at high field (i.e. 50-100 MV/m). Many publications in the domain of liquid metal emission sources show that very stable and strong emission sources, either ions or electrons, build up on metallic surfaces submitted to electrical fields through a mechanism that is slightly different from the usual localized breakdown evoked in accelerators. This mechanism involves the combination of electromigration and collective motion of surface atoms. The recent results obtained on 30 GHz CLIC (Compact Linear Collider) accelerating structures, altogether with the data exposed hereafter have led us to propose a complementary scenario (pre-plasma apparition), based on electromigration, which could explain early melting of large areas of the surface.