  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.
