| The application of 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 current at the apex vaporizes the emitting tip and the emitting current triggers a plasma in the vapor close to the surface. The plasma in turn melts the emitting site and makes it (hopefully) disappear. The conditioning process consists into “burning” the emitting sites one after another and numerous observation exhibit surface covered with molten craters that more or less overlap. In the case of radiofrequency (RF) applied fields, effect of fatigue are also considered due to the cyclic nature of the applied stress (Laplace forces). Nevertheless when dealing with RF cavities for accelerators, where higher and higher fields are now aimed, one can legitimately wonder if other physical phenomena should now also be taken into account.
In particular, we think that electromigration, especially at surfaces or grain boundaries cannot be neglected anymore at high field (i.e. 50-100 MV/m). In particular, intensive publication in the domain of liquid metal emission source show that very stable and strong emission sources, either ions or electrons, built up on metallic surfaces submitted to electrical field through a mechanism that is slightly different from the usual, localized breakdown usually evoked in accelerators. This mechanism involves the combination of electromigration and collective motion of surface atoms. In the case of emission source, this effect is looked for and has been extensively studied whereas in our case it is very detrimental to the possibility of reaching high fields.
In this paper we intend to bibliographically review in the literature some of the physical phenomena involved on metallic surfaces submitted to very high fields that could explain, in particular, early melting of large areas of the surface as was observed on 30 GHz CLIC (Compact Linear Collider) accelerating structures. We will not discuss the plasma, or plasma spot, formation at the surface as we consider it to be the next step into the formation of the vacuum arc. Yet it is very difficult to provide evaluation of the relative weight of each phenomenon; however, we hope to provide new angle of observation that could help us to better understand and possibly overcome the observed experimental limitation. Observation of recent results obtained at CTF3 (CLIC test facility) led us to propose additional or alternative scenarios that could be involved in the experimental observation.
Some of the data gathered hereafter let us think that the combination of RF and other interface/surface related phenomena tends to lower field limit, compared to what is expected considering the cohesion of bulk material.
We hope that this review, destined to the accelerators community, will provide new angles of vision on the way to determine the actual maximum field a material can sustain without damage, and how to mitigate the detrimental surface effects. In this paper we will mainly concentrate on the early stages of breakdown, before plasma apparition.
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